Speaker: Dr. Chris Marble, Associate Professor of Horticulture, University of Florida IFAS, Mid-Florida Research and Education Center, Apopka, FL
Moderator: Dr. Shimat Joseph, UGA Turfgrass Entomologist
Duration: 45:13
Series: Green & Commercial
Webinar Date: March 12, 2026
Introduction
Shimat Joseph: Okay, so let me introduce our first speaker. Our first speaker is Dr. Chris Marble. Dr. Chris Marble is an associate professor of horticulture at the University of Florida, Institute of Food and Agriculture Sciences, Mid-Florida Research and Education Center, located in Apopka, Florida. His research and extension program focus on weed management in ornamental plants, both in production and landscape plantings. Major research and extension effort focus on developing and implementing new and innovative methods of weed management in nursery production and landscape plantings and understanding how production and maintenance practices influence weed growth, reproduction, herbicide efficacy, and ornamental plant growth. Dr. Marble, the floor is yours.
Chris Marble: All right. Thank you very much. Thank you all for attending and thank you all for inviting me. I’m hoping that my screen looks good, sharing and everything. So we’ll go ahead and get started. Today I’m going to be talking about weed control in nurseries and greenhouses. Mostly, if you’re focused on weed control in landscapes, hopefully a lot of this will still apply to you as well. So the same principles and things like that. But we’ll talk about some of the latest research that I’ve done. I’ll go quickly through those results and then kind of how it’s applicable in real-world scenarios, because that’s kind of what my program does is everything that we do, it’s very applied and focused on developing solutions for growers.
Non-Chemical Control Options
Chris Marble: So the first thing that we are going to talk about here is non-chemical control options. So rethinking some of those non-chemical control options. Down in Florida, I mean, we have a lot of people that are growing palms and different tropicals and things like that, but everybody is going to have, you know, a few plants, whether you’re doing hydrangea, which is kind of the poster child for herbicide sensitive plants, or you’re doing a lot of annual and perennials and things like that. You can’t use herbicides in those situations, and so there are a lot of non-chemical tools that can be utilized, whether or not you’re using herbicides at all, that can improve control and that’s been a focus of my program over the last couple years.
But I do want to say this, that these non-chemical controls, they’re not an all-or-nothing tool. So you don’t have to incorporate these throughout the entire production system. If you have a specific area that you’re having more weed pressure or you can’t use herbicides or you can’t even use the same herbicides, you know, maybe you have a block of plants or a specific group of species that you can only use maybe one or two herbicides on those. You can incorporate some of these non-chemical controls to get better control there.
But all these different things should be part of an overall program, really, regardless of herbicide use. So things like sanitation and prevention, people are going to be, you know, putting those into play and that’s going to be part of their program regardless of whether they’re using herbicides or not. But some of these things that I’ll talk about, like rice hull mulch, for example, which is what we’ve done, the interest in that has really increased over recent years and so we’ve done a lot of work trying to answer some of the questions growers have had about that.
Hand Weeding Frequency Study
Chris Marble: Hand weeding is always going to be needed and it can usually be improved. So you think of something like hand weeding and it’s pretty straightforward. You know, you go through and you’re cleaning out the plants when weeds break through. But we actually did some research that was led by Joe Neal at North Carolina State, where we looked at hand weeding frequency and how hand weeding practices can be altered to get better control and actually reduce labor. We saw a 40 to 60% reduction in weeding cost by just weeding more frequently, but not weeding more in terms of time. So I’ll talk about how we did that study real quick.
So this is a study that we did several years ago. It was sponsored by the Southern IPM Center. We had blocks of plants. So this is just what we did in Florida. We carried it out a little bit differently in North Carolina, which is where Dr. Neal is at, and then Virginia, which is where Jeff Derr did his studies at. So they used different herbicide combinations and had a little bit different treatments. This is what I did in Florida. So we had two blocks of plants. The two herbicides that we chose to use were Tower and Gemini, which are two commonly used pre-emergent herbicides. And it was what was actually used at this particular nursery.
All we did was we had separate blocks of plants. We had several hundred plants in each block. They were all treated with those herbicides. Half of the plants were weeded every two weeks and then half weeded every eight weeks. So that’s like a typical weeding schedule that you’ll see in a container nursery where people are putting a herbicide out and then they don’t really get weeded until it’s time to reapply that herbicide. Then they go in, send people in to completely hand weed those and then reapply. In this case, what we did was is there was scouting that was done every two weeks. If there were fairly large weeds there in those pots, those were going ahead and being pulled. And then what we did was is over that the span, we repeated this over a 24 week period. And then we, every time that we weeded, we recorded the time that it took to weed those plants. Then we also recorded biomass that we were collecting in those plants as they were being weeded.
