When most people talk about weeds in their lawn, they are usually talking about broadleaf weeds. These can be perennials, such as dandelions and thistles, or annuals such as oxalis and spotted spurge. Broadleaf weeds can be treated throughout the growing season but spring is the time of year when they will be most prevalent.
What's the difference between broadleaf and grassy weeds?
Efforts to control crabgrass and other annual grassy weeds focus on preventing their growth in the spring and early summer with the use of pre-emergent herbicides. The effectiveness of pre-emergent herbicides has much to do with weather conditions.
Lawns with excessive annual grassy weed populations may take several seasons to be brought under control. Post-emergent sprays for crabgrass are available. Crabgrass, like most weeds, are "opportunistic plants." They will germinate and grow wherever there is a spot of bare ground, as is found in thin undermaintained turf. Much progress can be made on a crabgrass problem by providing adequate fertilization and irrigation, or in some cases overseeding the lawn with desirable turfgrasses, thereby allowing the desirable grasses to outcompete the weeds.
Perennial Grassy Weeds
There are some undesirable grasses that appear in a lawn that cannot be controlled. Quackgrass and dallis grass are examples of unwanted perennial grasses. There are no products available to us that will control perennial grassy weeds such as these without harming the desirable turfgrasses.
You can limit the spread of undesirable grasses by doing all you can to nurture your desirable grass with proper watering, fertilization and mowing practices.
Products are now available to control SEDGES in turfgrass. Sedges, which are neither true grasses, nor true broadleaf plants, can be identified by their lime-green color, their waxy, triangular leaves and their tendency to stick out above the turf.