Understanding Preemergence Herbicides for Common Weeds in Vegetables
A recent survey from one of our winter meetings cited pigweeds, marestail, lambsquarters, foxtail, morningglories, and Italian ryegrass as some of the most troublesome weeds in vegetable production. These weeds, and others like them can be especially difficult to manage in vegetables. Starting clean and including a good preemergence herbicide is key, but there may not be a one-herbicide-fits-all approach to managing all the weeds that may be present in a field. Understanding the species these herbicides target and other factors is essential for early season weed control.
Marestail and Italian ryegrass typically emerge prior to spring planting and can be managed using aggressive tillage such as plowing or disking, or with preplant herbicide applications. Herbicides such as glufosinate (Rely) and paraquat (Gramoxone) can be applied pre-transplant in many vegetable crops, but they are most effective when target plants are small, and multiple applications may be required. Other species tend to emerge later in the spring and may not be present at the time fields are prepped, so a residual herbicide should be applied prior to planting or transplant.
Residual herbicides such as pendimethalin, trifluralin, ethalfluralin, and bensulide primarily target grass species, but will also control certain small-seeded broadleaf species such as lambsquarters and pigweeds (Table 1). Preemergence applications of halosulfuron and imazosulfuron target several broadleaf species including lambsquarters, but will not control grasses. Halosulfuron does have activity on morningglories and yellow nutsedge, but only when applied as a postemergence application. Preemergence control of yellow nutsedge can be achieved using imazosulfuron or S-metolachlor, which also targets several grass and small seeded broadleaf weeds.
It is important to note that the higher rates may be needed to control certain weeds, but this may not be possible without exceeding maximum labeled rates. For example, clomazone can control morningglory at 1.25 lb./A, but the maximum rate for most vegetable crops is less than 1 lb./A.
Rainfall or irrigation will be needed to activate all of these herbicides, and the amount needed will vary with each herbicide. Herbicides such as clomazone require very little moisture to activate (~0.33 in.), while S-metolachlor requires between (0.33 to 0.5 in.), and pendimethalin and trifluralin require over 0.75 in.
If a single herbicide does not provide satisfactory control, tank mixing may be appropriate. Always consult the product label before tank mixing or applying any pesticide. For more information, a copy of the 2024/2025 Mid-Atlantic Commercial Vegetable Production Recommendations can be found at https://extension.umd.edu/resource/mid-atlantic-commercial-vegetable-production-recommendations-eb-236/.
| Herbicide | Trade Nameᵃ | Group # | Crops listedᵇ | Weeds controlled with PRE applicationsᵇ |
|---|---|---|---|---|
| halosulfuron | Sandea | 2 | cucumber, pepper, pumpkin, tomato, watermelon | pigweedsᶜ, marestail, lambsquarters |
| imazosulfuron | League | 2 | pepper, tomato, watermelon | lambsquarters, pigweedsᶜ, yellow nutsedge |
| ethalfluralin | Curbit | 3 | cucumber, pumpkin, watermelon | lambsquarters, pigweeds, crabgrass, fall panicum |
| pendimethalin | Prowl H₂O | 3 | pepper, tomato, watermelon | pigweeds, lambsquarters, morninggloriesᵈ, crabgrass, Italian ryegrassᵈ, foxtails |
| trifluralin | Treflan | 3 | cucumber, pepper, tomato, watermelon | pigweeds, lambsquarters, large crabgrass, foxtails, Italian ryegrass |
| bensulide | Prefar | 8 | cucumber, pepper, pumpkin, tomato, watermelon | crabgrass, foxtails, lambsquarters, pigweeds |
| clomazone | Command | 13 | cucumber, pea, pepper, pumpkin, watermelon | common lambsquarters, foxtails |
| S-metolachlor | Dual Magnum | 15 | pea, pepper, pumpkin, tomato | pigweeds, crabgrass, foxtails, Italian ryegrass, yellow nutsedge |
- ᵃ Mention of a product does not constitute a guarantee or warranty of products. Reference to trade names is made with the understanding that no discrimination is intended and no endorsement by University of Maryland Extension is implied.
- ᵇ Column does not constitute and exhaustive list. For more information refer to the product label.
- ᶜ Resistance documented to this herbicide group in Maryland.
- ᵈ Suppressed but not controlled when used at a rate of 1.9 lb. pendimethalin/A.
This article appears in May 2024, Volume 15, Issue 2 of the Vegetable and Fruit News.
Vegetable and Fruit News is a statewide publication for the commercial vegetable and fruit industries and is published twice a month during the growing season (April through October). Subscribers will receive an email with the latest edition.
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