Degree-days for predicting smooth crabgrass emergence in cool-season turfgrasses

Successful smooth crabgrass [Digitaria ischaemum (Schreber) Schreber ex Muhlenb.] management in turfgrass with herbicides depends on accurate application timing. Little information exists, however, regarding smooth crabgrass seed germination, and there are no reliable methods for predicting crabgrass emergence. The objective of this study was to utilize degree-day (DD) accumulation as a means of predicting smooth crabgrass 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 mowing height treatments (i.e., 3.7 or 6.4 cm). Air and soil temperatures 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 with a base temperature of 12°C. The mean soil temperatures during the 7-d period prior to seedling emergence were 15.6, 13.9, and 17.5°C in 1992, 1993, and 1994, respectively. Minimum soil temperatures during the same periods of each year ranged from 10.4 to 12.5°C. A mean soil temperature >22.8°C was required for the emergence of large numbers of seedlings. Smooth crabgrass first emerged between 42 and 78 DD; however, the major emergence period corresponded to a range of 140 to 230 DD. A cumulative percentage emergence model was developed based on DD accumulation, which was accurately described by a Gompertz distribution (r² = 0.96).