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July 25, 2023

LSU AgCenter's Weekly Message

Managing Weeds in the Vegetable Garden, Part 1: Preplant Decisions

At this time of the year, many people have lost their initial enthusiasm for their spring vegetable gardens. However, some heat-tolerant vegetables can still be planted, and it won’t be long before it’s time to plant many cool-season crops.

I’ve previously written about managing diseases and insects in the vegetable garden, as well as preventing deer damage. I think it’s time to address weeds.

Weeds can cause substantial yield losses. They compete with plants for water and nutrients, reduce the amount of sunlight that gets to vegetable plants, and in some cases serve as hosts of pest insects or disease-causing organisms.

As with diseases and insects, it’s best to prevent weed problems, when possible, rather than waiting to deal with them after they’re present.

This week’s article discusses decisions made before planting that affect weed management. Future articles will address ways to kill standing weeds before planting and approaches to prevent and manage weed problems once vegetables are in the ground.

First, choose a site suitable for vegetables. It should be well-drained and receive direct sunlight for most of the day. If the site isn’t a good one for vegetables, it will be harder for them to compete with weeds.

If you haven’t taken a soil test in several years, I suggest taking one to see if adjustments need to be made to pH or nutrient levels. If lime or sulfur application is needed to raise or lower soil pH, this is best done several months before planting.

If you don’t need a large space, you might consider growing in containers or raised beds. These tend to make weed management considerably easier.

If certain weeds predominate on an in-ground site, you may be able to choose a vegetable that allows application of an herbicide that’s effective against those weeds.

If the main weed problems are grasses, you might consider growing vegetables over which you can apply a grass-selective herbicide. There are herbicides that contain sethoxydim (e.g., Bonide GrassBeater II, Fertilome Over-The-Top II Grass Killer, Hi-Yield Grass Killer, Poast) and can be used over the tops of many (though not all) broadleaf vegetables. These can’t be used over corn, since it’s a grass.

On the other hand, if the most problematic weeds are broadleaf weeds, growing corn may allow application of a broadleaf-selective herbicide such as 2,4-D. (Note that some 2,4-D products marketed to home gardeners are not labeled for use around corn, and in Louisiana, pesticide applicator certification is required to purchase concentrated 2,4-D products in quantities greater than one quart. So, it may be challenging to find a product both labeled for corn and available for purchase without pesticide applicator certification.)

If there are too many difficult-to-manage weeds in an area, it may be best to avoid planting on that site. You could leave it out of production for a season and repeatedly use an appropriate non-selective herbicide and/or repeatedly till to reduce the weed pressure in that area. A variation on this is planting a cover crop that will allow application of an herbicide effective against the most problematic weed(s).

Be sure to read the label of any herbicide you are considering using, and follow label directions if you use it.

Let me know if you have questions.

Dr. Mary Helen Ferguson is an Extension Agent with the LSU AgCenter, with horticulture responsibilities in Washington and Tangipahoa Parishes. Contact Mary Helen at mhferguson@agcenter.lsu.edu or 985-277-1850 (Hammond) or 985-839-7855 (Franklinton).

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