So you would look at that and you would say, yeah, obviously you’re going to have less plants in the two-week interval. These are, you know, almost perfectly clean. That’s because these are being weeded basically four times what these are being weeded. That’s what you see in these pots here. So this is about a five-gallon terracotta pot here. And then the typical amount of weeds that will be pulled, two-week interval versus 16-week interval from the blocks. This has a terracotta pot back behind that. You just can’t see it because of how large the weeds got and then how many weeds we actually pulled out.
This is what was really eye-opening here is the biomass and then the cost per acre. So with the biomass, you know, we’re getting over a 9% reduction in weed biomass by weeding every two weeks. Those weeds were never able to mature and they were never able to go to seed if they’re being weeded every two weeks. And so our pressure just continually decreased over the span of that study, whereas, if we let the weeds go, those are two, you know, pretty effective pre-emergent herbicides, but no herbicide is going to give you 100% control. And so you would still have breakthroughs. And as those breakthroughs occurred within eight weeks in Florida in the summertime, you know, a lot of these weeds are going to be able to reproduce, set seed, you know, some of these species, somewhere between 10,000 to 100,000 seed on one plant, your pressure just continually increases. And so your herbicide has your margin of error gets smaller and smaller as that weed pressure gets higher. So our cost per acre dramatically decreased and continued to decrease when they were weeded more frequently.
Overall, the time savings on average is 66%. So in this case, these pots were really weedy. So we saved a lot of time by doing it every two weeks. Across the board, I would say the average at all the different nurseries that this was done at, the savings was about 40%. So, some nurseries, weeding by this method, where you’re not having to go through and if you have a little cotyledon-stage weeds there, we didn’t pull those because we know that we’re going to get those. We’re going to be back around to those pots within two weeks, so we didn’t have to meticulously weed every pot. We’re trying to get the biggest ones out and then just leave and be done with it.
So, some nurseries, when they have implemented this, they’re able to weed those blocks three to seven times faster by doing it this way. They’re not spending a ton of time getting those small cotyledon-stage weeds out. They’re just getting the large ones that are going to go to seed within the next week or two. And so we did this study, we found this, but then I started using it to when I do consulting trips for nurseries. I do consulting all across the country, and most of it’s in Florida, just through my extension appointment, but to develop cleanup protocols. There’s a lot of nurseries where they’ll have blocks or sections of plants that have just become completely inundated with weeds. And they’re trying to figure out, you know, how to get those back under control without spending a ton of money.
And so what they can do is kind of incorporate this same methodology where you’re going through, you’re just getting the big stuff out, and then you’re coming back and just continually hitting them. Getting them weeded, getting a pre-emergent herbicide down, see what breaks through, weed those again soon afterwards, and just keep going until that pressure starts to drop off.
This is just the same results. The publication where we kind of summarized all the results is frequent hand weeding saves money. If you just Google that, that publication will come up. The two authors on that are Barker and Neal, and that paper came out in 2016. But overall in Florida, 65% reduction in weeding costs.
This is kind of just an example of spurge and we’ve kind of come up with guides for different weed species whatever our nursery is dealing with. We’ll take some pictures and say you know this is about when we’re training people will say this is about what you can leave this is what you need to pull so you know weeds that are this small like if it was spurge those could be left those aren’t going to go to seed within two weeks. If they’re a few inches, bigger than maybe a half dollar or so, that’s the ones that you need to pull. Those small ones can be left. It allows you to weave more quickly. And they’re not going to be perfect, but perfect didn’t name the game. It’s just keeping them under control and getting them back under control.
Pre-Emergent Herbicides with Post-Emergence Activity
Chris Marble: Another thing too with these cleanup protocols is that if you’re weeding, you’re getting them as good as you can, good enough basically, and then putting the pre-emergent herbicide down. But with different pre-emergent herbicides, there’s been research over the past 10-15 years that we’ve seen that a lot of these pre-emergent herbicides that can be applied over the top of ornamentals have a lot of early post-emergence control. So this is just a few examples.
So if spurge was the big issue that you’re dealing with, using herbicides with dimethenamid-P, which would be like Tower and FreeHand, you can control spurge that’s already emerged with those pre-emergence herbicides. It’s going to, even if you don’t kill it, it’s going to stun it pretty significantly where you’re going to delay seed set. And indaziflam or Marengo, good for Oxalis, it’ll also stunt spurge, kill annual bluegrass. Isoxaben, that’s where most of this research was done with isoxaben because bittercress is very sensitive to that. You can kill bittercress up to the flowering stage with a labeled rate of Gallery or Gemini. Oxadiazon, Ronstar G, that’s the old Chipco G and a lot of other names, but artillery weed suppression. Dimension also has some, Oxalis, bittercress, and then your oxyfluorfen-containing products like OH2 and Biathlon. A lot of different species are suppressed with those.
