Lawn weed control can be quite a challenge in existing turf. In newly established turf, it can be an even bigger challenge. Weed control begins with taking the proper steps for establishing turf. This includes soil testing, proper seeding methods and good post-seeding care of the seedbed. The most helpful practice you can employ is to establish the turf at a time of year when weed and grass seedling competition is at a minimum. For cool-season grasses, fall is the proper time. Warm-season grasses should be established in late spring to early summer.
Mowing is an important part of weed control. Once the turfgrass has grown to the point where you can mow without injuring the turf (3 to 4 inches tall), do not hesitate to begin mowing. Removing the top growth forces the turf plant to produce basal tillers. This increases the density of the turf eliminating the space where weeds can establish themselves. A common mistake made by many homeowners is to let the turf grow excessively long which can weaken the turf. In addition, some broadleaf annuals cannot tolerate close mowing. Mowing in these situations provides excellent weed control without the use of herbicides.
If the above recommendations fail, pre- and post-emergence herbicides can be used. Annual grassy weeds can be particularly troublesome because they produce abundant seeds. Siduron is a preemergence herbicide that does not reduce the germination of desirable cool-season grass species. Siduron can be applied to the site following seedling or through the use of a combination product containing starter fertilizer and siduron. Pre-emergence herbicide applications are important if you need to establish grass seed in late spring or summer when weed competition is at its highest. Postemergence herbicides are difficult to use in new seedings. Grass seedlings are quite tender and can be severely damaged if the herbicide application is applied too soon. Also, injured seedling turf is not competitive against weeds and the resulting weed population may be heavier than if you did nothing at all. A general rule or thumb for using selective broadleaf herbicides in new seedings is that the application should wait until you have mowed the turf three times. By waiting until after the third mowing, the seedlings will have a chance to mature and become more tolerant of the herbicide.
In most cases, optimum cultural practices are sufficient in providing the weed control that is needed in new seedings. If these methods fail, chemical options are available.
This article originally appeared in the September 15, 1995 issue, p. 135.