Weed prevention priorities

Container nursery growers are at a disadvantage compared to some other cropping systems when it comes to weed control because, at least in container production, our only tools are preemergence herbicides, hand-weeding, and mulch. About 25% of growers spend over 100 hours per month on weed control, and with labor becoming more expensive and harder to find, it is becoming more important for growers to improve efficiency in any way possible. Weed control is also expensive. Estimates have shown that growers spend an average of $5,000 or more per acre on weed control, but this just includes chemical costs and labor. What is not included in these estimates is the opportunity cost that is lost when labor is diverted from profit generating/production related tasks (loading trucks, potting, propagation, etc.) to a corrective or profit reducing task like hand-weeding. While weed control is challenging, there are growers in many different parts of the country that have consistently clean nurseries. In each case, these growers are following similar practices that have been shown to work both in research and in the real world. In this article I will describe the five common things I have seen in the most consistently clean nurseries I have visited.

1. They scout daily and hand- weed frequently to reduce weed pressure.

The goal of improving a weed control program should be to reduce hand-weeding labor, but we want to reduce the total time spent weeding, not the frequency. Research out of North Carolina State has shown that weeding every two weeks reduces labor costs by an average of 36% (and some much more) compared with weeding only every eight weeks, or just before herbicides are reapplied (Barker and Neal., 2016). I have visited several nurseries that have adopted this approach, or a similar approach, and have seen tremendous improvements including cutting labor costs for weeding almost in half (or more) and reducing the total number of herbicide applications they need. How does this work? First, most nursery weeds spread exclusively through seeds, and they do not have time to produce seed in two weeks. This reduces the overall weed pressure throughout the rest of the season. This allows subsequent herbicide applications to be more effective because the “seed bank” or weed pressure is low. Secondly, it is time consuming to look for and pull very small weeds. If you keep a two-week schedule, crews can skip over very small weeds and only focus on weeding plants that may go to seed within the next two weeks. For more information including how to implement this in your nursery, see “Frequent Hand Weeding Saves Money” (bit.ly/hand-weeding-NCSU).

2. Sanitation is their most effective tool.

Proper sanitation is discussed in many different Integrated Pest Management (IPM) guides. In terms of weed management, the main principles to follow include:

• Hand weed frequently.
• Do not recycle potting media. Many growers do this to reduce costs, but savings on soil deliveries can be quickly lost by heavy weed pressure. I have seen very strange (and hard to control) weeds that aren’t normally in containers, such as dollarweed (Hydrocotyle spp.), yellow nutsedge (Cyperus esculentus) and horsetail (Equisetum spp.) show up in pots with recycled soil. If someone is determined to recycle soil, solarization or steam sterilization is one way to reduce pest pressure. If that is not an option or not feasible, at least avoid using recycled soil on herbicide sensitive plants like herbaceous perennials because you will not want to limit your preemergence herbicide options. Using recycled soil only for certain crops like trees in larger containers or in pot-in-pot systems could allow the use of directed postemergence herbicide applications.
• Use preemergence herbicides in non-crop areas (walkways, aisles, ditches, etc.). Weeds in non-crop areas will inevitably move into the crop if they are not controlled. Non-selective herbicides such as glyphosate (or possibly glufosinate if glyphosate resistant weeds are present) can be used to control most weeds. While these postemergence herbicides are relatively low-cost, the applicator is not. Tank-mixing a preemergence herbicide in with your post application can significantly improve control and reduce the total herbicide bill by reducing the number of applications that are needed throughout the year.
• Clean pots before reusing them. Pressurized water, heated water or steam can be used to reduce weed seed presence and/or viability.



• Moving liners and other plants in the greenhouse off the ground. Tables are expensive and not feasible in all cases, but simply moving plants in the greenhouse up on benches can help prevent weed growth in the crop. This also allows the use of postemergence herbicides or Marengo (indaziflam, a preemergence herbicide) to be used on the floor underneath the benches. Other preemergence herbicide options are available if there are no crops in the house when treatments have to be applied.
• Start with weed-free liners and keep liners weed free prior to potting.
• Stabilize plants whenever possible. Blown over pots can create several issues – first, the herbicide barrier in the top portion of the soil is broken, so weeds have greater opportunity to take over after a pot has been blown over. Probably a bigger issue is that the spilled soil is an ideal place for weed seeds to germinate.