Just to kind of show you, so this is Oxalis, the two to four leaf stage. This is four weeks after treatment with these different herbicides. These are all label rates of these herbicides. This is granular Marengo here. But you can see the non-treated control group. And then if we add Gallery and Dimension or Dimension, or we do a granular Marengo application, getting almost 100% control of small Oxalis there.
This is Gallery in bittercress. So you can see the non-treated control group. This was bittercress at the 6th and 9th leaf stage. So this is about to start flowering. And then we add Gallery and Dimension together. We’re getting, you know, almost 100% post-emergence control with these two pre-emergence herbicides. So you can see that if you were to just get those cleaned up good enough and get one of these pre-emergence down, you can take out some of those weeds that you missed while you were hand weeding. And then dimethenamid-P Broadstar also will have some control, but you see FreeHand and Tower and the effect that it has on spurge. Just some things to keep in mind if you’re battling the weeds and you’re trying to get areas back under control. Choosing some of these pre-emergence herbicides that have some post-emergence activity can really be a good route to go down.
Rice Hull Mulch Research
Chris Marble: So the next thing that we did, we’ve done a lot of work with rice hulls. People are really interested in these. They’re lightweight and hydrophobic. They’re easy to apply, which is one of the big reasons that people like to use them. And then they’re very small particle sizes. And what that does for you is that allows you to use lower depths and still get complete surface coverage of the substrate.
So this is just an example of that. So pine bark mini nuggets here. This is actually a that screen to about everything. I think it was like a three-quarters inch to a three-eighths inch size. And then all the fines were removed and using that as a mulch. And then this is like a landscape pine bark mulch. And you can see that if you put these out the same depth, these smaller particle sizes, you’re going to get complete surface coverage, which is what you have to have for weed control. Whereas with these larger particle sizes, you’ve got to go three inches with some of these landscape mulches to get sufficient coverage, just depending upon how big those particle sizes are. So pine bark nuggets great for the landscape but in a container not going to be really the best choice unless it’s a really large container. Small containers it doesn’t make a lot of sense.
This is definitely not a new concept. A lot of people have done you know research with mulches in nurseries. Second speaker today, Dr. Bartley has done a lot of work with this as well. But for rice hulls specifically, a lot of questions kind of came up when people started to use these. Number one, can you top dress your controlled-release fertilizer on top of the rice hulls and are you going to get any suppression of growth? Number two, if you do have small weed seeds that have germinated, can you still top dress those rice hulls on top or do they need to be weed-free pots? Can they be combined with herbicides? And then also, most of the research that’s been done with rice hulls has looked at greenhouse environments, specifically on two species, bittercress and Oxalis. And so people said, “I’m not going to put these in the greenhouse. I want them outside. How long are the rice hulls going to last? And then how well are they going to work on these outdoor species like spurge, for example, that not a lot of people had done work with rice hulls, spurge or some of these other ones.”
We stood out. Horticulture Research Institute was great in funding some of this research as well as the National Horticulture Foundation. So a shout out to them.
Fertilizer Placement Effects
Chris Marble: But this is some of the work we did with fertilizer placement. And then we did see that, you know, if you top dress on top of rice hulls, especially at higher layers, you can get reductions in growth of some ornamentals. So in this study, we had coleus and Vitex. They were potted up into two-gallon pots. Rice hulls were applied at zero inches, so no rice hulls, and then at one and two inches. And then we had fertilizer that was either incorporated into the media or it was top dressed, and that top dress was on top of that rice hull layer.
You can see with coleus in particular, when the fertilizer was incorporated, we had significantly higher growth than if the rice hulls were top dressed on top, which you see right there. Vitex, you also had a little bit of a growth difference. It was significant, but not as much so as with coleus, which was a faster growing species. Here you can see it across the board, incorporated versus top dress at all the different depths of rice hulls. Definitely when you get into two inches, much greater growth with the incorporation versus the top dress.
