By WALLY PECK, Master Gardener
An earlier weed article concentrated on garden weeds, in this article the emphasis will be on lawn and landscape weeds. Many people have remarked that there seem to be many more weeds appearing in lawns and areas surrounding property. Climate change and plants’ ability to evolve are changing our lawns. Everyone wants a perfect lawn. The question you have to ask is at what cost? I will concentrate on natural control; chemical control requires professional advice from a turf grass expert. A healthy, thick lawn discourages weeds.
There are several conditions that encourage grasses to dominate a lawn and suppress weeds. First is the pH of the soil. Turf grass grows best in a pH range of 5.5-7 for the grasses that grow in the north; acid soils allow mosses to dominate. Lime is generally used to adjust pH of soils here. A soil test for growing turf grass will tell you whether to and how much to apply.
Second, how deep is the organic layer of the soil? Often, lawns in newly developed areas have a very thin layer of soil on top of gravel or clay. The desirable soil usually has not been incorporated by tilling the layers together and adding a final layer of topsoil. Roots grow in the narrow top layer and never develop the deep roots that help them withstand drought and fight off weeds.
Third, weeds dominate if the soil is compacted or there is a thick layer of thatch. Compaction can be corrected by using a core aerator that removes plugs two or three inches deep. It is a messy job but the results are usually amazing. Plan to rent an aerator as purchasing a good one is rather expensive. Mid-August is a good time to do this. Dethatching, often called vertical mowing, is necessary when the thatch layer gets too thick.
Grassy areas may have the wrong grass mix for the amount of light received. Bluegrass does not like shade while fescues are more shade tolerant. Grass mixes that are mass marketed are usually wrong for this area. Get grass seed that is blended for the amount of sun and for northern growing conditions.
How turf grass is mowed is another key to a healthy lawn. Mow no shorter than three inches using a sharp blade. A sharp blade reduces the stress to the grass and taller grass allows grass to shadow weeds like creeping Charlie. Taller grass has deeper roots. If you always bag your clippings, you are eliminating a source of nutrition for the grass. A properly fertilized lawn with adequate moisture grows thick grass that crowds out weeds.
Finally, there is no substitute for weeding to eliminate the source of weed seeds. Manual pulling of creeping Charlie is best to control minor infestations. Dandelions and plantain both have roots that need to come out, the right tool really helps with those. Crabgrass is a tough one; it blooms and forms seed very early so hand weeding must also be done early.
If you do opt for chemical control, be aware that what goes on a lawn also goes on the feet of pets, children, birds and often ends up somewhere else after a rain or heavy watering. A broadleaf weed killer (since recalled) named Imprellis killed pines and spruce in yards and golf courses this last year illustrating the principle of unintended consequences.
Assistance for turf grass questions is available from U of M Extension. Use this link: www1.extension.umn.edu/garden/. For answers to gardening questions, call 444-7916 leaving your name, number, and question. A local master gardener will call to assist you.
