Although cultural controls can be very effective, there are times when good weed control requires chemical applications. The turfgrass type, and the weed and its life cycle, will determine what herbicide is best.
Preemergent herbicides kill germinating weed seeds, and generally have little or no effect on perennial turfgrasses. These products must be applied to the soil before weed seeds germinate, so timing is critical. Crabgrass begins to germinate when soils are moist and the temperature in the upper inch of soil reaches 55° to 58° F. Goosegrass germinates later than crabgrass so preemergent herbicide applications to control goosegrass should take place three to four weeks after the normal dates for applying crabgrass control materials.
Soil-applied herbicides can be applied as either liquids or granulars. They should be watered into the soil during or following application. Examples of preemergent herbicides are Pendimethalin, Oxadiazon, Prodiamine.
Postemergent herbicides are used for controlling weeds that have already emerged from the soil. They are either contact or systemic in nature. Postemergent-contact herbicides, like Diquat or Paraquat, affect only those plant parts that they contact, and are not translocated to other portions of the plant. Postemergent-systemic herbicides, such as Glyphosate or 2,4-D, are translocated throughout the plant, and are effective in controlling perennial weeds that can generate new foliage from underground vegetative structures. Postemergent herbicides are most effectively applied when weeds are young and growing vigorously.
Selective postemergent herbicides, like 2,4-D or Metsulfuron-methyl, are typically used to control annual, biennial and perennial broad-leafed weeds because they will kill many broadleaf plants without damaging grass plants. These herbicides can cause injury to trees, shrubs and flowers; thus they should be used with great care near these plants.
Nonselective postemergent herbicides, including Glyphosate and Diquat, kill all plants, both desirable and undesirable. These herbicides can be used to spot treat perennial grassy weeds that are not affected by selective herbicides.
Always read the entire label before applying any pesticide, as it provides detailed information on safety and proper use of the herbicide.
Understanding the characteristics of each of these products, and knowing how to fit them into a balanced management system, is an important part of developing an effective weed control program.