We looked into why that was kind of occurring. Really, we didn’t see a whole lot of differences with EC or how much of that fertilizer is actually releasing. There’s a lot of other things that we could have done. We just didn’t have the funding to do it. So we just took EC measurements and looked at nitrogen content, basically. But this is basically what we saw with that top dress treatment. We are having higher electrical conductivity, especially in those first few weeks after potting and those rice hulls what they’re doing is they’re drying very quickly after rainfall or irrigation and what does a controlled-release fertilizer need to release from its coating it needs moisture and so because it’s drying out quicker you don’t get the same release rate we think we need to do some more research with that but that’s just to say you know if you are using rice hulls be mindful of this. Fertilizer rates may need to change or, you know, using rice hulls on shorter term plants where they can just be incorporated and you don’t have to come back in and top dress on top of them because you could see reductions in growth.
Emerged Seedling Suppression
Chris Marble: Then we also looked at hand pulling prior to application. With rice hulls, they might be applied, you know, right at potting, which is what I recommend. Or they could be, you know, done with something like this where you have these rice hull blowers that will be blown into pots after they’ve already been in production. It depends upon how a nursery is set up, which one is going to make the most sense. And be the easiest to do. Stilt Pro has some cool rice hull blowers and things that you see in this photo here.
But we looked at this and we looked at several different weed species at different growth stages and we said, “Okay, what happens if we just put rice hulls on top of these after they already emerged? Is the rice hull still going to be effective on weed seeds that have emerged?” So here we have here, so this is Eclipta and Phyllanthus. We also had liverwort in this trial here. But we had five plants per pot and then we basically put rice hulls on it at several different layers. So we had no rice hulls, then we put it on a quarter inch, a half inch, one and two inches. And then we had two different stages of growth. We had Phyllanthus and Eclipta at the two to four leaf stage and then also at the cotyledon stage where we just had maybe one true or two true leaves on there like you have here.
And so we found that when you get to a half inch of rice hulls, you’re definitely getting a big drop off of the small stage of Eclipta, we almost got 100% control. When you get up to about one inch, you’re reducing the survivability of those weeds by over 95%. And so if you were putting rice hulls out onto, it’s going to be best to do it to weed-free pots. But if you had some small seedlings coming up, and you were top dressing the rice hulls at at least an inch, you could go ahead and put those down and then just look for breakthroughs and see what came through that because you are going to kill a lot of those small weeds if they’re not mature and large by that point.
We did this with bittercress and Oxalis as well, and we kind of saw the same thing where at that one-inch layer with those two species had basically 100% control. None of those small weeds were able to emerge up through that rice hull layer, if they had already germinated, which I’ll show you that in just a second. And here’s cotyledon stage. So this is just the example. So this is at two weeks after the rice hulls were applied, and the survivability decreases as rice hull depth increases.
Outdoor Performance and Seed Placement
Chris Marble: So another question that we get a lot of times is how are they going to perform outdoors? So in this study we had two different depths, a half inch and one inch, and then we had two seed placements. So we had rice hulls that were put above the mulch, that would be like if rice hulls were applied at potting, and then weed seeds were introduced on top of that mulch layer over time. And then we had seeds that were placed below the mulch, which might occur if the plants were potted and then rice hulls were blown in on top, you know, several weeks or months after the plants were out there in production.
And so we looked at species that hadn’t really been looked at in the literature before. Doveweed, crabgrass, spotted spurge, Eclipta, and long-stalk Phyllanthus. Here’s what we saw across the board. This was really clear cut. We’re working on this paper now to have this out. And this was easy to write up because the results were really clear cut.
When we put the seeds on top of that rice hull layer, we were getting 89% control of these weeds over a 12 week period. If they were below, in a lot of cases we had similar growth as we did in the non-mulched group. So you can see here, so this is long-stalk Phyllanthus, no mulch, no herbicide or anything. This is the seeds placed on top of the mulch where we’re getting about a 9% reduction in weed growth and then below the mulch basically no suppression of the growth. When those weeds had already emerged like in the previous slides I went over we kind of smothered those weeds but then if they hadn’t germinated yet they were able to germinate and grow up and emerge through the rice hull layer. Same thing with doveweed, same thing with crabgrass, Eclipta, and spotted spurge.
The takeaway here was to apply the rice hulls as soon as possible because the way their mechanism of control is is they’re drying quickly after rainfall or irrigation. They’re not holding much moisture at all and so it’s not allowing those weed seeds to germinate. They don’t have enough moisture to germinate. And so you want to get those rice hulls on as soon as you can because if the weed seeds land on top, you’re going to have a lot better success with that as opposed to if they’re already on the surface of the soil when the rice hulls are applied.
Outdoor Degradation and Comparison to Pre-Emergent Rotations
Chris Marble: And then we looked at degradation just over time outdoors. So after 12 months about 40% of what we applied remained. In this case, we had twice daily irrigation. We did have hurricanes. We weren’t directly hit by the hurricanes, but that was kind of an interesting thing.
