Webinar: The Latest in Nursery Weed Control
Speaker: Dr. Chris Marble, University of Florida IFAS
Webinar Date: March 12, 2026
Series: Green & Commercial
Questions: 12
Difficulty Distribution: 5 Recall (42%) / 5 Application (42%) / 2 Analysis (17%)
Question 1
Timestamp Reference: 4:00 – 6:30
Difficulty: Recall
In Marble’s multi-site hand weeding frequency study, what was the overall cost reduction achieved by weeding every two weeks instead of every eight weeks?
a) 10–20%
b) 25–35%
c) 40–60%
d) 75–90%
Correct Answer: c
Explanation: Marble reported a 40–60% reduction in weeding costs across all study sites, with the Florida site achieving a 65% reduction.
Source in transcript: Blocks 64–65 (~4:00), Block 113 (~7:36)
Question 2
Timestamp Reference: 10:30 – 12:00
Difficulty: Recall
According to Marble, which pre-emergent herbicide can kill bittercress up to the flowering stage at labeled rates?
a) Indaziflam (Marengo)
b) Isoxaben (Gallery, Gemini)
c) Dimethenamid-P (Tower)
d) Oxadiazon (Ronstar G)
Correct Answer: b
Explanation: Marble stated that isoxaben — the active ingredient in Gallery and Gemini — kills bittercress up to the flowering stage at labeled rates. Dimethenamid-P targets spurge, indaziflam targets oxalis, and oxadiazon suppresses artillery weed.
Source in transcript: Blocks 162–164 (~11:00–11:10)
Question 3
Timestamp Reference: 15:30 – 17:05
Difficulty: Recall
In Marble’s fertilizer placement study, what happened when controlled-release fertilizer was top-dressed on two inches of rice hulls?
a) Fertilizer released faster than normal, burning plant roots
b) Coleus and Vitex grew significantly less than when fertilizer was incorporated
c) Rice hulls absorbed the fertilizer and prevented any nutrient release
d) No measurable difference in plant growth was observed
Correct Answer: b
Explanation: Marble found that top-dressing fertilizer on two inches of rice hulls reduced growth in coleus (significantly) and Vitex (to a lesser degree). The rice hulls dry quickly after irrigation, which likely slows the release of nutrients from the coated fertilizer granules.
Source in transcript: Blocks 232–247 (~15:24–16:20)
Question 4
Timestamp Reference: 18:00 – 19:30
Difficulty: Recall
What percentage of already-germinated cotyledon-stage eclipta and Phyllanthus seedlings were killed when one inch of rice hulls was applied on top of them?
a) 50% or greater
b) 75% or greater
c) 85% or greater
d) 95% or greater
Correct Answer: d
Explanation: Marble reported a 95% or greater reduction in survivability of cotyledon-stage eclipta and Phyllanthus when one inch of rice hulls was applied. Bittercress and oxalis were controlled at nearly 100% at the same depth.
Source in transcript: Blocks 282–284 (~18:54–19:02)
Question 5
Timestamp Reference: 23:00 – 24:00
Difficulty: Recall
After 12 months outdoors with twice-daily irrigation, what percentage of the original rice hull layer remained?
a) About 10%
b) About 25%
c) About 40%
d) About 60%
Correct Answer: c
Explanation: Marble reported that about 40% of the originally applied rice hull layer remained after 12 months under Florida conditions, including twice-daily irrigation and hurricane-season rainfall.
Source in transcript: Blocks 324, 348 (~22:19–24:00)
Question 6
Timestamp Reference: 2:00 – 6:30
Difficulty: Application
A nursery manager has a block of containers treated with pre-emergent herbicides that gets weeded every eight weeks. Small weed seedlings appear between weeding cycles and produce seed before the next scheduled weeding. Based on Marble’s research, which change would most effectively reduce long-term weed pressure in this block?
a) Switch to a stronger pre-emergent herbicide
b) Weed every two weeks, removing only large weeds likely to seed soon
c) Apply rice hulls over the existing weeds
d) Double the pre-emergent application rate
Correct Answer: b
Explanation: Marble’s study showed that weeding every two weeks — pulling only large weeds likely to seed and leaving cotyledon-stage seedlings — reduced costs by 40–60% and steadily decreased the seed bank. The key was preventing seed set, not removing every small weed.
Source in transcript: Blocks 80–82 (~5:17–5:33), Blocks 117–119 (~7:47–7:58)
Question 7
Timestamp Reference: 12:50 – 17:05
Difficulty: Application
A grower plans to use rice hulls on two-gallon containers of coleus with controlled-release fertilizer. Based on Marble’s findings, what should the grower do to avoid reducing plant growth?
a) Apply rice hulls at two inches and top-dress fertilizer on the surface
b) Incorporate fertilizer into the media below the rice hull layer
c) Use liquid fertilizer instead of controlled-release granules
d) Wait until the coleus are established before adding rice hulls
Correct Answer: b
Explanation: Marble found that top-dressing controlled-release fertilizer on rice hulls reduced coleus growth because the hulls dry quickly and slow fertilizer release. He recommended incorporating fertilizer below the rice hull layer or limiting rice hulls to short-term crops where incorporation is practical.
Source in transcript: Blocks 240–247 (~15:46–16:20), Block 261 (~17:26)
Question 8
Timestamp Reference: 20:00 – 22:00
Difficulty: Application
A nursery applies rice hulls to containers several months after potting, when weed seeds are already present in the substrate below the surface. Based on Marble’s seed placement study, what weed control outcome should the nursery expect?
a) Strong control — weed seeds below the hulls will be smothered
b) Moderate control — about 50% suppression of seeds below the hulls
c) Poor control — seeds already in the substrate will germinate and emerge through the rice hull layer normally
d) Variable control depending on weed species
Correct Answer: c
Explanation: Marble’s study showed clear-cut results: rice hulls controlled weeds effectively when seeds landed on top of the layer (89% control), but seeds placed below the hulls germinated and grew with essentially no suppression. The rice hull mechanism is moisture deprivation at the surface, not a physical barrier to emergence from below.
