Speaker: Dr. Chris Marble, Associate Professor of Horticulture, University of Florida IFAS, Mid-Florida Research and Education Center, Apopka, FL
Moderator: Dr. Shimat Joseph, University of Georgia
Webinar Date: March 12, 2026
Duration: 45:13
Series: Getting the Best of Pests (GTBOP) — Green & Commercial
CEU Categories: 10 (Private Applicator), 21 (Plant Agriculture), 24 (Ornamental and Turf), 27 (Right-of-Way), 32 (Regulatory)
Summary
Dr. Chris Marble presented results from his nursery weed management research program at UF IFAS in Apopka, Florida, covering non-chemical control strategies, rice hull mulch, and herbicide application efficiency.
Marble opened with a hand weeding frequency study conducted with Joe Neal (NC State) and Jeff Derr (Virginia Tech). Nursery blocks treated with Tower and Gemini pre-emergents were weeded every two weeks instead of the standard eight-week cycle. Workers pulled only large weeds likely to seed within two weeks, leaving cotyledon-stage seedlings for the next pass. The result across all sites was a 40–60% reduction in weeding costs — 65% at the Florida site — because the seed bank declined with each cycle. He also reviewed early post-emergence activity of several pre-emergent herbicides: dimethenamid-P (Tower, FreeHand) on spurge, indaziflam (Marengo) on oxalis, isoxaben (Gallery, Gemini) on bittercress up to flowering, and oxadiazon (Ronstar G) on artillery weed.
The middle section addressed rice hull mulch. Top-dressing controlled-release fertilizer on two inches of hulls reduced coleus and Vitex growth, likely because the hulls dry too quickly for the fertilizer coating to release properly. A separate study showed one inch of rice hulls killed 95% or more of already-germinated eclipta and Phyllanthus seedlings, though seeds beneath the hulls emerged normally. The suppression mechanism is moisture deprivation, not physical barrier — Paul Bartley contrasted this with heavier wood and bark mulches that block emergence by weight. Outdoors, about 40% of the applied layer remained after 12 months, and hulls matched standard pre-emergent rotations for the first six months before degradation reduced their effectiveness. Marble also presented substrate stratification — coarser bark on top, fertilizer placed below — which eliminated liverwort for over 20 weeks and cut bittercress by 40–60%.
The final section covered herbicide rotation principles using the 2017 Southeast Pest Management Guide, two new bare-ground products (StrataCor/pethoxamid from SePRO and SureGuard Extra/flumioxazin + pyroxasulfone), and the LEAP project — a USDA-funded nationwide study comparing application equipment efficiency. Data from four nurseries showed a liquid boom covering 17 acres with one employee in six hours versus a granular crew of three covering 1.3 acres in five hours at double the chemical cost. Drone application covered a half acre in three minutes. Marble closed with a call for collaborators and a note about UF’s four-week weed management training course offered in English and Spanish.
Video Chapters
0:00 Introduction — Dr. Chris Marble
1:00 Non-Chemical Weed Control Options in Nurseries
2:00 Hand Weeding Frequency Study Design
4:00 Cost Savings from More Frequent Weeding
7:00 Cleanup Protocols for Heavily Infested Blocks
9:30 Training Crews — Which Weeds to Pull, Which to Leave
10:30 Early Post-Emergence Activity of Pre-Emergent Herbicides
12:50 Rice Hull Mulch — Properties and Grower Questions
15:30 Fertilizer Placement Effects with Rice Hulls
18:00 Rice Hulls on Already-Emerged Weed Seedlings
20:00 Rice Hull Performance Outdoors — Seed Placement Study
22:00 Moisture Deprivation as the Weed Control Mechanism
23:00 Rice Hull Degradation Rates and Depth Recommendations
24:20 Substrate Stratification for Liverwort and Bittercress
27:20 Developing a Herbicide Rotation Program
30:00 New Bare-Ground Herbicides — StrataCor and SureGuard Extra
31:00 LEAP Project — Herbicide Application Equipment Efficiency
34:00 Liquid Application Volumes and Canopy Penetration
36:00 Granular vs. Liquid Equipment Economics
37:30 Drone-Based Granular Application
39:00 UF Weed Management Training Course
40:00 Q&A — Substrate Stratification and Pot Temperature
43:00 Q&A — Rice Hulls vs. Wood/Bark Mulch Mechanisms
Questions & Answers
Q: How much can a nursery actually save by weeding more often?
A: In Marble’s multi-site study, weeding every two weeks instead of every eight reduced costs by 40–60% across all locations. The Florida site saw a 65% reduction. Workers moved three to seven times faster per pass because they only pulled large weeds likely to seed — cotyledon-stage seedlings were left for the next round. Over 24 weeks, the seed bank declined steadily in the frequent-weeding blocks, while the eight-week blocks saw increasing pressure as breakthrough weeds reproduced.
Q: Which pre-emergent herbicides also control weeds that have already emerged?
