Planting time. Winter squashes are frost-tender, warm-season annuals. Sow squash seeds in the garden–or set out seedlings started indoors–only after the soil has warmed to at least 60°F, usually no sooner than 3 weeks after the last frost in spring. Start squashes indoors as early as 4 weeks before the last average frost date in spring. Sow seed indoors in biodegradable peat or paper pots that can be set directly in the garden so as not to disturb or shock plant roots. Winter squashes grow best in air temperatures ranging from 50° to 90°F; established fruit will ripen in temperatures as high as 100°F but flowers will drop in high temperatures. Winter squashes require 60 to 110 days to reach harvest.