January 28, 2008

Controlling Crabgrass

By Jane Morse, University of Florida/IFAS Extension Agent, Pinellas County

crabgrass If crabgrass has taken over your lawn, early February is the time to put out a pre-emergent herbicide. Pendimethalin (sold as Pendulum, Pre-M, Turf Weedgrass Control, Halts Crabgrass Preventer) provides excellent control of crabgrass and is safe at the recommended rate on mature, actively growing grass (Bahia, Bermuda, St. Augustine and Zoysia). Apply herbicide when daytime temperatures reach 65 to 70 degrees Fahrenheit for 4-5 days in a row, or usually about the same time that azalea plants start blooming in the spring. Plan to reapply herbicide about 2 months later for season-long control.

If weeds are a recurring problem in your lawn, follow these steps to have a healthy, thick lawn that will out-compete most weeds:

1. Start off with the right grass. Bahia is best suited for sandy, acidic soils that are subject to drought. St. Augustine is better suited for mildly acidic to alkaline soils that are subjected to salty conditions. Also choose grasses based on the amount of care that you are willing to provide. Bahia has a low maintenance level, St. Augustine a moderate level, while Zoysia and Bermuda have a high maintenance level. For areas of dense shade choose a shade-tolerant ground cover or use mulch.

2. Mow at the right height. Set those mower height adjustments high for St. Augustine and Bahia grass which should be mowed at a height of 3 to 4 inches. Dwarf St. Augustine grasses (Delmar, Jade, Seville) are mowed at 2 - 2 ½ inches. Bermuda is mowed at ½ to 1 ½ inches and Zoysia 1 - 2 inches. Proper mowing height and frequency will get rid of many annual weeds.

3. Mow often. Only 1/3 of the leaf blade should be removed each time the lawn is mowed. The shorter it’s mowed, the more often it needs to be mowed. Repeatedly removing too much of the grass blade (more than 1/3) will eventually kill the grass. Keep the mower blades sharp for the best cut. Mow when grass is dry.

4. Water only when it needs it. When 30% of the lawn starts to show symptoms of wilt (i.e. folded leaf blades, bluish-gray color, foot-prints that last for more than 10-15 minutes, and soil is dry), water the grass (unless rain is expected in the next day). Apply ½ to ¾ inch of water each time the lawn is watered. Overly wet lawns promote sedges, spurges and dollar weed, as well as root rots. When watered and mowed correctly the grass will develop a deep root system and will not require water as often. Water in the early morning when dew is still present. Watering late in the evening promotes disease development.

5. Fertilize correctly. Lawns that have been over-fertilized are much more prone to getting chinch bugs, brown patch, grey leaf spot, pythium blight, powdery mildew and thatch. Under-fertilized lawns are prone to getting take-all root rot, dollar spot and rust. Of course, if the lawn gets attacked by these insects and diseases, large areas die off, leaving a perfect place for weeds to sprout.

Fertilize St. Augustine two weeks after the start of spring regrowth using a complete fertilizer (16-4-8) containing slow-release nitrogen (e.g., Isobutylidene diurea [IBDU], Sulfur-coated urea [SCU], urea formaldehyde, cottonseed meal, or poly-coated sources). Sewage sludge products are also a slow-release nitrogen source, but avoid them if you have palms. Apply 1 pound of nitrogen to 1000 square feet of lawn (divide 100 by the first number on the fertilizer bag to get the pounds of fertilizer to apply per1000 sq. ft.) e.g. 6.25 pounds of 16-4-8 fertilizer per 1000 square feet. Apply ferrous sulfate or a chelated iron source in July. For Bahia grass apply a complete fertilizer with slow-release nitrogen in March and August and an iron source in May.

6. Scout. Watch turf closely for symptoms of disease or insect attack. Keep track of any problems on a calendar and note the location where symptoms first appeared. Knowing when to expect a certain disease or insect pest (i.e., chinch bugs, brown patch, etc.) will help to catch problems early before much damage can be done. Pest problems should be greatly lessened or non-existent when proper maintenance steps are used. Using these steps will lessen the need for pesticides, fungicides and herbicides and help to keep our environment clean.

For more information call the Extension Service’s Horticulture Help Line at 727-582-2110, visit our website at:; or visit the University of Florida Turfgrass website at

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