It has been a challenging year to harden-off transplants with cold temperatures, rain, then sunshine. The transition from the greenhouse to home gardens involves changes in light, temperature and wind. Vegetable transplants benefit by a gradual "hardening" off period before they are transplanted into the customer’s garden. Gradual exposure to outdoor growing conditions and reduced watering at the end of the growing period with some protection from wind and temperature but full exposure to light can increase the survival rate of planted transplants. Three to six days are adequate to acclimate transplants.
Care must be taken to not "over-harden" young transplants. Cool-season crops exposed to very low temperatures can result in bolting (in cabbage) or buttoning (in broccoli or cauliflower). Warm-season crops generally are hardened at temperatures higher than those of cool-season crops. Cold temperatures can set back warm-season crops and can induce disorders such as catfacing in tomatoes when cold temperatures occur about 3 weeks before bloom.
Tina Smith, UMass Extension and Leanne Pundt, UConn Extension