Rice hulls, the big complaint of them outdoors is that they’re going to blow out of the pots, and we have definitely seen that. They can definitely blow out of the pots. But in these big storm events, what comes with hurricanes? A lot of rain. And if those rice hulls are wet, they tend to stay put. It’s when we irrigate in the morning and then we have these strong winds in the afternoon where those rice hulls are dry. That’s when they’re blowing out of those containers. They’re ideal for short term crops where herbicides can’t be used. Something that’s going to finish in six to eight months. Long term crops, I would probably opt for or recommend something like a pine bark mulch because it’s going to last longer than rice hulls would. And you’re not going to have the blowout and things.
But they provide a control similar to a standard pre-emergence herbicide rotation. We just finished this work where we looked at rice hulls applied at different depths over a 12-month period and compared it to some standard pre-emergence herbicides. They perform similarly as those pre-emergence herbicides up until about the 6-month mark. Then the herbicides provided a lot better control because those rice hulls had degraded to a level where weeds were able to break through.
So this just kind of shows the degradation over time. So this is rice hulls applied at a half inch, one inch, and two inches, and then kind of how they broke down. By the time we got to 12 months, we had about 40% of what we originally applied, you know, depending upon the starting. So a good depth, you know, that I recommend for small containers is shoot for an inch or so. That’s a happy medium where you’re getting good weed control and it’s not, you’re not reducing, you know, potential root volume like you would if you went at two inches or something.
Substrate Stratification
Chris Marble: Another thing that we’ve looked at over recent years is substrate stratification. This was kind of pioneered by Jeb Fields’ research who’s at University of Florida now. But this is basically filling containers and layers with different substrates with different particle sizes and physical properties. Jeb came up with this concept and the idea that he had was he put really coarse mulch or coarse bark at the bottom of the container and had finer material at the top of the container and the idea was is that it would increase the water use efficiency and fertilizer efficiency by doing it that way. You can look up a lot of his information to go into more detail on that.
We took that concept and I talked to Jeb and Jim Owen and some of the people that have worked with that. And then we said we could probably flip this and see if there’s a weed control benefit where we put a more coarse mulch or coarse bark on top, not coarse like a mulch would be, but just a larger particle bark with some of those fines removed so that it’s not holding as much moisture and then not have fertilizer in that very top layer. But the ornamental that’s potted into that, you know, is still sufficient for those to root into it.
So this is the standard where it would just be a homogeneous mixture. And this would be like a stratified version where you have coarser bark at the top. The first two or three inches or so doesn’t have any fertilizer in there. And then the plants are potted in for larger liners. The liner can still access those nutrients, but then when weeds germinate, they have to grow for a while before they actually get down to the fertilizer layer. And they don’t have as much moisture. The top part of the bark doesn’t have a high enough moisture-holding capacity to support some weed species.
So we’ve looked at this. I’m not going to go into the specific differences that we did in this study in terms of stratification. It was basically different bark sizes and fertilization regimes and things. The main thing here is that all of these stratified treatments, liverwort will not grow on top of that. Liverwort has to have nitrogen, it has to have moisture to be able to grow. Even if you sub-dress fertilizer, liverwort won’t grow on those containers. It will grow very minimally. It might establish, but it’s not going to form a thick mat like it would if that fertilizer is incorporated or top-dressed. So stratification was excellent for liverwort control. We had over 20 weeks perfect weed control when the substrates were stratified.
Bittercress, not as good as liverwort, but we definitely did reduce bittercress growth by 40 to 60% with our stratified treatments, which you see all here in between these two yellow lines. These are our industry standard treatments here.
Herbicide Rotation Principles
Chris Marble: And so now I’m going to get into the last part of the presentation is just some keys with developing a rotation and talk about some of the research that we’re doing now. So when you’re using herbicides, I can go and I can give a talk on all the different herbicides and efficacy charts and all those different things that we’ve looked at. But I try to give people the tools and say, you know, we know all these herbicides work. You have a lot of options, but it’s picking the right herbicide for the job that you needed to do. So developing a rotation is a big part of that.
When you’re developing a herbicide rotation, you want to look, use guides to see what all is labeled. This is the 2017 Southeast Pest Management Guide. I tell people to this particular guide because we went through all the herbicide labels and we showed, you know, what is labeled on what. So which herbicides are labeled on which ornamental species.
