By Pam Rosendal, Colorado Master Gardener

Summer is here. Trees are green and flowers are blooming, and so are those darn weeds! Every year homeowners fight weeds in their lawns and every year they come back. With a little detective work and some changes to your lawn care program, weeds can be kept at bay.

Of course, the best defense against weeds in the lawn is good maintenance. A thick healthy lawn will choke out most weeds over time. That means proper watering, fertilizing and mowing. Refer to CSU’s fact sheet on https://extension.colostate.edu/topic-areas/yard-garden/lawn-care-7-202/.

Photos left to right: black medic, utahpests.usu.edu; prostrate spurge, scotts.com; prostrate knotweed, msuturfweeds.net

Certain types of weeds in the lawn, called indicator weeds, can signal what may be going wrong. Prostrate spurge, knotweed, mouse-ear chickweed or annual bluegrass can indicate that the soil is compacted. To correct this, core aerate in the spring and fall to break up the compaction.

Photos left to right: yellow woodsorrel, amazon.com; mouse-ear chickweed, better-lawn-care.com; annual bluegrass in lawn, turffiles.ncsu.edu

Crabgrass, yellow wood sorrel and white clover are a sign that the mower is set too low. Recommended mower height is 2.5 to 3 inches. Taller turf grass shades the ground so weed seedlings have a harder time germinating. 

Insufficient water encourages dandelions, black medic and bindweed. Check your irrigation system to make sure it’s properly covering all lawn areas. Use a catch-can test to determine if there is a coverage problem. If poor coverage isn’t the problem, you may need to increase the amount of water. Programming your system with a cycle and soak setting can reduce water runoff.

On the flip side, overwatered lawns invite annual bluegrass, crabgrass, violets and ground ivy. Make sure the sprinklers aren’t running too long or too often.

Over application of fertilizer can encourage henbit, yellow wood sorrel and annual bluegrass. Fertilize turf once in May to mid-June, once in September and once in November, using the amount recommended for your turf type.

Insufficient fertilizer and too much water can create ideal conditions for black medic, plantain and white clover. If these weeds are a problem, cut back on the water and fertilize.

Herbicides are an option for controlling weeds until the changes you’ve made to your lawn care take effect. If you have only a few weeds, spot treat them rather than applying herbicide to the entire lawn. Apply herbicides on a calm, clear day when the temperature is between 50 and 85 degrees. Don’t apply if rain is predicted in the next 12 hours and don’t irrigate for 12 hours after application. And, most importantly, always read and follow all directions on the herbicide label. 

With a little time and effort, you can have the best looking, nearly weed-free lawn in the neighborhood. If you need additional help, or have questions, call the Master Gardener volunteers at 303-730-1920 or e-mail mastergardener@arapahoegov.com.

Read more details on indicator weeds here.