Controlling Grassy Lawn Weeds

There are three ways of controlling grassy lawn weeds like crabgrass, goose grass, and annual bluegrass.

First, the Cultural Approach directs you how to mow, fertilize, water, and reseed your lawn.

Second, a Preemergent Herbicide prevents weed seeds from germinating.

Third, a Post Emergent Herbicide will eradicate just the grassy weeds, not your turfgrass.

1. Cultural Approach

The way you mow, fertilize, water, and reseed your lawn can help keep weeds in check and reduce the need for chemical controls.

  • Mow High and Mow Often. Mowing high means keeping your grass on the longer side of its optimal height. This keeps the soil cooler and provides shade that restricts the growth of annual weeds. Weed seeds on the soil surface need the heat of the sun to flourish. Scalping your lawn is an open invitation for weeds. Second, once weeds have already invaded your lawn, frequent mowing will keep them in check. A weed can't form seedheads when its topmost growth keeps getting lopped off.
  • Fertilize at the Correct Times. The goal is to feed your lawn, not your weeds. Cool season grasses should be fertilized in early spring and late fall. Fertilizing cool season grasses in the heat of the summer will only promote more weeds. Warm season grasses should be fertilized at the height of their growth period in the summer. Avoid feeding in the cooler spring or summer when the weeds are likely to emerge.
  • Water Deeply and Infrequently. There are weed seeds hiding out in your lawn just waiting for the right conditions to emerge. Those seeds grow best when kept damp with light frequent watering.
  • Reseed in the Fall. The fall is the best time to reseed for several reasons. Grass has nine months to get its roots deep and to get more established before facing the summer heat. It has a better chance surviving than grass planted in the spring. In the North, crabgrass and other weeds complete their life cycles in the fall and die out. So they aren't there to compete with the new seedlings for space, water, and soil nutrients.

2. Preemergent Herbicide

A preemergent herbicide application prevents seeds from germinating. The chemical works by creating a thin film over the ground, which covers seedlings. Sprouting weeds are killed as they contact the herbicide barrier.

Apply the preemergent herbicide 10 to 14 days before the weed seeds are expected to germinate.

Crabgrass begins to germinate when the temperature in the upper inch of soil reaches approximately 58-60 degrees Farenheit. A good rule of thumb in the spring is to apply pre-emergent herbicide during forsynthia bloom, before the lilacs bloom, or when the early blooming magnolia petals fall.

For goose grass, use the preemergent product 3 to 4 weeks after the normal application date for crabgrass.

Your local garden center or county extension agent should know precisely when to apply preemergent herbicide for controlling grassy lawn weeds.

The downside of a preemergent herbicide is that it prevents ALL seeds from germinating. This includes stopping regular grass seed from germinating. Some landscapers, like myself, choose to fertilize WITHOUT using a preemergent in the spring. We prefer give the existing grass a chance to fill in and treat future weeds with a post emergent.

If you do use a preemergent, remember two important things:

  1. Do NOT plant any new grass seed for another 6-8 weeks. Again, this is a common mistake because the preemergent stops grass seeds from germinating.
  2. Do NOT aerate or thatch your lawn after this application. Aerating or thatching will break this thin chemical film, negating the application.

3. Post Emergent Herbicide

Use a post emergent herbicide in the summer to kill just the grassy weeds but not the real grass. Check the label for selectively controlling grassy lawn weeds like crabgrass, goose grass, or annual blue grass.

One application is rarely enough. Expect to apply 2 or 3 applications.

These products also tend to cause severe discoloration of regular grass, especially fine fescue and bentgrasses. The grass does recover, but it will look lousy for a week or two.

If you get sick of controlling grassy lawn weeds or don’t want to be exposed to pesticides, have a local lawn treatment company do it. I like how you can sign up online for a free estimate.

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