Just to kind of give you an example of how you can go about doing this for your own operation. Gardenia. When you have a F or C, that means it’s labeled for use in field or container. And so you can see all the different options that you have labeled for use in gardenia. A lot of plants that you have, or cultivars too, might not be in those labels. So testing is always key. And I recommend, you know, if you’re using a new herbicide, test it out on a small group of plants first and see if it’s going to work for what you need it. We do a lot of testing, you know, at my research and other people do this research, but nothing, you’re never going to get as good enough data as you do at your own nursery, basically.
But then when you have all your different options, you want to kind of separate them out by mode of action group. You know, you have a bunch of 14 plus three combinations. These are all good herbicides, but they wouldn’t be good rotation partners because they work very similarly and they’re going to control the same general weed spectrum. Same for these isoxaben combinations. These are all good herbicides. All of them really good on bittercress, for example, but they’re not going to be good rotation partners. You just want to look at your herbicide options and make sure that you’re choosing options from different mode of action groups and you’re choosing ones that are effective for the weed that you have at that time as well.
So knowing which weeds you target, you can also go back to that guide, you can fix spotted spurge, and you can look and see what all is ranked as good for spotted spurge and say, “Okay, so during March, June and July, May, June and July, spurge is my number one problem, so I have to have options for spurge in that time frame.” And so I could look here and I could say, okay, FreeHand is a good option or Marengo is a good option. So I’m going to put those into my rotation at that time frame because I need those for spurge. That’s my biggest issue at that time frame.
And so then you just kind of do that throughout the year. You figure out what your worst weeds are during your different time frames and then your different options that are labeled on those plants, and you can kind of structure your rotation program that way. So it’s just kind of a piecemeal process.
This is just for using the example of gardenia, but gardenia is going to fall in line with a lot of those other ornamentals. You can kind of look and group all those together. Most of your woody ornamentals are going to be pretty tolerant to a wide range of these pre-emergent herbicides.
New Bare-Ground Herbicides
Chris Marble: Just a quick herbicide update. Two herbicides to be on the lookout for. Two bare ground options. StrataCor or pethoxamid is a new one that has come out. Broadleaf grass sedge controller suppression. That’s a new SePRO product. And then SureGuard Extra, which is flumioxazin, they added pyroxasulfone to that, for broadleaf grass control, and it has a long residual.
You can see in this particular trial that we did, this is SureGuard Extra. SureGuard is a really good herbicide, and I recommend it all the time, but we have seen in a couple instances where we get longer control with SureGuard Extra. So just two to keep in mind.
LEAP Project: Herbicide Application Equipment Efficiency
Chris Marble: The latest thing that we are kind of doing now is looking at the efficiency of different herbicide application equipment. So I’m on a nationwide grant. It’s a USDA funding grant. We call it the LEAP project. We’re looking at labor efficiency automation and labor engineering automation and production. So we’re trying to increase the adoption of different technologies and automation in the nursery industry.
With herbicide application practices, there’s a lot of different herbicide options available. The bottleneck is often getting the herbicides out in time and at the correct rate. That’s typically due to labor issues. You might have weather. A lot of people that I’ve talked to, they say, you know, we know what to do. We know what to apply. It’s just getting the herbicide out in time. It takes too long to apply to the nursery, depending upon how they’re doing it. And so we’ve started to look at these different forms of herbicide application equipment and what can be used to improve efficiency, basically.
Granular vs. Liquid Formulations
Chris Marble: Granular formulations are still the predominant formulations of herbicides. We have more herbicide options with granules. People are very familiar with those and then you have a good safety profile with a lot of these granular products. Here’s just an example. The granules, you have a lot more options than you do with liquid. These are just your over-the-top liquid options. You have basically three mode of action groups, whereas with granulars you have many that you can do over the top. But it’s not going to be the most efficient way to apply herbicides in a lot of cases.
So, that’s what we wanted to do. So, with this project, I’m looking at herbicide application, stuff we’re looking at comparing the labor needed and the economics of things like, you know, as simple as a, you know, a homemade shaker jar all the way up to a drone basically for granular applications and then for liquids from a backpack sprayer all the way up to air tech, you know, air assisted boom sprayers like you see in this slide here. So we’re trying to get baseline data and see, you know, for people that are using this different type of equipment, what is the equipment cost? And what is the return on investment for this type of equipment? So if somebody invested in a huge, big capital investment of an air tech sprayer or something, how quickly are they going to recoup that cost based upon their particular nursery infrastructure? So that is what we are working on.
Application Volume and Canopy Penetration
Chris Marble: Another part of this is just looking at, say someone’s just using a typical boom sprayer, you can use a wide range of application volumes with that. And so, you know, some people might go very low application volume and make it more efficient, and then some people go very high because they feel that they get a lot better canopy penetration and coverage with that. And so we’ve been looking at that.
