When I harden my tomato plants, I wait until the sprouts have reached 7” in height, nighttime temperatures stay about 55° F and soil temperatures are 65° F. Once these three factors are in place, I work through the following steps: • Daily place the sprouts in an outdoor spot that is shady but wind-blocked  • Bring the sprouts inside before the sun goes down • Move the sprouts to a different location if they appear injured by heat, sunlight, wind, or other outdoor conditions • Continue this process for up to two weeks • Transplant the sprout if it is in a healthy state after working through these steps for two weeks