Herbicide Timing for Container Weed Control

James Altland, Ph.D.
North Willamette Research and Extension Center
Oregon State University

Introduction

This article discusses timing of preemergence herbicide applications, which is perhaps the most critical component of successful herbicide management. Use of sound sanitation and cultural practices, along with proper herbicide timing, will result in the most effective weed control.

Herbicide timing cannot be described in terms of calendar dates. Nurseries are diverse in their operations, with each having their own schedule and procedures for potting, overwintering, pruning, etc. This article will make specific recommendations for herbicide timing relative to events such as potting and overwintering, and these recommendations can then be applied to any nursery regardless of their calendar schedule.

At a minimum, I recommend applying herbicides in the spring as plants are removed from overwintering, or soon after potting up. I also recommend an application prior to overwintering. One or two additional applications will probably be necessary sometime in the middle of the growing season. Some growers have shown that through good sanitation and a little hand-weeding, only two applications each year are necessary, though most routinely make a total of three or four applications each year.

Apply herbicides prior to weed seed germination

The most important rule for herbicide application, the rule that trumps all others, is that preemergence herbicides must be applied prior to weed seed germination. Preemergence herbicides will not control weeds present at the time of application. One notable exception is sprayed-applied Goal (oxyfluorfen), and though it will kill weeds less than 4 inches tall, it is limited to field use and some container-grown conifers. Existing weeds in containers must be hand-weeded prior to application. Weeds present at the time of herbicide application will continue to grow and produce seed, thus perpetuating the problem.

Even these tiny oxalis (compare their size to the blue fertilizer prills) will continue to grow after application of preemergnece herbicides. Preemergence herbicides must be applied prior to weed germination.

Herbicides at potting

Herbicides can and should be applied shortly after potting. When potting liners or shifting plants in to larger containers, allow 2 or 3 irrigation events (about 1 inch of water) to occur prior to herbicide application in order to allow the substrate (media) time to settle. Some herbicide labels instruct waiting 2-4 weeks prior to applying herbicides when potting bareroot plants into containers (check labels for specific instruction). If herbicides are applied immediately after potting, before time is allotted for settling, macropores in the substrate may allow herbicides to channel and make contact with plant roots, thus causing injury or stunting. If herbicides are withheld for too long of a period after potting, weed seed germination may occur.

Herbicide degradation and reapplication

Herbicides degrade over time. How long a herbicide persists is dependent on several factors including: light, temperature, and substrate moisture. Several processes are responsible for herbicide degradation and include: photodegradation, chemical degradation, microbial degradation, leaching, and volatilization. Photodegradation occurs when ultraviolet (UV) light breaks chemical bonds of the herbicide active ingredient. Secondary molecules resulting from the cleavage of the parent molecule are generally less effective in providing weed control.

Microbial degradation occurs when soil microorganisms use the herbicides as a food source. Virtually all pesticides are organic compounds comprised mostly of carbon, hydrogen, nitrogen, and sulfur. These compounds are a food source for microorganisms. When the same chemical is applied repeatedly, build-up of a group of microorganisms that preferentially feed on the chemical may occur, making the chemical less effective or reducing its longevity of control. This is particularly important with preemergence herbicides which are supposed to have longevity in container media and soil.

Chemical degradation occurs when a non-biological chemical reaction cleaves the herbicide active ingredient into non-active secondary molecules. The most common form of chemical degradation is hydrolysis.

Volatilization is the process by which chemicals go from a solid or liquid state into a gaseous state. Clearly, if herbicides volatilize to a gas and are released into the atmosphere, they will not be available to provide weed control. Herbicides are most prone to volatilization immediately after application. This is why incorporation of the herbicide, usually by irrigation, is so critical. Most preemergence herbicide labels recommend irrigation immediately after application. An important note, this is different from postemergence herbicides in which labels recommend a dry period after application to allow the herbicide to be absorbed by weed foliage.

