How Recent Rain Helps Improve Fall Herbicide Weed Control?

By Scout Nelson

South Dakota experienced a hot and dry 2026 growing season, creating difficult conditions for crops, forage, and weeds. Recent rainfall across the state brought needed moisture and may also create better conditions for fall herbicide applications. The improved moisture and mild temperatures can help noxious weeds become active again before the first hard freeze.

Eric Jones, Assistant Professor and SDSU Extension Weed Management Specialist, along with Graig Reicks, Jill Alms and David Vos, explained that herbicide performance depends on the growth and condition of weeds. Postemergence herbicides generally work best when they are applied to actively growing weeds.

Dry weather can place weeds under stress even when they appear healthy. During drought conditions, weeds may slow down their growth and develop a thicker waxy layer, called the cuticle, on their leaves. This layer helps plants hold water but can also make it harder for herbicide droplets to enter the plant.

Herbicide activity can also be reduced when a plant is under severe stress. When weeds are not carrying out normal growth processes, the herbicide may have more difficulty reaching and affecting its target. Recent rain helps reduce some of these problems by providing moisture needed for active plant growth.

The weather forecast is also important when planning fall weed control. Mild temperatures after rainfall can allow weeds to continue growing before a killing frost. Fall applications can be useful for controlling perennial noxious weeds because these plants begin moving nutrients and other resources from their leaves toward their roots and underground stems as winter approaches.

Canada thistle and leafy spurge are examples of perennial weeds that prepare for winter in this way. Herbicides applied during this period can move with these resources toward the roots and rhizomes. Because roots and rhizomes help perennial weeds survive and return the following year, reaching these parts of the plant is an important part of long-term weed control.

Farmers do not always need to wait for a light frost before making a fall herbicide application. Changes in day length and cooler temperatures can also tell plants that winter is approaching. A useful weather window is when daily high temperatures remain around 50 to 60°F for at least two days before spraying and at least five days after treatment.

Timing becomes more difficult after the middle of October because long periods of suitable temperatures have become less common. Farmers should also watch for hard freezes. If a hard freeze happens before an application, plants should be checked after about 48 hours. If their green tissue becomes brown or discolored, the plants may have been damaged or killed by the freeze. Herbicide should not be applied to dead plant tissue because the product cannot move through the plant properly.

Weather should also be checked after spraying. A hard freeze soon after application can reduce herbicide movement and affect control. Careful planning can help farmers make better use of available weather conditions.

Scouting is another important part of fall weed management. Farmers should examine fields, pastures and rangelands before applying herbicides. The size and growth stage of weeds can strongly affect treatment results.

For example, absinth wormwood should be treated before its leaves develop a gray-white appearance. Biennial thistles, including bull and musk thistles, can be treated during the rosette stage or before they produce buds. Canada thistle should be treated before the budding stage. Leafy spurge is best treated when yellow bracts are present, before seed pods develop.

If weeds were mowed or clipped earlier in the year, fresh regrowth may be at a useful stage for treatment. However, weeds that have received no control during spring or summer may be too mature for an effective fall herbicide application.

Herbicides are only one part of a strong weed management plan. Farmers can also use mowing, rotational grazing, and biological control methods. For example, leafy spurge beetles can help reduce leafy spurge populations. Continued scouting can identify new weed infestations and show whether additional control is needed.

Farmers should review current SDSU Extension Weed Control: Noxious Weeds and Weed Control: Pasture and Range publications for more information on specifics of herbicides and weeds.

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