A: Marble identified several. Dimethenamid-P products (Tower, FreeHand) stunt or kill emerged spurge. Indaziflam (Marengo) controls small oxalis, stunts spurge, and kills annual bluegrass. Isoxaben (Gallery, Gemini) kills bittercress up to the flowering stage at labeled rates. Oxadiazon (Ronstar G) suppresses artillery weed. Dimension has some activity on oxalis and bittercress. Oxyfluorfen-containing products (OH2, Biathlon) suppress a range of broadleaf species after emergence.
Q: Does top-dressing fertilizer on rice hulls affect plant growth?
A: Yes. Marble found that when controlled-release fertilizer was top-dressed on two inches of rice hulls, coleus grew significantly less than when fertilizer was incorporated into the media. Vitex showed a smaller but still significant difference. The rice hulls dry quickly after irrigation, which likely slows fertilizer release from the coated granules. Marble recommended either incorporating fertilizer below the rice hull layer or limiting rice hull use to short-term crops where incorporation is practical.
Q: Can rice hulls be applied over pots that already have small weeds emerging?
A: At one inch of rice hulls, Marble’s team saw 95% or greater reduction in survivability of cotyledon-stage eclipta and Phyllanthus. Bittercress and oxalis at the same depth were controlled at nearly 100%. The hulls smother small, already-germinated seedlings effectively. Weed-free pots at the time of application are still the best scenario, but one inch of rice hulls will kill most small seedlings that are present.
Q: How do rice hulls control weeds — is it the same mechanism as wood mulch?
A: No. Paul Bartley pointed out during Q&A that his master’s research with Charles Gillum found wood and bark mulches controlled weeds better when seeds were placed under the mulch, while Marble’s data showed rice hulls worked better when seeds landed on top. Marble explained that heavier mulches physically block weed emergence — they are too heavy for seedlings to push through. Rice hulls are too lightweight for that. They work by drying rapidly after irrigation, depriving weed seeds on the surface of the moisture needed to germinate. Seeds already in the substrate below rice hulls germinate and emerge normally.
Q: How long do rice hulls last outdoors?
A: About 40% of the original applied layer remained after 12 months under Florida conditions with twice-daily irrigation and hurricane-season rainfall. Rice hulls performed comparably to standard pre-emergent herbicide rotations for the first six months. After that, degradation reduced the layer enough for weed breakthrough, and herbicide treatments provided better control. Marble recommended rice hulls for short-term crops finishing in six to eight months and pine bark mulch for longer-term production.
Q: What depth of rice hulls does Marble recommend for containers?
A: About one inch for small containers. That depth provides good weed suppression without significantly reducing root volume the way two inches would. A half inch gives partial control — a noticeable drop in weed counts — but one inch is the threshold where control becomes consistent across species.
Q: What is substrate stratification, and how well does it work on liverwort?
A: Substrate stratification means filling containers in layers with different bark sizes and placing fertilizer below the coarse top layer. The coarse bark on top holds less moisture and contains no nitrogen. Liverwort requires both moisture and nitrogen to grow, so stratified containers eliminated liverwort for over 20 weeks. Bittercress was reduced by 40–60%. Marble credited Jeb Fields at UF for pioneering the stratification concept for water and fertilizer efficiency, then adapted it for weed control by flipping the coarse layer to the top.
Q: How should growers choose herbicides for a rotation program?
A: Marble recommended the 2017 Southeast Pest Management Guide as the starting point. Growers should identify their worst weed species by season, look up which herbicides are labeled for their ornamental species, then group those options by mode of action. Rotation partners should come from different mode of action groups — putting two Group 14+3 combinations back to back gives similar weed control spectra without rotation benefit. Marble emphasized that no two nurseries he has consulted with use the same rotation, because crop lists, weed pressure, and infrastructure vary too much.
Q: What are the new bare-ground herbicides Marble mentioned?
A: Two products: StrataCor (active ingredient pethoxamid), a new SePRO product labeled for broadleaf, grass, and sedge control or suppression. And SureGuard Extra, which adds pyroxasulfone to flumioxazin (the original SureGuard active ingredient), providing broadleaf and grass control with a longer residual than SureGuard alone.
Q: How does herbicide application equipment compare in efficiency?
A: Early LEAP project data from four nurseries showed that a granular application used three employees for five hours to cover 1.3 acres at $5,500 in chemical cost. A liquid boom sprayer covered 17 acres with one employee in six hours at roughly half the chemical cost. A drone applied granular herbicide over a half acre in three minutes at 200 pounds per acre. The capital cost of boom sprayers and drones is higher, but Marble’s economic modeling suggests many nurseries would recoup the investment quickly through labor and chemical savings.
Additional Resources
- Publication: “Frequent Hand Weeding Saves Money” — Barker and Neal, 2016
- Reference: 2017 Southeast Pest Management Guide (herbicide-by-ornamental species tables)
- UF IFAS Weed Management Training Course — four-week course, eight lessons, offered in English and Spanish, typically fall/winter
- LEAP Project — USDA-funded; Marble seeking collaborators for equipment efficiency data collection at nurseries nationwide
- Contact: Dr. Chris Marble — Google “Chris Marble UF IFAS” for contact information
Processed for the UGA Center for Urban Agriculture / GTBOP Archives