So this is basically the canopy penetration of liquid applied herbicides through different types of ornamentals at different application volumes ranging from 25 gallons per acre up to 200 gallons per acre. And then you can see that the canopy penetration upon that initial application definitely increases as you go up higher. And that’s pretty obvious. If you use 200 gallons of water per acre, you’re probably going to get better coverage and better application bottom than if you went at 50. So that’s pretty straightforward.
What we’re doing now is we’re taking this, you know, what we actually saw on spray cards, and then we’re putting that in efficacy studies that we’re looking to see following irrigation, are we still getting the same efficacy at these lower application volumes? So far, the early data that we’ve done suggests that you do have to, if you do increase those application volume, it’s going to take a lot more time to make those applications and a lot more water, but you’re going to get improved control by doing that. Next time I speak, I’ll have some of that data available for that.
Baseline Equipment Data
Chris Marble: So we’re doing a lot of this. We’re still doing it at nurseries, mostly in Florida, but really throughout the Southeast in the US. So I’ll be going out to Oregon to measure some different types of herbicide application equipment. This is just kind of some of the data we’ve been doing.
So this is average of four nurseries that we took. So we look at different herbicides that they’re using, how much actual product they’re doing, how many employees that they’re using to apply that. How much area are they covering, you know, per man hour, and then the chemical cost of that application. So in this case with a granular application, you know, $5,500, three employees per five hours, so 15 man hours, and covered 1.3 acres. As opposed to a liquid application, this was several different booms kind of averaged together, but six-hour application time with one employee, and they covered 17 acres at about half of the chemical cost.
You can definitely change a lot of these variables depending upon the type of equipment you use. These liquid boom sprayers, and especially if you factor in the cost of the tractor, much higher cost than a belly grinder spreader, but based upon your specific operation, is it going to make sense? Are you going to recoup that cost? In a lot of cases, the answer is yes. You’re going to recoup that cost pretty quickly by investing in that more efficient equipment.
Drone Applications
Chris Marble: We also did drones. We’ve only done this at a couple places because relatively new technology where a lot of people weren’t actually doing this, but a half an acre in three minutes with this at a 200 pound per acre application rate. So two passes, putting 100 pounds in the hopper there with that drone, and then a very efficient type of application.
So I say all that to say this, I still need collaborators. I’m still interested in speaking to people on this and taking this baseline data. Allow me to come during normally scheduled herbicide applications. All I do is for liquids, I’ll measure how much area is being treated, put some spray cards out basically, area treated in the amount of time, the pot size species, accuracy of that. I share all that with you too. I don’t publish anything without the grower’s permission. I don’t put any kind of personal or business identifiers on any of this. It’s just nursery one, two, three, four, five, etc. But it’s, you know, the more data that we can get, the better information that we can develop to give to other growers to help everybody improve, basically. “Rise and Tide Lifts All Ships” is kind of the motto there.
Kind of as a thank you for people that allow me to come out if they’re interested and I haven’t done this for them before. I do come up with, I can come up with herbicide plans for people. I’ve done this for several different people where they’ll give me their crop list. I tell them what all is labeled, and then come up with a treatment plan or some options that they could follow. Whether they want to do a liquid program or granular or combination based upon the weed species that they have and the plants that they’re growing. Because, you know, I can give, you know, examples of rotations and say, you know, this is an example of rotation. People were going to take that and they say, this is the rotation that Chris said, you know, will work. The stuff that I show on my slides, it could be the best rotation that you’ve ever used in your nursery. It could be horrible.
I’ve never used the same rotation for any two nurseries that I’ve ever consulted with or done a farm visit with. Everything is very specific to that particular operation. There’s a lot of nuance and different factors that have to go into that. So just showing you an example of what I’ve done for people that have helped us out with collecting this type of data.
Training Courses and Contact Information
Chris Marble: I also want to point out that we do offer, we have several greenhouse training courses at University of Florida. I have one on weed management. Every year I do mine about the fall to winter time frame, but it’s a four week course where I do eight different lessons. So there’s two lessons per week. And I try to cover all different aspects of weed control and nursery production in greenhouses, and give people the tools that they need to manage weeds more effectively and develop a program for their specific operation. And it’s offered in English and Spanish as well. So if you had Spanish speaking employees, they could take the course and we can provide all kinds of course support and answer questions and all that in Spanish as well as English.
That’s all that I have. If you’re interested in learning anything more about the research that we’re doing or if I can help you with anything, feel free to reach out, email my mobile. That’s my personal mobile phone. I give it out to everybody. Feel free to call it or text it if I can ever help with anything.
