Spring Weeds in Grass Hay and Pasture: What Can Be Controlled Now?

Now that spring is upon us, many are taking notice of weeds in grass pastures and hayfields. With the colorful array of weedy flowers, some wonder if this is a good time to spray. The short answer is "it depends." Just because certain weeds are more obvious now because of flowers does not mean it is necessarily a good time to spray. As you scout, note the types of weeds you observe and understand their life cycles to determine whether it is a viable time to spray herbicides. For certain winter annual and biennial weeds, now may or may not be the correct time to apply an effective herbicide to prevent flowering and seed production.

Broadleaf Weed Considerations

For example, many winter annuals, such as mustard species, common chickweed, and deadnettle/henbit, have already flowered or set seed. Since they are in their later stages of maturity, now is not a good time to spray. (Spraying them at this point would just be an expensive recreational activity.) However, if certain mustards have not yet set seed, mowing can help to clip off their seedheads to stop seed production. If your pasture is "yellow" with buttercup flowers, it's too late to get effective control from herbicides. In general, the best time to apply herbicides to control buttercup is in the early spring (mid-March through April). But if weeds such as marestail, fleabane, and biennials, including poison hemlock, musk and plumeless thistle, burdock, wild carrot, etc., are still in the vegetative stage, now would be a good time to spray before they start to get taller and flower.

Management of perennial weeds such as Canada thistle, horsenettle, common milkweed, and smooth bedstraw, and woody perennials such as multiflora rose and autumn olive is best performed a bit later in early summer after plants reach the bud-to-bloom stage. But an even better time to manage perennials is late summer and early fall, using systemic herbicides after they have been mowed a couple of times during the growing season.

The most common herbicides used for control of many broadleaf weeds in grass hay/pasture this time of year are the plant growth regulator herbicides such as 2,4-D ± dicamba (Clarity, etc.), triclopyr products (Crossbow, Remedy Ultra, etc.), and clopyralid (Stinger, PastureGard, etc.). In addition, products containing metsulfuron (Cimarron, other generic formulations, etc.) can provide good control of many broadleaf weeds in the spring. (Be cautious, if forage grasses were recently seeded and are not yet established, many of these herbicides can cause severe crop injury.) Refer to Table 2.6-11 in the 2025 Penn State Agronomy Guide for ratings of these herbicides on many different weed species.

Weedy Grass Considerations

Weedy annual grasses such as crabgrass, foxtails, panicum, and Japanese stiltgrass, among others, are becoming problematic in grass forage settings. Unfortunately, these weedy grasses start to germinate at different times in the spring. Japanese stiltgrass can germinate in early spring, a couple of weeks or more before crabgrass and the other species, thus an early application of pendimethalin will be necessary for initial control. Large crabgrass and the other mentioned grass species typically germinate around 200 GDD (growing degree days); however, Japanese stiltgrass begins germination much earlier. Prowl H2O or Satellite HydroCap must be applied before weed germination in spring, or in-season between cuttings, otherwise weeds will not be controlled. If grassy weeds escape, Plateau can be applied in-season at low use rates (e.g., 4 fl oz/acre) to control these annual weedy grasses if they are less than 5 inches tall. However, significant injury to certain forage grass species such as timothy and tall fescue can occur (see label for more details).

Keep in mind that pendimethalin products do not control established perennials, such as roughstalk bluegrass (Poa trivialis), which is becoming more of a problem in forage fields across the state. Fall and early spring are the best times to apply effective herbicides to control this weed, but unfortunately, there are no effective products that control roughstalk bluegrass in grass forages. However, in alfalfa settings, using Gramoxone/paraquat, Raptor, or Select/clethodim after the first cutting generally provides 85-90% control.