Weed Control in Containers

by Carl E. Whitcomb, Ph.D.


For all growers, gardeners and nurserymen alike, there is an ongoing battle to discourage the unwelcome weed guests that not only steal nutrients and water, but may also be hosts to pests and diseases that will move on to attack the crop.

Weeds have been defined in various ways, but one of my favorites is that "a weed is a thoroughly successful plant." In spite of all that you do to discourage weeds, they grow and flourish, and continue to cause problems and restrict growth of your crops. Controlling weeds in field nursery stock or in the garden is challenging, but in containers the problem is even worse.


Consider that no plant is native to a container, since a container is a totally artificial environment created for our convenience. What works in Mother Nature's soil rarely works the same way in the unique environment of a container.

The mix, or growth medium, in a modern container nursery contains NO field soil, but is rather a mixture of pine bark, sand , peat or other material. Becasue of the ever-present, perched water table at the bottom of the container, coarse materials are necessary to provide drainage and avoid root suffocation from excess water, while retaining sufficient water to support the crop between irrigation cycles. Further, because we have restricted the root system of the plant to the volume of the container (and that root system is quite limited relative to the root system the plant would develop in the landscape) water must be reapplied frequently. Water loss from the container is accelerated by the fact that the temperature of the growth medium in the container is typically 20 to 50 degrees warmer than soil in a landscape, and this heat "cooks out" the water and restricts root growth. Taken altogether the result is that lots of water, applied frequently, is necessary to grow plants in containers.

In my early research of trying to sort out the factors involved and most effective ways to control weeds in containers, two key points surfaced:

1. The high content of organic material in the growth medium (pine bark, peat, etc.) binds or adsorbs, like iron filings sticking to a magnet, substantial quantities of herbicides, thus the rates must be higher than for the same herbicide applied in the field

2. The more water soluble the herbicide, the faster it moves downward through the mix, and the greater the likelihood of damaging the crop.

More than 30 years of study in the area have amplified my understanding to give the following simple rule for container nurseries.

Use only preemergent herbicides with water solubilities LESS than ONE ppm.


None of the compounds listed (see sidebar) will last more than about two months on the surface of a container due to elevated temperatures and microbial activity. YOU MUST watch the calendar carefully, and also check for the first presence of germination of weed seeds.

REAPPLY REGULARLY. In nearly all cases it is better to be a bit early than to wait too long and have to pull weeds by hand.

Factor and Barricade must be applied at high rates in order to have sufficient soluble herbicide to control weeds. Remember, if too little of the herbicide is soluble, weed control will be poor, but if too much is soluble, weed control may be good, but there is a danger of stunting the crop.

The compounds currently available that fit this requirement are:

Barricade, Factor (prodiamine) @ 0.013 ppm water solubility
Goal (oxyfluorfen) @ 0.1 ppm
Treflan (trifluralin @ 0.3 ppm
Prowl, Pendulum, Southern WeedGrass Control (pendimethalin)@ 0.5 ppm
Ronstar (oxadiazon) @ 0.7 ppm
Gallery (isohexaben) @ 1.0 ppm

Rout is a combination of Goal and Surflan
Snapshot 80 DF is a combination of Gallery and Surflan
Snapshot 2.5 TG
is a combination of Gallery and Treflan
Ornamental Herbicide II is a combination of Goal and Prowl

Surflan (oryzalin), which appears in Rout and Snapshot 80DF, has a solubility of 2.6 ppm. I do not recommend its use either alone or in combination with other compounds since crop stunting is common.


The practical sequence that works well is to water the plants in well by hand after potting up new liners or shifting to a larger pot. Then the next day, apply the preemergent herbicide and water it in by sprinkler irrigation. Because of the low solubility of the suggested compounds, they do not move below about 1/2" deep into the growth medium. Since nearly all weed seeds require light to germinate, it is essential that the herbicide remain on the surface where it is needed, and away from the roots of the crop.

Sanitation and control of weeds in the whole area of plants being grown in containers is also a critical part of the weed control program. The more weed seeds available to travel onto your plants in containers, the more likelihood you will have weed problems.

If you recycle your water, my very strong advice is that ANYWHERE on the property you shoulduse only those herbicides with water solubilities less than one ppm . For example, the soil sterilant herbicide Hyvar X has a water solubility of 815 ppm, and goes wherever water goes, causing damage wherever it shows up. Roundup is not likely to cause damage because it adheres to soil and organic particles very strongly and is broken down quickly by microbes.

NEVER, NEVER let anyone talk you into using an herbicide at your nursery until you know its solubility. Further, ALWAYS do a small test area before treating the entire nursery. When reading the results of an herbicide test, look to see if they had a double control. A double control is one where one control plot receives no herbicide and the weeds are allowed to grow, while the second control receives no herbicide but is kept free of weeds by hand. Only with this second control can you judge whether or not the herbicides in the test are stunting the crop.

Weeds will always be with us, but with a good control program, they need not be a serious problem.

Carl Whitcomb, Ph. D. is president of Lacebark, Inc., Publications & Research, Stillwater OK.
Tel. 405 377 3539

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