Another thing that I want to point out too is with the LEAP project is we’re looking for people that are still using manual irrigation, turning valves on and off. We’re looking for people to work with to and why people are doing that or if they’ve looked at different technologies that didn’t work for them in terms of irrigation controllers and stuff. So point that out as well. But thanks to the people that sponsored a lot of the research that I shared in the talk, Horticultural Research Institute, and then the Specialty Crop Research Initiative as well. So with that, hopefully I have time to answer any questions that anyone has. But I really appreciate your time and attention and for the opportunity to speak today.
Questions and Answers
Shimat Joseph: Thank you so much, Dr. Marble, on these topics. It’s the idea of topics you discussed today. I think this is time for any questions you may have. You can use the Q&A box there and type your question, and I can ask Dr. Marble, and that worked out very well in the past. Thank you. Since we are waiting for questions, maybe let me start with one question.
Substrate Stratification and Temperature
Shimat Joseph: So you showed in one of your data, excellent picture that showed where you put some coarse substrate on the top and the finer one on the bottom. Very interesting that you’re seeing a lot of very good weed control, especially some species that you indicated, right? So how does that translate into temperature in the pot in that particular arrangement?
Chris Marble: So I haven’t looked at temperatures with that. So like when Jeb is doing his, he has seen cooler temperatures in the pot because he is basically, I’m not going to describe it better, so somebody can just go and look at Jeb’s research. He’s the substrate guru and everything. So Jeb Fields, you can look up that information for anybody that’s interested in that, but he has seen cooler temperatures with the way he’s doing it. But for what we’re doing, we’re just using it in the top, you know, two or three inches of that container, like for a one gallon pot. So it’s not a significant amount of that, of the entire pot. So we’re not really decreasing the temperature at all. I would imagine it’s not really increasing it either. But we haven’t looked at that closely, but, you know, 80% of that, that substrate in that pot is going to be just like a typical pot would be. So I doubt there’s much impact on temperature. I don’t think it’s providing a benefit from that standpoint.
Shimat Joseph: Sure. For definitely there is a moisture playing a big role, right? You know, less moisture in the top makes sense.
Mulch Mechanisms: Wood/Bark vs. Rice Hulls
Shimat Joseph: Any questions you have, please type it. Sometimes you need some time to think about it. Digest because a lot of information was thrown at you.
Chris Marble: Yeah, and feel free to anybody you can read. If you think of something at some point, feel free to email me. If you weren’t able to write it down when I had the slide, you can Google Chris Marble UF IFAS and my contact information will come up.
Shimat Joseph: Right, right. Let’s wait for one more minute and see if there is any questions there. If not, you know, we’ll take a break and we’ll come back at four. That’s what you’re going to do. So let’s take one minute and see if anybody has any more questions. And you’ll go from there.
Paul Bartley: I got a comment for you, Chris.
Chris Marble: Okay.
Paul Bartley: Some of the work you pointed out from my time in my master’s work with Charles Gillum, we had way better weed control when the weed seed was under the wood and bark mulch. And you had much better weed control when it was above the rice hulls.
Chris Marble: Yeah.
Paul Bartley: So it’s just not a great…
Chris Marble: The weight. Your mulch was a lot heavier than the rice hulls. We’ve seen the same thing with wood chips and stuff like that. We’ve seen, when we did, especially like shredded, I forgot what all y’all had in y’alls, but we did work with shredded melaleuca, for example, which is, they call it flora mulch down here. It’s like a shredded wood mulch. Weeds will germinate on top of that. It’s just, it holds a lot of moisture and then it’s able to germinate. But the rice hulls are so lightweight that it’s not providing any kind of physical barrier to emergence. We smothered some of them when they had already germinated like in those studies. But wood and bark and those heavier mulches provide a much better physical barrier to emergents than the rice hulls do. Rice hulls are basically working just by reducing the available moisture within that layer. I just thought that was a fascinating interaction. Great info to distinguish between different mulches and how they work more efficiently. They all have different mechanisms of control and so yeah what works for one doesn’t necessarily work for another one.
Shimat Joseph: Okay. So it looks like we don’t have any questions. So thank you so much, Chris, for the talk. And I really appreciate your time and everything.
Chris Marble: Yeah, thank you for having me.
Shimat Joseph: Thank you.
Transcript processed for UGA Center for Urban Agriculture / GTBOP Archives
Source: Corrected SRT (Stage 1) — GTBOP_Transcript_2026-03-12_NurseryWeedControl.srt (670 blocks)