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Dealing With Crabgrass and Quackgrass in the Lawn


Dealing With Crabgrass in the Lawn
Of all lawn weeds, crabgrass is the most common and bothersome. It is probably the only weed that has no virtues and that everyone hates! Even though it is truly a grass, it doesn’t blend in with lawn grasses in color, habit or texture and can be so aggressive that it out-competes desirable grasses. Understanding the life cycle of crabgrass will help you deal with it more effectively.

What exactly is crabgrass? Crabgrass (Digitaria sanguinalis) is an annual grassy weed with broad blades that have a rough texture and are a lighter green than turf grasses. Since it is annual, it germinates in the spring, develops during the summer leaving lots of fresh seeds, then dies in the late fall. The plants that are there this summer will not be back, but their seeds will be.

What does it look like? Crabgrass is a low-growing weed with lots of stems that form flat clumps. The clumps spread out and root along the stems at joints. The blades are coarse, pointed and short. As the plants get older, they send up branched seed heads that have thousands of seeds. Looking out over your yard, crabgrass shows up as patches of a much lighter green than the rest of your grass.

How did it get into my lawn? Crabgrass seed can be spread by several methods, but most often by birds that eat the seed, then drop it in your yard as they fly over. From a single seed or two, you can end up with thousands of plants in just a season. Another source of the seed is borrowing mowers that have not been cleaned after use. Sometimes, unfortunately, it can even be introduced in grass seed that was not thoroughly cleaned.

Will crabgrass grow anywhere? While it prefers full sun, crabgrass is highly adaptable and even grows where the soil is poor, dry, compacted and nutrient poor. That means lawns that have only minimal care are very likely to develop a crabgrass problem.

When I see it in my yard, what can I do? Early in the season, small patches can be easily pulled since they have a very shallow root system. During the summer when it is actively growing, there are few chemicals that can be used to minimize crabgrass growth. You may decide not to spray, since the plants you are treating will die on their own during the winter. But sometimes crabgrass can get so bad it chokes out your turf grasses. Most selective herbicides are ineffective on crabgrass, since it is a true grass. There is one post-emergent herbicide for crabgrass that can be used during the summer, but it will leave patches of dead crabgrass that may be more unsightly than living crabgrass.

Post-emergent crabgrass herbicides are strong chemicals and should be used conservatively and carefully.

How do I prevent crabgrass next spring? Crabgrass preventatives are pre-emergent herbicides that work by interrupting the seed germination process. Since they remain effective for 30 days, it is important to apply it when it will do you the most good. Depending on how quickly the ground warms up in the spring, plan on applying crabgrass preventer about May 1. Most crabgrass preventers are a combination of fertilizer and pre-emergent herbicide to help strengthen the turf grasses and make them more competitive.

How long do I have to wait before I can seed grass? You should wait three weeks to seed after using a post- emergent herbicide such as Trimec. After seeding, grass should be old enough to have been mowed three times before using post-emergent herbicides.

Pre-emergent herbicides remain in the soil for months, affecting all seed germination, not just crabgrass. While you could put down grass seed 30 days after using a crabgrass preventer, you will get much better germination if you wait until fall.

Dealing With Quackgrass in the Lawn
Quackgrass is another common lawn weed that offers nothing desirable and is very difficult to eradicate. In fact, it is often classified as one of the most objectionable grasslike weeds. Like crabgrass, it doesn’t blend in with the lawn and can choke out desirable turf grasses.

What exactly is quackgrass? Quackgrass (Agropyron repens) is a perennial and has a really tough root system. Seedlings can be pulled easily, but once established, quackgrass is tough. If you can pull a clump, it probably is crabgrass, not quackgrass.

What does it look like? Quackgrass has long, tapered blades around 1/3 inch wide attached to hollow stems. The plant is much more upright growing than crabgrass. The seed heads are single, upright stems, not branched like crabgrass. Quackgrass has a strong root system with white, fleshy rhizomes that spread out from the main plant, sending up new plants.

How did it get into my lawn? The most common way for weed seeds to enter a yard is birds. Quackgrass seed is a favorite of sparrows and other birds and some of it passes through their systems to be deposited in your yard as they fly by. Weed seed can also be introduced by winds and contaminated grass seed. Once a plant is in your yard and goes to seed, hitting it with the lawn mower spreads it to the surrounding area.

Will quackgrass grow anywhere? Although it prefers a sunny location, quackgrass is very adaptable and will grow almost anywhere.

When I see it in my yard, what can I do? You must be aggressive in dealing with quackgrass, since it is aggressive in your yard. Unfortunately, there are no selective herbicides for quackgrass and with its rhizomatous root system, it is almost impossible to pull. The only real alternative in dealing with quackgrass is killing it with a nonselective herbicide such as Roundup or Finale. Keep in mind that these herbicides will kill anything with leaves, and apply them carefully.

Can I prevent quackgrass next spring? Quackgrass, like crabgrass, leaves thousands of seeds in the lawn for the new season. Pre-emergence herbicides, such as crabgrass preventers, will inhibit germination of quackgrass seed. Such seeds germinate as the soil temperature reaches around 55-60 degrees in spring, so timely application is important.

How long do I have to wait before I can seed grass? After using Roundup or Finale to kill the quackgrass while it is actively growing, you can re-seed within 7 days. If you are using a pre-emergence herbicide in the spring, you need to wait a minimum of 30 days before seeding grass. Again, preemergent herbicides continue to affect germination rates after the 30 days, so waiting until the fall seeding season, will be more successful.

Additional Bachman's Information
Lawn Weeds
Lawn Care Calendar
Herbicide Basics
Bachman's Lawn Fertilizers
How to Use Garden Sprayers
Calculating, Measuring and Spacing

© Bachman's 2007

 

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