The grass sample that "P.L." provided to our office is known as Blanket or Rabbit Crabgrass, Digitaria serotina. This mat-forming annual weed has very short leaf blades about one inch long, and forms a very tight carpet in weak areas of local lawns. The sample also showed a few seed heads hidden tightly in the sample. These seed will serve as a source of reinfestation come next spring.

"E.S." brought in another grass plant called Annual Jewgrass, Microstegium vimineum. This summer annual is a freely branching grassy weed that reproduces by seed in very shady areas. It’s reportedly a native of Asia, and is found throughout the Southeast U.S.

Turf weed management is a matter of first providing good cultural practices for your lawn. The basics can be boiled down to (1) proper irrigation, (2) proper fertilization, and (3) proper mowing. After those conditions are met, if weeds are still a problem, consider if turf is the right plant for that location. If turf is adapted to the intended area, and weeds persist, then is the time to break out some "landscape medicines", which are better know in the industry as pesticides.

First, let’s review basic turf cultural management issues. Established turf should receive ½ to ¾ inches of water at each irrigation event, either from rainfall or your sprinkler system. Then, the system should NOT be turned on again until the grass shows signs of first wilting. Check to see if the sprinklers reach all areas of the lawn with 50% overlap. Ask us for more help in calibrating your irrigation system if it need adjustment.

Poor irrigation creates wet spots that encourage pests that can stress your grass, and encourages those weeds that prefer moist conditions. Blanket Crabgrass likes moist disturbed areas and low wet places. Lots of other common lawn weeds also thrive in wet areas. Moisture management equals weed control.

Second, established turf needs to receive a complete granular fertilizer two or three times a year. A ratio of 3-1-2 is ideal for our area with most of our turfgrasses, which translates into a 16-4-8 analysis. Apply granular fertilizers at the rate of one pound of actual Nitrogen per 1000 square feet of turf. If you do the math, a 30 pound bag will cover 5000 square feet of lawn. Use as much slow release as you can afford. October is a target month for this application to be made in our area. Grass with inadequate nutrients can not outgrow weeds that may appear in your lawn.

Third, mowing of Bahia and St. Augustine turfgrasses needs to be at the proper height. Proper mowing should only take off the top third of the grass leaves. When mowing is completed, the turfgrass species should still measure three to four inches deep. If you are sure that your lawn is a dwarf variety of St. Augustine, or some other kind of specialty turfgrass, the final cut height recommendation may be less than this.

Cutting turf shorter than recommended mowing heights removes excessive leaves needed for the plants to make food. It opens up bare soil that allows weeds seeds to sneak in and take over. And, close shaves result in more shallow roots, which need more frequent watering. With the two weeds mentioned here, normal (high) mowing heights will discourage these weeds, but short mowing opens them up to more sun and actually helps the weeds to grow better.

Another situation that encourages weeds is trying to grow turf where it is not going to get the right conditions for good growth. Turfgrasses needs sunlight to grow; if they don’t get sufficient light, then will not be able to compete with weeds. Certain St. Augustine grass cultivars have the most shade tolerance of any of our adapted lawn species, but they still need several hours of sunlight to do well.

In the case of our second letter, the owner reported that her yard was under the shade of many well established oak trees. Short of severe thinning of the trees or their removal, growing grass under the shade of trees is always going to be an uphill battle.

After all cultural conditions have been met, some herbicides are available to aid turf management. Many effective herbicides can be used to control weeds before they germinate, called preemergents, while others may have use on weeds that have already appeared, the post emergent weed killers. Current University of Florida Pest Management recommendations  for controlling crabgrass list (below) several products than can be used on St. Augustinegrass.

Many of these pesticides come prepackaged with fertilizer. The use of these so-called weed and feed products are difficult to apply as the correct rate for the herbicide may differ from an appropriate rate for the fertilizer. Best results for preemergent herbicides occurs in the early spring. Care must be used to avoid injuring the trees and shrubs that grow near treated turf. Several of these can only be legally applied to turfgrass by licensed pest control companies.

Once the weed appears, post-emergent products must be used to mange weeds. The only postemergent herbicide labeled for St. Augustinegrass that was effective against crabgrass was removed from the market recently. This is a major reason for the epidemic of crabgrass in area lawns.

Another strategy that can be used is to eliminate all of the turf, weeds and grass, by using a non-selective herbicide in a specified problem area. Use of a weed killer such as glyphosate (e.g. Roundup™) will kill everything and leave dead spots in the lawn. Once the plants are dead, they can be removed and a new section of sod can be reinstalled.

In summary, proper culture can eliminate these grassy turf weeds and reduce the need to use weed killers. If you need to use pesticides, be sure to read and follow all label directions. If you need additional information on turfgrass weed management, visit your county Master Gardeners, or call or stop by your county Extension office. For those with other questions about Florida Yards, contact me - my phone number is 863-763-6469 and you can send e-mail to dfculbert@ifas.ufl.edu.

Trade names, where used, are given for the purpose of providing specific information.  They do not constitute an endorsement or guarantee of products named, nor does it imply criticism of products not named. The Florida Cooperative Extension Service - Institute of Food and Agricultural Sciences is an equal opportunity/ affirmative action employer authorized to provide research, educational information, and other services to individuals and institutions that function without regard to race, color, sex, age, handicap, or national origin. Florida Cooperative Extension Service / IFAS /University of Florida. Millie Ferrer, Interim  Dean.  Last update: 07/30/2009.  This page is maintained by Dan Culbert 

Partridge-Telenko, Darcy E.,  Unruh, Bryan  and Brecke, Barry J.   Crabgrass Biology and Management in Turf (ENH-1134).  Gainesville:  UF/IFAS Extension Service, June 2009. http://edis.ifas.ufl.edu/EP395

Unruh, J. Bryan, ed.  2009 UF Pest Control Guide for Turfgrass Managers.    Gainesville: UF/IFAS Extension Service, 2009.