3. They use non-chemical controls.

Certain ornamental species are notoriously sensitive to preemergence herbicide applications (e.g. hydrangea, herbaceous perennials, succulents, tropicals, etc.). These untreated areas can quickly become “hot spots,” or areas where weed growth is concentrated and spreads to other parts of the nursery. Mulch like rice hulls, pine bark nuggets, wood chips, and many others are going to cost much more than preemergence herbicides, but they cost much less than hand-weeding. If the mulch is applied properly (about 1 to 2 inches depending on the situation), a single application of mulch will usually outperform a single herbicide application. Another way I have seen some growers reduce weed pressure is by subdressing fertilizer. While it is usually easier to topdress or incorporate fertilizer, subdressing is a method of “layering the fertilizer” at potting in a single layer several inches below the media surface. When fertilizers are subdressed or layered, the pot is filled halfway at potting, fertilizer is applied in a single layer, and then the rest of the potting soil is added along with the liner. This results in a 2- to 4-inch (or greater) layer of potting soil that contains no fertilizer, and so no nutrients are available to weeds germinating on the soil surface. In research at the University of Florida, growth and seed production of common container weed species has been reduced by 40% to over 90% by subdressing with no negative impacts to several different ornamental species. The benefit diminishes later on in the production cycle when a topdress application is needed to finish off the crop, but subdressing can still provide a benefit early on in the production cycle when plants are most sensitive to weed competition.

4. They select herbicides based on their specific nursery and rotate.

There are over 20 different preemergence herbicide products labeled for over-the-top use in container plant production, but in reality we only have just a few different modes of action (MOA) including Weed Science Society of America’s herbicide MOA groups 3, 14, 15, 21, and 29. With most herbicides, growers are limited to around two applications per year, but in most cases an application is needed every 8 to 10 weeks during the growing season to keep weeds under control. This means that multiple herbicides are going be needed. Another thing to keep in mind is that all herbicides have weak areas, and if you only use one product or just one (or two) MOA, you will quickly figure out the weed(s) that your current herbicide is weak on. To get the best weed control, you need to rotate through different MOA throughout the year, timing the herbicides when they work best for your specific weed spectrum each season. Avoiding back-to-back applications of the same herbicide can also improve crop safety. A list of common preemergence herbicides including their MOA and general weeds they control are included in Table 1. Weed efficacy guides, such as the “2017 Southeastern U.S. Pest Management Guide for Nursery Crops” (bit.ly/SE-pest-control-guide) can be used as a reference for which herbicides are most effective on different weeds, and on which crops these herbicides can be applied. An herbicide rotation should be based on the specific conditions at a particular nursery. I have published a lot of examples of rotations that would work great for some growers but would yield horrible results for others because of differences in ornamentals and weed species. When developing your own rotation, the first priority is making sure the herbicide is labeled for use on the ornamental, or you have determined it is safe by testing it on a small group of plants. The next priority is selecting an herbicide that is highly effective on the most troublesome weed during a particular time of year. As several herbicides are likely effective, the next step would be to select the best option that controls both the worst weed and then the other most common two or three species that are problematic at the same time. For the next application, an herbicide with similar efficacy but a different MOA could be chosen. This process is repeated throughout the year until you have your program developed. Implement that program, take notes, and reevaluate and make changes as needed. Some of the most successful nurseries may utilize four or five (or more) different preemergence herbicide products, using each to take advantage of the products’ strengths such as crop safety or efficacy of key weed species during different times of year.

5. They make weed prevention an overall priority.

The most consistently clean nurseries all make weed management one of their top priorities. They understand and have seen how much money can be saved by preventing weed issues compared with correcting weed issues. Weed populations (and the associated costs) do not increase linearly, they increase exponentially. In nursery weed control, it is definitely true that a dime spent on prevention is worth a dollar of the cure. With labor being more expensive and harder to find, weed control programs are one area where minor changes can make a big difference.

Chris Marble is associate professor at the University of Florida’s Mid-Florida Research and Education Center, marblesc@ufl.edu.