Two cultivars of onions, `New Mexico Yellow Grano' and `Midstar' were seeded in single bed plots in mid-October 1985 and 1987 for overwinter transplant production. Plots were covered with spunbonded polyester (POL) or tunnels of clear polyethylene (CLR) or microperforated polyethylene (PER) (1985 only) in early November and compared to uncovered controls. Temperatures were monitored 5 cm above the soil surface under the covers in each plot with three parallel-wired thermocouples. Heat unit (HU) accumulation (number of degrees by which the daily mean temp exceeded 0°C) was recorded for each plot and compared with onion plant size. HU accumulation by mid-February 1986 under CLR, POL and PER was 139%, 131% and 113%, respectively, of that over bare ground. In mid-March 1988 cumulative HU under CLR and POL were 192% and 125% of those over bare ground. Plant diameter varied with variety but increased linearly with cumulative HU for all varieties. `New Mexico Yellow Grano' reached the minimum 4 mm size for transplanting at about 1800 HU while `Midstar' required only 1500 HU. CLR produced useable transplants by early March and the other covers by late March. Numbers of useable transplants per meter of bed in mid-March ranged from 3-6 in uncovered plots to 102-153 under PER tunnels, 185-203 under POL and 263-301 under CLR tunnels. CLR tunnels appear to provide sufficient HU accumulation to produce onion plants for transplanting in early March in West Texas.