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Released: March 27,  2000

Horticulturist Gives Top Reasons
Weeds Invade Kansas Lawns

MANHATTAN, Kan. ó The best way to prevent lawn weeds is to grow strong, vigorous grass, according to Kansas State University horticulturists.

So, why arenít lush lawns common across a state thatís been good at growing grass since giant mastodons led the way for migrating buffalo herds?

Are Kansas homeowners doing too little? Too much? Or, do they just have bad timing?

The answer is "All of the above ó and then some," said K-State Research and Extension specialist Ward Upham. He listed the following as strong contenders for a Top Ten list of reasons weeds invade todayís Kansas lawns:

1. Wrong kind of grass. Bluegrass wonít thrive in western Kansas. Buffalograss doesnít like the east. Bermuda wonít thrive anywhere thatís shady. And, any grass not suited for its location will slowly decline, creating an opening for weeds.

2. Compacted soil. This is a hidden stress. Its effects on roots make grass unable to compete well with weeds.

"Unfortunately, homeowners who didnít start their lawn from scratch often overlook this possibility. Many other problems can develop if turf has a poor foundation ó especially if that base is clay-heavy or silty soil with little to no organic content," the horticulturist said.

3. Not mowing often enough. Research for years has shown that lawns get thinner each time a mower lops off more than a third of grass bladesí height. The taller grass grows before itís cut to recommended height, the greater the damage.

4. Mowing too low. K-State scientists have pinpointed the heights at which different types of turf tend to perform well. Bermuda, for example, can be lush and healthy cut 1 inch high. The most popular Kansas turf is K-31 tall fescue, which performs best at between 2.5 and 3.5 inches. But with either extreme, turf thatís shorter than its recommended height range will let sunlight reach weed seeds.

"A similar problem can develop if you donít keep mower blades sharp," Upham said. "Dull blades shred leaf tips, making your lawn both unattractive and less resistant to stresses and pests."

5. Improper fertilizing. Fertilizing too much or too little can benefit weeds more than grass ó as can fertilizing at the wrong time(s).

6. Overwatering/Over-Frequent Watering. With deep and well-established root systems, Kansasí common lawn turfs can survive a surprising amount of heat and drought (although some will go dormant). If watered too often, however, those grasses will "get lazy" and develop shallow roots that are

unable to do well, either competing with weeds or handling heat and drought. Overwatering also fosters lawn diseases, weed seed germination and thatch buildup.

7. Insect and disease injury. Pests thin turf. Weeds soon follow.

8. Thick thatch. Thatch is a peat-moss-looking layer of runners, stems and surface roots just on top of the soil. If less than a half inch thick, itís rarely a problem. But, some turfs (particularly bermuda, bluegrass, zoysia and creeping bentgrass) can develop a much thicker layer. In turn, those lawns will send more and more roots into thatch, rather than soil. Over time, the lawns will deteriorate as shallow roots make them vulnerable to invaders and sensitive to heat, cold and drought. The thatch also may reduce the impact of any soil-applied weed and insect controls.

"Often you have to get down on your knees and look closely, to discover a lawn has excess thatch," Upham said. "Thatch problems are not something youíre likely to notice while simply walking across the grass ó except for the fact that a thick thatch layer can make a lawn feel Ďspongy.í"

9. Excessive wear. Lawn areas used for heavy foot traffic or recreation can end up with worn turf and compacted soil. A need for intense weed control and lawn management can result.

10. Environmental stress. Kansas weather alone can weaken and thin lawns ó just as it damaged grasses when the buffalo roamed. Tornadoes, floods, wind, hail, sleet, heat, drought and untimely freezes all take a toll.

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K-State Research and Extension is a short name for the Kansas State University Agricultural Experiment Station and Cooperative Extension Service, a program designed to generate and distribute useful knowledge for the well-being of Kansans. Supported by county, state, federal and private funds, the program has county Extension offices, experiment fields, area Extension offices and research centers statewide. Its headquarters is on the K-State campus in Manhattan.

Kathleen W. Ward
Communications Specialist  

K-State Research & Extension News

Upham is at 785-532-1438