Managing Weeds in the Vegetable Garden, Part 4: Pre- and Post-Emergence Herbicides

(News article for August 19, 2023)

Previous articles in this series have discussed planting decisions, site preparation, and non-herbicide approaches to reducing weed pressure during the growing season. All of these are important. Herbicides used immediately before or after planting and while vegetables are in the garden are important tools in some cases, but they should not be relied upon alone to manage weed problems.

Preemergence herbicides prevent weeds from becoming established as seeds germinate. Most of them do not kill weeds that have already emerged, and they do not kill seeds.

There are preemergence herbicide products sold in home garden quantities that contain the active ingredient trifluralin. Some are labeled for use prior to or after planting specific vegetables. These are effective against some annual grasses and small-seeded broadleaf weeds. The timing of application varies by vegetable and by whether transplants or seeds are used, so read the label carefully.

An important part of using trifluralin products is mixing them into the top 2 inches (approximately) of soil within 24 hours of when they’re applied. You can use a garden rake or a tiller set to work the soil shallowly to do this.

There are other pre-emergence herbicides labeled for use around certain vegetables. Some of these provide weed control for a longer period than trifluralin products do, but many are only sold in large quantities.

Most herbicides have either pre-emergence or post-emergence activity but not both. There are a few exceptions. Halosulfuron-methyl products, which work well against yellow and purple nutsedge, are among the exceptions.

Where yellow or purple nutsedge is a significant problem, a halosulfuron-methyl product can help prevent nutsedge from coming up, even from nutlets. Sandea, Profine 75, and Stadia are labeled for use on a number of vegetables. These are pricey, but usage rates are low, so one container goes a long way. Allowed uses and timings of application vary by vegetable, by whether it’s seeded or transplanted, and by whether or not plastic mulch is used, so read the label carefully.

Post-emergence herbicides kill weeds that have already emerged and are visible.

Since most vegetables are broadleaf plants, grass-selective herbicides with the active ingredient sethoxydim can be used over the tops of many types of vegetables. Such products include Poast, Bonide GrassBeater II, Fertilome Over-The-Top II Grass Killer, and Hi-Yield Grass Killer.

If you’re growing sweet corn (a grass) and can find a 2,4-D product that’s both labeled for sweet corn and available for purchase without pesticide applicator certification, this is an option for broadleaf weed management.

As mentioned, the halosulfuron-methyl products also have post-emergence activity on nutsedge. The previously mentioned halosulfuron-methyl products can be used either in row middles only or in both rows and row middles with certain vegetables.

A handful of non-selective, non-systemic herbicides – including ones that contain pelargonic acid (e.g., Beloukha Garden Herbicide and Scythe) or combinations of caprylic acid and capric acid (e.g., Bonide Captain Jack’s Deadweed Brew, Homeplate, FireWorxx, and Suppress) – can be used in row middles while vegetables are present.

Some glyphosate products, which are non-selective and systemic, are labeled for row middle application for certain vegetables, if a hooded or shielded sprayer or wiper applicator is used. Be aware that, because glyphosate moves systemically in plants, if even a tiny amount of the product drifts onto vegetable plants, significant plant damage can occur.

Even when you have herbicide options, it’s important to stay on top of weed management rather than waiting until weeds are large to try to control them. Postemergence herbicides are generally more effective against weeds when they’re small, and weeds allowed to grow large have more time to compete with vegetables for water, nutrients, and sunlight.

Be sure any herbicide you’re thinking of using is labeled for the vegetables around which you plan to use it, and read and follow label instructions carefully.

Let me know if you have questions.

Contact Mary Helen Ferguson.

More vegetable gardening resources are available here.

Reference to commercial or trade names is made for the reader’s convenience and with the understanding that no discrimination or endorsement of a particular product is intended by LSU or the LSU AgCenter. In some cases, other brands may be available.

9/8/2023 7:46:35 PM
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