Extension

Mingying Xiang, PhD, Professor of Turfgrass Science / Turf Extension Specialist and Dennis Martin, PhD, Professor of Turfgrass Science / Turf Extension Specialist

Oklahoma has experienced unusually hot conditions this fall, and average soil temperatures remain high. This has led to a common question: Should fall preemergence herbicide applications be postponed and, if so, for how long?

There is no single answer for every property. Although temperatures remain high, days are becoming shorter. Shaded, irrigated, and moist areas may already have cooler soil and favorable conditions for winter annual weed germination, even while nearby areas in full sun remain hot and dry.

For this reason, September 15 remains a useful target for fall preemergence applications in Oklahoma. It does not have to be viewed as an exact deadline, but it is a good time to begin application plans. Weed germination is influenced by several interacting factors, including soil temperature, moisture, rainfall, irrigation, shade, turf density, site history, and weed species. A cool front followed by rain or irrigation can also trigger germination quickly.

Consider a Split Application When Permitted

Because this year’s warm conditions may contribute to a prolonged germination period, a split application may help extend control when permitted by the product label. An initial application around September 15 may target early germination, while a second labeled application can extend residual control later in the season. Split applications are not appropriate for every product or property. Follow the label for application rates, intervals between treatments, annual or seasonal limits, target weeds, turfgrass restrictions, and seeding or overseeding intervals. Never exceed the maximum seasonal rate.

Proper Activation is Necessary

Application is only the first step. Most preemergence herbicides must be moved from the turf canopy into the upper soil layer where weed seeds germinate. Apply the amount of rainfall or irrigation specified on the label. Products left on the canopy too long may be lost through sunlight degradation, volatility, or other processes.

Before applying:

• Identify the target weeds.
• Select a product labeled for those weeds, the turfgrass species, and the site.
• Check whether weeds have already emerged.
• Calibrate the application equipment.
• Apply the correct rate uniformly.
• Activate the product according to the label.
• Check seeding and overseeding restrictions.

A product that controls annual bluegrass may not provide the same control of chickweed, henbit, annual bromes, or other winter annual weeds. If seedlings have already emerged, a labeled postemergence herbicide will also be needed.

For more information, see HLA-6421 Controlling Weeds in Home Lawns and HLA-6420 Lawn Management in Oklahoma.