Source in transcript: Blocks 311–316 (~20:55–21:37)
Question 9
Timestamp Reference: 24:20 – 27:00
Difficulty: Application
A greenhouse grower’s primary weed problem is liverwort forming thick mats on container surfaces. Herbicide options are limited by the ornamental species grown. Based on Marble’s research, which non-chemical approach would give the grower the longest period of liverwort-free containers?
a) Apply one inch of rice hulls to each container
b) Use substrate stratification with coarse bark on top and fertilizer placed below
c) Hand weed every two weeks to prevent liverwort establishment
d) Switch to pine bark nugget mulch at three inches
Correct Answer: b
Explanation: Marble reported that substrate stratification — coarse bark on top with no fertilizer in the upper layer — eliminated liverwort for over 20 weeks. Liverwort needs both moisture and nitrogen to grow; the coarse top layer provides neither. Rice hulls work differently (moisture deprivation only) and were not tested specifically for liverwort duration in the data Marble presented.
Source in transcript: Blocks 376–384 (~26:26–26:54)
Question 10
Timestamp Reference: 27:20 – 30:00
Difficulty: Application
A nursery grower is building a herbicide rotation for gardenia containers. The grower’s current program uses Rout (oxyfluorfen + oryzalin, Group 14+3) followed by BroadStar (flumioxazin, Group 14). Based on Marble’s rotation principles, what is the problem with this sequence?
a) Neither product is labeled for gardenia
b) Both products come from the same or overlapping mode of action groups, providing no rotation benefit
c) Rout and BroadStar cannot be applied in the same calendar year
d) Granular products should not be followed by other granular products
Correct Answer: b
Explanation: Marble emphasized that rotation partners should come from different mode of action groups. Putting two Group 14+3 or Group 14 combinations back to back covers a similar weed control spectrum without actual mode of action rotation. The grower should select a product from a different group for the follow-up application.
Source in transcript: Blocks 415–421 (~28:49–29:08)
Question 11
Timestamp Reference: 31:00 – 37:00
Difficulty: Analysis
Marble’s LEAP project data showed that a liquid boom sprayer covered 17 acres with one employee in six hours, while a granular application used three employees for five hours to cover 1.3 acres. Beyond the area-per-hour difference, what factor in Marble’s data makes the economic case for liquid boom sprayers even stronger?
a) Liquid herbicides have longer residual activity than granular formulations
b) Liquid boom sprayers require no calibration
c) The chemical cost for the liquid application was roughly half that of the granular application
d) Granular herbicides are being phased out by manufacturers
Correct Answer: c
Explanation: Marble reported that the granular application cost $5,500 in chemical cost, while the liquid boom sprayer application covered far more area at roughly half the chemical cost. The efficiency advantage of liquid booms extends to both labor (one employee vs. three) and chemical expense.
Source in transcript: Blocks 513–520 (~34:35–35:57)
Question 12
Timestamp Reference: 20:00 – 22:00, 43:00 – 44:46
Difficulty: Analysis
Marble’s rice hull research and Paul Bartley’s Q&A comment reveal contrasting weed control mechanisms for different mulch types. Rice hulls controlled weeds better when seeds were on top of the layer, while Bartley’s wood/bark mulch research found better control when seeds were under the mulch. What does this contrast tell us about selecting mulch type for a specific weed management scenario?
a) All mulch types work identically regardless of application timing
b) The choice of mulch type should account for the mechanism of control — rice hulls deprive surface seeds of moisture, while heavier mulches physically block emergence from below
c) Rice hulls are always superior because they are lighter and easier to apply
d) Wood mulch should only be used in landscape settings, never in nursery containers
Correct Answer: b
Explanation: Marble explained that rice hulls work by drying rapidly and depriving surface seeds of germination moisture, while Bartley noted that heavier wood and bark mulches physically block seedlings from pushing through. A grower whose primary problem is weed seed introduction from above (wind, irrigation) would benefit from rice hulls, while a grower with an existing seed bank in the substrate might get better control from a heavier mulch that blocks emergence.
Source in transcript: Blocks 311–322 (~20:55–22:14), Blocks 642–659 (~43:29–44:38)
Quiz Summary
| Section | Questions | Coverage |
|---|---|---|
| Hand weeding frequency (0:00–10:30) | Q1, Q6 | Study results, practical application |
| Pre-emergent post-emergence activity (10:30–12:50) | Q2 | Product-species matching |
| Rice hull fertilizer effects (12:50–17:05) | Q3, Q7 | Research findings, grower recommendation |
| Rice hulls on emerged seedlings (18:00–19:30) | Q4 | Specific data point |
| Rice hull outdoor performance (20:00–24:00) | Q5, Q8, Q12 | Degradation, seed placement, mechanism comparison |
| Substrate stratification (24:20–27:00) | Q9 | Liverwort control application |
| Herbicide rotation (27:20–30:00) | Q10 | Mode of action principles |
| LEAP project equipment (31:00–37:30) | Q11 | Equipment economics analysis |
Quiz questions produced for UGA Center for Urban Agriculture / GTBOP Moodle Courses
Source: Corrected SRT — GTBOP_Transcript_2026-03-12_NurseryWeedControl.srt (670 blocks)
Archive Package — GTBOP_Archive_Summary_2026-03-12_NurseryWeedControl.md