Gardening Guru: Summer Trouble: Weeds in Lawns and Landscapes

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

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 through its midrib. Leafy spurge invades areas of sparse grass 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, leafy spurge reduces the uniformity and quality of the turf and attracts ants with its seeds. The milky sap that drains from its cut stems is a skin irritant.

Spurge germinates in open spaces from March to October. The main method of managing leafy spurge is prevention, as it is difficult to control once established. Although it is an annual, its prolific seed production (several thousand per plant!) Makes it a noxious weed.

It is very important to hand pull or weed new leafy spurge plants before they produce seeds. That means at least a weekly weed walk in the garden to stay on top. Once leafy spurge has established itself, changing cultural practices such as fertilizing or irrigation will not control it. However, increasing the mowing height to 2 inches or more in tall fescue reduces the risk of initial invasion.

Pre-emergent herbicides (before the weed is visible) are useful in reducing leafy spurge in lawns and landscapes and work best if applied in late winter or early spring before seed germination.

Post-emergence herbicides will not control larger, more mature spotted spurge plants once they are established in the beds. Treat leafy spurge in driveways and unplanted areas with a broad-spectrum herbicide, then apply several inches of mulch anywhere the leafy spurge has gone to seed.

Crabgrass is a weed familiar to most people; some mistakenly call it watergrass, but there is another weed that has this common name. The leaves of the seedlings are light green and smooth and are clearly visible in the lawn with their lighter green color. Crabgrass often forms patches in lawns, and the flowering stems are similar to those of Bermuda grass. Crabgrass seeds begin to germinate around March 1 in the valley and continue through spring, summer, and fall until soil temperatures begin to cool.

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

So try to do the opposite when you take care of your lawn! Don’t water too often. Mow fescue lawns to a height of at least 2 inches and Bermuda grass lawns to at least 0.75 inches. Fertilize fescue lawns in the spring and fall (not summer). Fertilize Bermuda grass lawns in summer, not early spring. Clean lawn mowers after using them in weed-infested sites.

Crabgrass is easy to control with preemergence herbicides but difficult to control with postemergence herbicides. The same herbicides that work on leafy spurge will also prevent crabgrass from germinating. For the most effective pre-emergence herbicides, it is best to apply them about three weeks before the crabgrass germinates. It really is never too late to apply pre-emergence, but you won’t kill existing weeds. If you choose a weed and feed product, make sure it contains a preemergence herbicide (often marketed as a crabgrass prevention agent).

Only a few active ingredients in post-emergence herbicides will have a detrimental effect on existing crabgrass in lawns. For more information on these weeds, download a pest control note at ipm.ucanr.edu/PMG/PESTNOTES/index.html.

Remember, your goal is to become a better gardener and not depend on herbicides.

The master gardeners will be live to answer your questions on Saturday, September 3, from 10 a.m. to 1 p.m. at the Ace hardware store in Visalia. You can also contact them at 559-684-3325, or visit their website at ucanr.edu/sites/UC_Master_Gardeners.


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