The two most annoying lawn weeds for many people are crabgrass and dandelion. However, close runner-ups would be chickweed, plantain, and nutsedge. You may discover others in your lawn. Any that have been a severe problem in the past, are likely to be a problem again if nothing is done. If any action is to be taken against them this season, it should be done promptly.
For the control of crabgrass or other annual grasses, preemergence herbicides are the most effective. They must be applied just before germination time since they are ineffective after the seedlings have appeared above the soil and developed green leaves. Since weather conditions vary with each season, one of the best ways to time an application is with the flowering of the bright yellow shrub known as forsythia. By the time the bush completes flowering, crabgrass begins germinating. Preemergence herbicides are often included in lawn fertilizers, so a single application will serve to prevent crabgrass as well as fertilize the lawn.
Some of the preemergence materials that are often used, either alone or in combination with other materials, are DCPA (Dacthal), siduron (Tupersan) and Bensulide (Betasan, Weedgrass Preventer or other tradenames). You will find many other possibilities in products. A more recent material that has been introduced for preemergence weed control in lawns is a byproduct of the corn wet-milling industry. This corn product is said to give from 60 to 90 percent control of many lawn weeds when spread over the lawn.
The effectiveness and duration of these preemergence herbicides is somewhat dependent on the weather. Extended rainy periods often reduce their effectiveness by reducing their active periods. In some cases heavy rains can splash the soil enough to also reduce effectiveness. Therefore, a second application may be necessary after extremely wet springs. If not done, a later germination of seeds may result in weeds even though earlier germinating seeds were killed.
Since most chickweed has already germinated and is growing vigorously, another approach to its control is necessary. A preemergence material must be used in the fall to reduce its development. At this time if any control is given, a post emergence material safe for lawn use should be selected. Here again, there are many products available. As the season progresses, chickweed becomes more and more difficult to kill. Post emergence materials with combinations of several herbicides are most effective. Some of the combinations you may find will contain MCPP + 2,4-D + dicamba or others. Although chickweed will be primarily in sunny areas, do not use any product containing dicamba within the dripline of trees or shrubs.
Dandelion and plantain are two of the most persistent lawn pests. They should be treated with herbicides only when they are growing most rapidly. This is normally in spring and in fall in our climate. 2,4-D is the most common herbicide used for their control although there are many others. When using 2,4-D in spring, be very careful of grapes, tomatoes and many other ornamental and vegetable plants that are very sensitive to it. A newer product for control of these pests, as well as clover, black medic and some other problem weeds, is a combination of Triclopyr + clopyralid (Confront).
Another difficult lawn pest is nutgrass. It does not appear as early as others, but in late spring this sedge rapidly produces grass-like tufts that grow faster than the lawn grasses. It requires post emergence treatment, and since not all the nutlets that exist in the soil may grow in a single season, it may take several years of late spring and summer treatment to eliminate it. One of the materials for its control is bentazon (Basagran or Lescogran). Other post emergence materials are also available in combination such as MCPA + MCPP + dichloroprop.
In areas where other plants or grasses are not growing, non-selective herbicides may be used for hard-to-kill weeds. Spot treatments or edging of beds with a material such as glyphosate (Roundup) are becoming more common. It is a method for killing such grasses as tall fescue, quackgrass or common bermudagrass. In some cases there is no easy control for perennial weeds and the only solution is to dig them out by hand.