Differences in Thermal Time Requirement for Germination of Three Turfgrass Species

Abstract

Temperature is among the most influential environmental factors for germination and establishment of grass species. We compared germination response to suboptimal temperature for the turfgrass species slender creeping red fescue ( Festuca rubra L. var. littoralis Vasey), perennial ryegrass ( Lolium perenne L.), and Kentucky bluegrass ( Poa pratensis L.). Two cultivars of each species were germinated at five constant temperatures (8, 12, 16, 20, and 24°C), and germination was recorded one to three times per day. Final germination percentage was little affected by temperature, indicating that the base temperature for germination ( T b ) is relatively constant within seed populations. Consequently, germination response to temperature was analyzed by a nonlinear regression method, which combined the thermal time model and the four parameter Weibull function. The analysis provided biologically significant parameters for comparing the cultivars and species. T b only varied slightly between species, from 2.6°C for red fescue and Kentucky bluegrass to 3.6°C for perennial ryegrass. Thermal time to 50% of final germination was 63.9 degree‐days for perennial ryegrass, 43.8 for red fescue, and 115.6 for Kentucky bluegrass, and thermal time from 25 to 75% of final germination was 14.9, 11.1, and 35.0 degree‐days. Thus, Kentucky bluegrass requires a longer thermal time to germinate and has a larger variation in thermal time requirement within a seed lot. Consequently, low soil temperature results in a relatively slower germination of Kentucky bluegrass, possibly resulting in a poorer competitive ability of this species. This suggests that poor establishment of Kentucky bluegrass may partly be due to a larger thermal time required for germination.