With spring gardening season right around the corner, lawn questions have been rolling in to the Home & Garden Information Center (HGIC). Here I’ll address some of the most common questions about weeds and overseeding.
Dealing with Winter Weeds
In late winter/early spring, we typically see winter annual weeds in thin, under-fertilized, wet, or shady areas. These weeds germinated in the fall and will die as the weather warms up later in the spring. In my observations, this has not been a particularly bad year for winter annuals. They are favored by wet, mild winters and I think we had just enough “bitter cold” in January and a fairly dry stretch through December and January to reduce populations.
Typical winter annual weeds include chickweed, henbit, purple deadnettle, and shepherd’s purse. Options to address these winter annual weeds include hand pulling, spot spraying with a broadleaf herbicide, or waiting until they die once weather climbs to the 60’s and 70’s on a regular basis. For perennial weeds like dandelion which will start to re-emerge later this month, hand-pulling or spot spraying are the best methods for control.
Crabgrass is native to the tropics and in Maryland is considered a summer annual since it germinates in spring once soil temperatures reach 55-57° F. The typical phenological (plant-based) indicator of 50°F soil temperature for applying crabgrass pre-emergent products coincides with forsythia near or at peak bloom. I know a lot of homeowners consider applying crabgrass pre-emergent somewhat of a rite of spring lawn care without thinking much about some of the other factors involved in crabgrass control. I would encourage you to think more about this than automatically going out to apply herbicide.
There are other important lawn management factors that contribute to reducing crabgrass encroachment in the lawn. As a summer annual, crabgrass likes areas that are warm and sunny and takes advantage of open spaces. In a lawn, this would include bare and thin areas where the grass is either non-existent or struggling. So what does this mean when it comes to maintenance practices?
Most importantly, “mowing high” and fertilizing correctly to encourage dense grass cover are two of the most important maintenance practices you can do to combat crabgrass invasion. Tall fescue lawns should be mowed at 3 – 3 ½”. This helps encourage a deeper root system and creates a cooler, shadier microenvironment at the soil surface. University of Maryland research has shown that plots mowed at 3 ½” with dense grass cover typically only had a few percent crabgrass even when no crabgrass pre-emergent products were applied.
Overseeding Bare Areas
While seeding is most successful in the fall, overseeding thin or bare areas in the spring to help increase density is an option. Tall fescue grass seed doesn’t need as warm of temperature as crabgrass, so you may consider trying to overseed in late March/very early April before crabgrass germination gets into full swing. Keep in mind, however, that any pre-emergent product you apply for crabgrass will also prevent tall fescue seed from germinating.
If you’re interested in learning more about lawn care, I will be presenting a “Bay-Friendly Lawn Care Practices” workshop and tour of the ‘Grass Roots’ turfgrass exhibit at the U.S. National Arboretum in Washington, DC on Saturdays, April 7 and May 12 from 10 a.m.-noon. In this workshop, I present a comprehensive strategy for lawn care best maintenance practices for the year in an integrated pest management framework. If you would like to register, please contact me at firstname.lastname@example.org.
By Geoff Rinehart, Lecturer, Turfgrass Management, Institute of Applied Agriculture, University of Maryland