DEGREE-DAYS FOR PREDICTING SMOOTH CRABGRASS EMERGENCE IN COOL-SEASON TURFGRASSES

Successful smooth crabgrass [Digitaria ischaemum (Schreber) Schreber e x Muhlenb.] management in turfgrass with herbicides depends on accurat e application timing. Little information exists, however, regarding sm ooth crabgrass seed germination, and there are no reliable methods for predicting crabgrass emergence. The objective of this study was to ut ilize degree-day (DD) accumulation as a means of predicting smooth cra bgrass seedling emergence in cool-season turfgrasses. Smooth crabgrass seedlings were counted and removed from field plot grids on a weekly basis from 1 Apr. to 31 Aug. 1992 to 1994 in turf maintained at two mo wing height treatments (i.e., 3.7 or 6.4 cm). Air and soil temperature s at the thatch surface and 2.5 and 5.0 cm below the soil surface were monitored. Soil temperature at the 2.5-cm depth provided the highest correlation with emergence and was used to quantify DD accumulation wi th a base temperature of 12 degrees C. The mean soil temperatures duri ng the 7-d period prior to seedling emergence were 15.6, 13.9, and 17. 5 degrees C in 1992, 1993, and 1993, respectively. Minimum soil temper atures during the same periods of each gear ranged from 10.4 to 12.5 d egrees C. A mean soil temperature >22.8 degrees C was required for the emergence of large numbers of seedlings. Smooth crabgrass first emerg ed between 42 and 78 DD; however, the major emergence period correspon ded to a range of 140 to 230 DD. A cumulative percentage emergence mod el was developed based on DD accumulation, which was accurately descri bed by a Gompertz distribution (r(2) = 0.96).