Spring Lawn Care - Pre-Emergence Herbicide Application for the Home Lawn
submitted by Sarah Browning, Nebraska Extension Educator
This spring's extremely warm weather has plants blooming almost a month ahead of schedule and gardeners scratching their heads. So many of the normal spring garden tasks are associated with dates on the calendar, and this year's unusual conditions are throwing everything into question. One of the biggest questions in gardener’s minds today, is when they should make the first pre-emergence herbicide application on lawns and landscape beds.
Pre-emergence Herbicide Timing
It is confusing, since Nebraska Extension turfgrass specialists for years have recommended April 20th to May 5th as the target dates for the first application. That recommendation is based on the germination time of crabgrass, which occurs when soil temperatures reach 55 degrees F several days in a row. A few hours or even a single day of warmth is not enough to induce germination; several days of 55 F degrees soil temperature is required. Typically Lancaster County soil temperatures have reached the "55 degrees for several days" mark in the last week of April or first week of May.
However, all that is out the window this year. Based on data from the High Plains Climate Center, soil temperatures in Lancaster County have been, on average, between 51 and 52 degrees for the last week. That’s 13-14 degrees higher than normal. Factoring in added heat effects from driveways, sidewalks and reflected heat from your home’s siding, along with the micro-climate benefits of urban landscapes, conditions are very favorable for crabgrass to begin growing. (Check out current average soil temperatures for yourself at Nebraska Extension’s Hort Update website, http://hortupdate.unl.edu.)
So it’s time the first pre-emergence herbicide application to be done now, but with such an early first application a single pre-emergence application will not provide adequate weed control all summer. Do-it-yourself homeowners should plan to make a second pre-emergence application approximately the first week of June.
And remember, pre-emergence products are often impossible to find by mid-summer, so buy all the products you need for this season’s weed control in spring and store them in your garage until you need them.
Pre-emergence Herbicide Products
Several pre-emergent herbicide chemicals can be found in products available to homeowners. To figure out which one you are buying, look for the chemical name in the active ingredient statement on the front of the fertilizer bag:
- trifluralin and benefin (Team)
- dithiopyr (Dimension)
- pendimethalin (Pendelum, Pre-M)
- prodiamine (Barricade)
Each product has a specific residual length. Follow label directions for the product you buy, and reapply the pre-emergence herbicide as directed for continued weed control throughout summer.
If you are concerned that some crabgrass has already germinated before your herbicide application was made, then consider using dithiopyr. When applied at the full label rate, dithiopyr will kill crabgrass seedlings if applied before they reach the tillering stage.
Pre-emergence herbicides should be watered in immediately after application with at least 0.5 inches of water.
Weed Control in New Seedings
If you are reseeding or overseeding your lawn this spring, the only pre-emergent herbicide available to homeowners that can be used with new seedings is siduron, commonly sold as Tupersan. This herbicide will provide good control of annual grassy weeds like crabgrass and foxtail, yet still allow the grass seed to germinate.
Weed Control and Mowing
Also, keep in mind that mowing height plays a significant role in your lawn's weed problems. Research at the UNL Turfgrass Research Facility has shown that raising the mowing height from 1.5 inches to 2.5 inches in a Kentucky bluegrass stand decreased crabgrass infestation from 80% to less than 15% respectively. This is one of the primary reasons why a mowing height of 3.0 inches season-long is recommended on all lawns in Nebraska.
There is also a direct relationship between mowing height, and potential turfgrass rooting depth. So choosing a taller mowing height enables your lawn to develop a deeper root system and draw water from a larger soil area during hot, dry summer conditions. Choosing a taller mowing height will reduce crabgrass germination, even with no application of pre-emergent herbicide.
The information on this Web site is valid for residents of southeastern Nebraska. It may or may not apply in your area. If you live outside southeastern Nebraska, visit your local Extension office