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42 March 2015 Landscape and Irrigation www.landscapeirrigation.com Landscape and Turf MainTenance Summer annual weeds such as crabgrass and goosegrass com- monly invade turf. Crabgrass and goosegrass complete their life cycle in one year, germinating from seed in spring, growing throughout summer, and setting seed in fall. Summer annual weeds invading athletic fields need to be controlled in order to maximize turf quality. An effective means for controlling summer annual weeds is the use of preemergence (PRE) herbicides in spring. A list of preemergence herbicides labeled for use on warm- and cool- season turfgrasses is presented in Table 1. Weed-control programs centered on the use of PRE herbicides offer many benefits to turf managers compared to eradicating these weeds with postemergence (POST) herbicides after they become established. For example: * Turf managers have more herbicide options to control sum- mer annual weeds PRE than POST. * PRE programs are often more economical than POST pro- grams that can require numerous sequential applications. * Several PRE herbicides are available on fertilizer carriers allow- ing for granular applications to be made instead of liquid sprays. ThIngs To remember when usIng Pres 1) Application timing: Be sure to apply PRE herbicides before weeds have emerged from soil (i.e., before they are visible). These herbicides do not prevent weed seed germination; rather they pre- vent germinated seedlings from developing into mature plants. Considering that the timeframe between weed seed germination and weed emergence can be quite short, it is often recommended that PRE herbicides be applied once soil temperatures are favor- able for crabgrass seed germination. Turf managers should make their first PRE herbicide application as soon as soil temperatures (at approximately 2 inches) measure ≥ 55°F for a minimum of 3 days in spring. Researchers studied how the blooming of 74 different or- namental plants in spring corresponded with the emergence of crabgrass in turf. They concluded that blooming of border for- sythias is a helpful indicator of when to apply PRE herbicides for crabgrass control. Border forsythias produces distinctive yel- low blooms at soil temperatures similar to those that facilitate crabgrass seed germination and emergence. Thus, turf managers should be sure to apply PRE herbicides before forsythia plants have completed flowering each spring. 2) Irrigation:A key to effectively controlling weeds with PRE herbicides is to water them into the soil after application. Most labels require that 0.25 to 0.50 inches of irrigation or rainfall be applied within 24 to 48 hours after application. These herbicides are absorbed by germinating weed seedlings in the soil, so mov- ing them into the rootzone is critical. Failure to irrigate after ap- plication can also lead to material being lost due to volatilization. On turf without irrigation, try to time PRE herbicide applica- tions around a period of rainfall. 3) Split applications: Split (also referred to as "sequential") ap- plication programs of PRE herbicides tend to provide more con- sistent control of summer annual weeds throughout a growing season, particularly in southern climates. These programs typically apply the total amount of active ingredient for the season in two equal rate applications spaced 8 to 10 weeks apart. A single herbi- cide application in spring for PRE control of crabgrass will slowly be broken down by soil microbial activity over the course of a sum- ■ by J.T. Brosnan, G.K. Breeden, J.c. sorochan, and a.W. ThoMs Make Use of Preemergence Herbicides