With herbicides used in nursery crops, microbial and photodegradation are the primary means of herbicide degradation. Because of degradation, herbicides should be reapplied every 60 to 90 days if the canopy of the plant is not sufficient to exclude weed growth.

Herbicides applications prior to overwintering

If containers will be covered or set inside an enclosed structure, herbicides should be applied 2 to 4 weeks prior to covering (check label for specific recommendation). Again, all herbicides volatilize (turn to gas) at some level, and are especially prone to volatilization soon after they are applied. The major fear of using herbicides inside an enclosed structure is that the volatilized herbicides will injure plant foliage by being directly absorbed by the foliage, or by codistillation of the herbicide on the foliage during condensation. To prevent potential injury after herbicide applications, plants should remain outside 2 to 4 weeks before covering.

Weed growth while plants are overwintering can be devastating. Winter annuals such as bittercress, oxalis, fireweed, and common groundsel can overwhelm a crop while in overwintering unless an herbicide or some other form of weed control is used (note that oxalis is perennial, so it is not technically a 'winter annual').

Herbicide applications in early spring

Crops coming out of overwintering from the previous year require an herbicide application. The best time to make the application is while plants are spaced pot-to-pot. Consider the following example that compares herbicides applied to containers spaced pot-to-pot verses containers that have been spaced out.

You have 75,000 one gallon shrubs (each pot is 6 inches wide). Spaced pot-to-pot they take up roughly 0.4 acres, requiring 80 lbs of herbicide (assuming a rate of 200 lb/acre). Once spaced out with 3 inches between each container (assuming no aisle ways) the plants will take up roughly 1 acre, requiring 200 lbs of herbicide. By spacing the containers your herbicide cost jumps 250%!

Now let’s consider the environmental impact of applying herbicides to non-spaced verses spaced containers. When the containers mentioned above are spaced pot-to-pot, roughly 79% of the herbicide will fall into the containers while 21% falls between the containers and onto the ground, where it can potentially be washed away with irrigation or rain. When spaced 3 inches apart, roughly 35% of the herbicide falls into the containers while 65% falls between the containers! These numbers are based on calculations of the container surface area. Actual field measurements have been made that verify their accuracy, though the actual percent of herbicide falling between containers varies based on plant height, canopy shape, etc. Nonetheless, it is clear that application of herbicides to containers spaced pot-to-pot makes sense both financially and environmentally.

Figure 1. Percent of herbicide that falls either in or between containers, depending on contianer spacing.

Containers spaced pot-to-pot.	Containers spaced 3 inches apart.	Containers spaced 5 inches apart

79% falls in container, 21% between	35% falls in container, 65% between	23% falls in contianer, 77% between

Dinitroaniline herbicides

There is some concern over using products containing dinitroaniline (DNA) herbicides at potting. This class of chemical includes trifluralin, prodiamine, pendimethalin, benefin, and oryzalin. DNA herbicides are root inhibiting chemicals used in many herbicide products. While most crops are tolerant of DNAs regardless of when they are applied, some crops are sensitive at potting (azaleas, herbaceous perennials, and some ornamental grasses, for example), and are subject to lodging, poor root development, and stunting. Use of products that do not contain a DNA (see list) is recommended if previous experience or prior knowledge does not provide you with information on crop safety at planting.

Summary

Herbicide timing is critical, both in terms of providing effective weed control and minimizing potential crop injury. Always check herbicide labels to make sure you are following manufacturer recommendations regarding application timing. Herbicides are most effective when applied at the proper rate, the proper time, and used in conjunction with good sanitation.

Disclaimer: This article is for educational purposes only. Mention of a specific product should not be interpreted as an endorsement, nor should failure to mention a product be considered a criticism.

Dr. James Altland is a nursery crop extension specialist at the North Willamette Research and Extension Center. He can be reached by email at James.Altland@oregonstate.edu or calling (503) 678-1264.