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 gardeners minds today, is when they should make the first preemergence herbicide application on lawns and landscape beds?
It is confusing, since University of Nebraska - Lincoln 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 in eastern Nebraska 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, average soil temperatures at four inches are 8 to 22 degrees F higher than normal across Nebraska, making conditions very favorable for crabgrass to begin growing. So UNL turfgrass specialists recommend that the first preemergence herbicide application be done now in eastern Nebraska, and within the next week for western Nebraska. Furthermore, they recommend that a second application should be planned on almost all lawns in the first week of June. Single applications will produce adequate control on most lawns in “typical” years.
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)
Pendimethalin (Pendelum, Pre-M)
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.
Also, keep in mind that mowing height can play 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 .
For more information on managing your lawn, visit UNL Turf iNfo at http://turf.unl.edu. Check out "It's Time to Mow" for tips on how to keep you lawn looking its best this summer.
Find out your soil temperature at the Backyard Farmer website, http://byf.unl.edu. Click on "Nebraska's local Soil Temperatures."