Weed control is most effective when herbicides are applied at the proper rate and proper time, and in conjunction with good sanitation. Keeping weeds at bay helps stop the spread of some insect pests and provides a more marketable plant at retail.
The first step to effective weed management is sanitation. Start with potting mixes that are free from weed seeds. Control weeds that have wind-dispersed seeds around the perimeter of the site. If weeds emerge in containers, remove them before they produce seeds. The more weed seed allowed to contaminate containers, the higher the probability that the weeds will germinate in areas where the herbicide barrier has been weakened or disrupted.
Maintaining a chemical barrier
Preemergence herbicides form a chemical barrier over the surface of containers. Though each herbicide controls weeds differently, preemergence herbicides provide control at the point where germinating seeds emerge through the chemical barrier.
If the chemical barrier is incomplete or disrupted, there will be a gap where weed seed can successfully germinate and grow. Several common practices can disrupt the chemical barrier, including but not limited to poking holes in the barrier with fingers or hands while moving containers, dropping containers, and allowing containers to blow over. Minimize all these activities to avoid disrupting the chemical barrier. Teach work crew members about this, because they are typically responsible for moving and working near the containers.
Pulling uncontrolled weeds also creates gaps in the chemical barrier. Pull weeds before they go to seed. Soon after removing weeds from an area, apply an herbicide to create a complete chemical barrier and prevent germination of more weeds.
Selecting preemergence herbicides
Preemergence herbicides are applied before weeds emerge, to prevent weed growth. This is in contrast to postemergence herbicides, which kill weeds after they have emerged and are actively growing. Base your selection of herbicides primarily on three criteria: the crop to which the herbicide will be applied, weed species to be controlled and herbicide solubility. Other considerations include the importance of rotating herbicide chemistry and the choice of granular versus spray-applied herbicides.
Crop tolerance of herbicides
Selecting an herbicide based on the crop being grown is critical. Every herbicide label describes how the product should be used, and which plants it can be applied to safely based on experimental tests. Chemical manufacturers make every effort to ensure that plants listed on labels can be treated safely. However, not every environmental or cultural situation can be predicted or accounted for when testing products. Therefore, before using a new herbicide, or using a familiar herbicide on a new crop, conduct a small trial to ensure the plant and herbicide are compatible under conditions specific to your production system (regardless of whether or not the plant is listed on the label).
Apply herbicides before weed seed germinates
The most important rule for application timing is that preemergence herbicides work best if applied before weed seeds germinate. Most preemergence herbicides will not control weed plants that are present and visible at the time of application. Two notable exceptions are spray-applied Goal (oxyfluorfen) and SureGuard (flumioxazin). These herbicides will kill weeds less than 4 inches tall. However, they are limited to field use and some container-grown conifers.
Existing weeds in containers must be hand-weeded before application. Weeds present at the time of herbicide application will continue to grow and produce seeds, thus perpetuating the problem. Applying preemergence herbicides to containers where weeds are growing is a costly mistake, wasting herbicide and the labor needed to apply it.
Herbicides at potting
Herbicides should be applied soon after potting. When potting plants into larger containers, before applying an herbicide, irrigate two or three times to settle the substrate (media). For best results, apply herbicides after recently potted crops have received about 1 inch of irrigation or precipitation. When using certain herbicides, wait 2 to 4 weeks before application when potting bareroot plants into containers (check labels for specific instructions).
If herbicides are applied immediately after potting, before settling, macropores in the substrate can allow herbicides to channel and make contact with plant roots, causing injury or stunting. If herbicides are withheld for too long after potting, weed seed may germinate.
About the authors: Ed Peachy is an associate professor at Oregon State University’s Horticulture Department and Lloyd Nackley is a nursery researcher at Oregon State’s North Willamette Research and Extension Center.