Developing Sustainable and New Alternative Non-chemical Weed Control Strategies for Container Nursery Growers

Weed Control. Weeding time was low at both 1 and 2 MAT but increased sharply at 3MAT, coinciding with higher rainfall that is typical for Florida in the summer months (Figure 2). Control (weed biomass) decreased at 4 MAT and then stayed relatively constant for the remainder of the trial. All treatments reduced weed biomass relative to the nontreated at 1 MAT but by 2 MAT the sawdust treatment had similar weed biomass as the nontreated (Table 2). It is also important to note that the herbicide treatment provided a benefit through 3 MAT but was similar to the control group on all other evaluation dates.

Figure 2. Means and standard error bars for weeding times across all treatments by month after trials were initiated (MAT).

When examining total weeding time, the plastic treatment provided the most benefit, however treatments containing pine bark (pine bark alone or pine bark + slurry) resulted in similar reductions in weeding weed (66 to 91% reductions). Sawdust and herbicide were the only treatments that did not significantly reduce weeding time (Table 3; Figure 3). All treatments significantly reduced weed biomass relative to the non-treated, but again, the plastic treatment and pine bark treatments provided the greatest benefit.

Figure 3. Mean weeding time and standard error by treatment across all locations and evaluation dates.

Plant Growth. Plant growth was largely unaffected by mulch treatments (Table 4). For all three species, all mulch treatments resulted in plants with similar growth rates or size compared with the non-treated control group. Minor differences were observed amongst mulch treatments with podocarpus and elm height, but differences were not observed visually and were potentially the result of differences in liner quality or other cultural practices.

Soil Moisture. Due to system malfunctions, soil moisture data could not be analyzed statistically due to lack of replications with some treatments. However, soil moisture levels remained relatively constant and within acceptable levels for most of the study (Figure 4). Moisture sensors were also placed on the opposite side of drip emitters and were on the top 2in. of soil, which also contributed to lower moisture levels than might be expected in some cases. Most mulch treatments resulted in higher volumetric water content compared with non-mulched controls throughout the experiment but the increase in soil moisture did not correspond to increased growth.  

Figure 4. Soil moisture levels under each mulch layer averaged bi-weekly across two trial locations.

As illustrated with cumulative weed biomass, all of the mulch materials evaluated provided a reduction in weed growth in comparison with the non-mulched control. For the entire six months of the experiment, treatments containing pine bark or the plastic mulch tended to provide the most benefit. Several other treatments provided more significant reductions in weed biomass or handweeding time, but the benefit was only realized in early evaluation periods. For example, hardwood, the slurry, and the herbicide treatment provided a benefit through 3 MAT but were similar to the non-treated at 4 MAT. For hard wood, the small particle size of the shredded wood material held more moisture, which likely contributed to greater weed germination. The same was true for sawdust that also held more moisture and decomposed more rapidly than pine bark. The herbicide treatment provided a benefit through 3 MAT but was less effective than some other mulch materials by as early as 4 MAT as it would have needed to be reapplied at approximately three months to continue to provide a benefit. The slurry provided a high level of control through 2 MAT but by 3 MAT, the material shrunk (Figure 4), exposing a large portion of the container media surface which reduced weed control. Adding the slurry to pine bark also did not provide any additional benefit compared with use of pine bark mulch alone.

Overall, growers wishing to use mulch for long-term weed control in container production could utilize pine bark nuggets or potentially wrap pots in plastic. Both of these mulching methods reduced weeding time and weed biomass by 60 to over 90% compared with using no mulch material at all and outperformed the use of a single preemergence herbicide application. While these treatments were the most effective, other mulch materials that were evaluated also provided a benefit and could potentially be utilized. The costs for these materials will likely be highly variable depending upon availability and grower location. The material cost for pinebark was about $0.16 per #7 container at 2 inches deep, hardwood was about $0.12, and plastic cost was about $0.023.  This would not include the labor to apply the materials. In comparison, material costs for a preemergence herbicide range about $0.008 to $0.012 per application to a seven gallon pot. However, in most cases preemergence herbicides are broadcast at a cost of approximately $200 per acre and up to 80% of the material falls in-between the pots.  In Florida, during the spring-fall growing season, a preemergence herbicide application would be applied approximately every 5-6 weeks to maintain expected weed control. Other ongoing research at the University of Florida has shown that mulch particle size and depth are more important to weed control than mulch type. Thus, other mulch materials not evaluated here could also be utilized if they were available and processed in large (> 0.75 in.) particle sizes and did not hold high levels of moisture. An additional consideration is that these mulch materials were applied at approximately a 2 in. depth in all cases. This was difficult to accomplish on-farm as growers will typically completely fill pots to maximize root volume, and there will be no container “lip” to hold mulch materials on the media surface. In very large containers, growers could potentially utilize a labeled herbicide immediately after potting and then topdress pots with mulch once soil had settled. In very large containers, leaving a shallow rim in containers for mulch application would likely have little to no impact on overall plant growth, but this has not been thoroughly investigated.

Conclusions:

Due to small sample size available in 2017, few statistical differences were noted among the treatments, especially when considering growth data. However, as this was designed to be a 2-year trial, additional data points will allow us to better determine the effects of these mulch treatments on both weed control and ornamental crop growth. We also determined that adding tackifier to mulch significantly increased the amount of mulch that stayed in the pots, especially for the sawdust treatment in which some of the mulch blew away once it dried. Additional data points are also needed for moisture sensor data due to two incidences when loggers malfunctioned or were dislodged from pots following hurricane conditions. Trials for 2018 are schedule to be installed at the two grower locations on April 17, 2008 and will follow procedures described above. In addition to trials at grower farms, the experiment will also be repeated at the Gulf Coast Research and Education Center (using funds from Dr. Marble’s program) to display at the field day if pots from grower locations cannot be moved for the field day. While this trial will be used primarily for educational purposes, it will be replicated and data will be collected as described above to provide us additional data points in a more controlled environment and also to provide additional data for publishing in a peer-reviewed journal article.