Simplified equipment for low-humidity treatment (LHT) was developed by installing a light source and heaters in a small refrigerator. Then, LHT was applied to rootstock squash (Cucurbita moschata Duch.) seedlings for 24 h before they were cut and cucumber scions grafted onto them. Average air temperature, relative humidity, and vapor pressure deficit in the LHT equipment were 30°C, 15%, and 3.6 kPa, respectively. Dry weight and percent dry matter of the rootstock shoot at end of LHT were significantly higher than those of seedlings grown in a greenhouse at a temperature of 15/29°C (night/day) and vapor pressure deficit of 0/2.2 kPa (night/day) instead. Leaf conductance of the seedlings that received LHT was 0.39 times that of those not receiving LHT. Grafted cuttings were obtained by grafting cucumber scions onto the squash rootstock receiving or not receiving LHT. The grafted cuttings were planted in growth medium and then grown for 7 days in a plastic-film tunnel. Total fresh weight, root fresh weight, and root number of the plants grafted onto the rootstock that had received LHT were 1.14, 1.36, and 1.44 times those of plants grafted onto rootstock not receiving LHT. LHT apparently enhanced the growth after planting by causing a decrease in the transpiration rate of the rootstock, as reported previously. A similar experiment, conducted under artificial lighting, showed similar results for percent dry matter, leaf conductance, and root fresh weight after planting.