Technical Abstract: When overseeding cool-season forages into warm-season pasture, farmers are advised to sow into short residue in order to facilitate establishment. However, practical constraints may result in overseeding being made on untrimmed warm-season grass stubble. Whether this increased cover changes the temperature environment around the seedling sufficiently to influence its rate of development is not clear. Experiments were undertaken during the cool season (November-April) of 2004-05 to measure the effects of warm-season pasture residue on soil temperatures and phyllochron in Italian ryegrass. Soil temperatures at the surface or at 2 cm below surface were measured under warm-season grass residues of 6.5 or 20 cm height (equivalent to above-ground biomass of 1040 or 2520 kg.ha-1 respectively). Cumulative leaf appearance on marked plants was also measured with no surrounding residue (NR), 6.5 cm residue (R), or 6.5 cm residue covered with black landscape-cloth (RLC). Mean soil surface temperature was greatest without residue and decreased by 0.7 ºC under 1040 kg.ha-1 residue and by a further 2.0 ºC as residue increased to 2520 kg.ha-1. Mean daily maximum temperature and daily temperature range were also decreased, but mean minimum temperature was increased with increased residue amount. Mean phyllochron was 183, 168 and 141GDD leaf-1 for NR, R and RLC treatments, respectively. Surface residue can alter soil temperature sufficiently to affect phyllochron in Italian ryegrass.