Troublesome Summer Weeds in Lawns and Landscapes

Mid August signals the half way mark of our region’s hot season and lawns and landscapes are in peak demand for water. Because most soils are harboring a treasure trove of seeds, along with water and fertilizer comes weeds.

One of the best control measures for weeds in lawns is to maintain a competitive stand of grass. When open areas develop in lawns due to stress, disease, lack of fertility, insects, or traffic abuse, light penetrates to the soil surface, allowing weed seeds to germinate. Once most weeds get started it is difficult to control them with cultural practices.

Before planting an area with turf or ornamentals you might want to take a little extra time to prepare your soil. Covering the soil with sheets of clear plastic (soil solarization) for 4-6 weeks during the summer effectively reduces the number of weeds. Soil temperatures can reach over 140 degrees and that kills the seed.

Two very common weeds in summer lawns and landscapes are spotted spurge and crabgrass.

Spotted spurge grows close to the ground, often forming a dense mat. Its dark green leaves grow in pairs called “opposites” and are less than half an inch long. Frequently a red spot will mark the leaf halfway down its center vein. Spurge overgrows sparse turf areas and low-growing ground covers, invades open areas in gardens and landscapes, and can grow in sidewalk cracks. In addition to reducing the growth of desirable plants, spotted spurge reduces uniformity and quality of turf and attracts ants with its seed. The milky sap that exudes from its cut stems is a skin irritant.

Spurge germinates in open spaces from March through October. The primary method of managing spotted spurge is prevention since it is difficult to control once established. Although it is an annual, its prolific seed production (several thousand per plant!) makes it a pesty weed.

It’s very important to hand-pull or hoe new spurge plants before they set seed. This means at least a weekly weed walk through the garden to keep on top of it. Once spotted spurge establishes itself, altering cultural practices such as fertilization or irrigation won’t control it. However, raising the mowing height to 2 inches or more in tall fescue reduces the chance of initial invasion.

Preemergence herbicides (before the weed is visible) are helpful in reducing spotted spurge in lawns and landscapes and work best if they are applied in late winter or early spring before seeds germinate.

Postemergent herbicides will not control the large, more mature spotted spurge plants once they are established in flowerbeds. Spot treat spurge in pathways and unplanted areas with a broadsprectrum herbicide, then apply several inches of mulch wherever spurge has gone to seed.

Crabgrass is a familiar grassy weed to most people; some mistakenly call it watergrass, but there is another weed that bears that common name. Seedling leaves are light green and smooth and are conspicuous in the lawn with their lighter green color. Crabgrass often forms patches in lawns and the flowering stems are similar to those of bermudagrass. Crabgrass seed starts germinating around March 1 in the valley and continues throughout spring, summer, and fall until soil temperatures start to cool down.

A thick lush lawn will prevent crabgrass from becoming a problem. Lawn care practices that favor the development and spread of crabgrass are: frequent, shallow irrigations; mowing lawns too short; edging lawns too close; fertilizing lawns at the wrong time of year; and lawn mowers spreading it from site to site.

So try to do the opposite when you take care of your lawn! Don’t water too frequently. Mow fescue lawns to at least a 2-inch height and bermudagrass lawns no lower than .75-inch. Fertilize fescue lawns in spring and fall (not in summer). Fertilize bermudagrass lawns in summer, not in early spring. Clean lawn mowers after using them in weedy sites.

Crabgrass is easy to control with preemergent herbicides but difficult to control with postemergent herbicides. The same herbicides that work on spurge will also prevent crabgrass from germinating. For preemergent herbicides to be most effective it is best to apply them about three weeks before crabgrass germinates. It is really never too late to apply a preemergent, but you won’t kill any existing weeds. If you select a weed and feed product, then be sure it contains a preemergent herbicide (often marketed as crabgrass preventer).

Only a few active ingredients of post emergence herbicides will have a detrimental effect on existing crabgrass in lawns. For more information on these weeds download a Pest Note from http://ipm.ucanr.edu/ PMG/PESTNOTES/INDEX.HTML.

Just remember that your goal is to become a better gardener and not to rely on herbicides.

Are you interested in becoming a Master Gardener? The UC Master Gardener program of Tulare/ Kings Counties is recruiting! Our next class runs from January 19 through June 8, 2022. Applications will be available online in August and must be turned in by October 27. We will be holding a mandatory orientation on Wed., October 13 @ 3:00 to share what the training course and the volunteer commitment entail. Please call our office (684-3343) with any questions… we look forward to talking with all interested gardeners! Check us out at:

https://ucanr.edu/sites/uc_master_gardeners/become_a_ Master_gardener/

The Tulare-kings County Master Gardeners will be available to answer your questions in person:

Ace Hardware, Visalia - 1st Sat./every month, 10 am-1 pm

Luis Nursery, Visalia - 2nd Sat./every month, 10 am-2 pm

Hanford Farmer’s Market - 4th Thurs, May - Sept., 5-8 pm, 7th ST. and Irwin Downtown Hanford

Visalia Farmer’s Market - Saturdays, 8-11 am, 2100 W. Caldwell Ave (behind Sears Building) Questions? Call us: Master Gardeners in Tulare County: (559) 684-3325, Tues & Thurs, 9:30-11:30