Crabgrass: A Weed's Weed!

According to noted author, Dave Barry, crabgrass can grow on bowling balls in airless rooms and there is no known way to kill it that does not involve nuclear weapons. Crabgrass isn’t quite that bad. As a matter of fact, scientists in Arkansas are using crabgrass to clean up crude oil in land around thousands of oil wells in Southern Arkansas. The process is called phytoremediation, (phyto means “plant” and remediate means “clean up”).

Crabgrass attracts bugs or microbes to its roots, which in turn consume the oil. The oil is food for the microorganisms. As crabgrass roots grow through the soil they excrete chemicals, which attracts more critters. High populations of microorganisms are found near the roots. So crabgrass pulls in these microbes to devour the oil.

This pesky weed has many names, some which are printable such as finger grass, crowfoot, purple crabgrass and Polish millet as well as some, which are unprintable as exclaimed by someone finding it in their lawn.

Crabgrass was introduced into the United States in 1844 as a potential forage crop. Prior to that, it was grown as a grain crop in China in 2700 B.C. For you urban types, a forage crop is food grown for cattle. Believe it or not, folks in Oklahoma have developed the only commercially available crabgrass variety in the United States called ‘Red River’. This variety provides excellent grazing for cattle and produces quality hay.

As an urban, horticulture educator for the past 20 years in the great Chicago metropolis of bungalows with pristine weed-free green lawns, I have been asked every imaginable question about weeds; crabgrass is usually the most popular topic of conversation followed by the invasion of wicked Creeping Charlie; but that is another article.

Life Cycle
Crabgrass is a summer annual. Seeds will germinate during spring and summer. Crabgrass seeds will germinate in mid-spring when soil temperatures are greater than 55°-60°F for seven to ten consecutive days. During the summer, seeds will continue to germinate up to temperatures of 95°F. These seeds can stay viable in the soil for thirty years.

As days become shorter in late summer, crabgrass goes into its prolific reproductive phase. Finger-like purple seed heads will form until killed by frost. A single crabgrass plant can produce over 150,000 seeds during the growing season. That’s about 10,000 to 20,000 seeds per square foot. So, you can see why control is difficult.

How to Control Crabgrass
Quality turf can compete well with weeds and is best achieved by properly preparing planting soils, planting turf grasses suitable for the given situation and by using standard cultural practices (mowing, watering, fertilizing and cultivating.

Follow these cultural recommendations:

Proper mowing is critical to turfgrass health and appearance. Turfgrasses mowed too short become open, inviting weed invasion. Mow grass at 2.5 to 3.0 inches. After mowing a site infested with crabgrass, rinse the mower to remove seeds, to avoid transferring them to an uninfected site. Mow frequently so as not to remove more than 1/3 of the leaf blade at one time. That may mean mowing twice a week in the spring and once a week during the summer.Water deeply and infrequently. Most grasses need an inch to an inch and one-half of water per week. Water to wet the soil to the depth of rooting. When irrigated lightly and frequently, weed seeds (especially those of annual weeds) can germinate and readily develop into mature weeds because the soil surface is never allowed to dry out.

Fertilizers maintain turf grass density, vigor and color. Inadequate nitrogen fertilization leads to open turf that is readily invaded by weeds. Inadequate amounts of other minerals in the soil, especially potassium, phosphorus, iron and sulfur, can also reduce turf color, disease resistance and stress tolerance. Conduct soil tests and supply these elements as recommended by test results. It is important, however, not to supply excessive amounts of phosphorus to established turf. Excessive amounts of phosphorus at the soil surface can encourage development of germinating weed seeds.

Crabgrass likes to invade bare soil or areas where the turf is thin, avoid practices that damage turfgrass stands in late spring or summer. If possible delay turf establishment or renovation work until fall to avoid crabgrass problems. This also holds true for practices such as dethatching. Just remember, crabgrass infestations are a symptom of poor turf, not a cause of poor turf.

All these cultural practices, if followed, will help to crowd out crabgrass seedlings.

Pre-emergence or Post-emergence: That Is the Question
Pre-emergence herbicides are applied in the spring in many areas. Timing is critical. Often you will see forsythia bloom as the time to apply pre-emergence herbicides, but forsythia flower petal drop is probably a better indicator. Unfortunately, this occurrence can vary greatly from year to year based on weather. So, if your timing of application is off, poor crabgrass control may result.

Many pre-emergence herbicides also need rain or have to be watered in to be activated. If after an application the turf does not receive rain or is not irrigated, reduced control will result.

With pre-emergence you will have to treat the entire lawn; even when sections of the turf may not need it. Another issue with pre-emergence is that seeding of lawns will be delayed for a long period of time because the germinating grass seedlings will be damaged by the pre-emergence weed killer.

Post-emergence herbicides can be effective. Apply the weed killer before the crabgrass plant sends out tillers. Tillers are side shoots. A post-emergence herbicide is more effective at the 2-4-leaf stage of early growth. As crabgrass matures, it is much harder to control.

Crabgrass must be actively growing for the post-emergence herbicide to be highly effective. Do not apply herbicides when the plants are under stress such as drought.

Turf scheduled for a post-emergence herbicide application should not be mowed for a few days before and after the application. Try to schedule the application so as to allow for a day of no rain after the application. This assures maximum uptake of the herbicides by the leaves.

There are a number of effective pre-emergence and post-emergence herbicides available. Certain post-emergence herbicides are at risk of causing injury to turf, so, remember to always read, understand and follow pesticide label directions for the safest and most effective method of control.

Natural Alternative
Over my twenty-year career, I have noticed a shift or growing concern from consumers concerning the use of chemicals for their lawns and gardens. With regard to weed control in lawns, I have received numerous inquiries about the use of ‘corn gluten meal” as a natural pre-emergent for crabgrass control.

Corn gluten meal is a by-product of processing corn for animal feed. The meal is 60% protein and 10% nitrogen, so it is also a source of nitrogen fertilizer. It must be applied before crabgrass germination. An application is made in the spring, followed by a second application in late summer. During the first few seasons of application corn gluten does not usually provide crabgrass control that is acceptable as most synthetic herbicides. Subsequent use each season may provide improved control.

A philosopher once said, “Work is the crabgrass of life.” For many professionals and homeowners that is so true, but with numerous controls available, the battle with crabgrass can be won.

For additional information about crabgrass check out the Lawn Care section of the University of Illinois’ Hort Corner website at http://www.urbanext.uiuc.edu/hort.

Spring 2003
Hort Shorts | Hort Tips | Crabgrass: A Weed’s Weed! | Health & Household Tips | Perennial Plant of the Year 2003 | Mulch Reminder | Using Organic Fungicides | Bug Bites: A Bug Bite is Not Always a Bug Bite | Lawn Care Calendar | Cybergarden Sites | Pondering Early Planting? Think Lettuce | Eggology | Soup for Supper | Protein-Rich Diets and Weight Loss

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