Hardening off is simply acclimating plants to outdoor conditions. Seedlings grown indoors have been coddled – you’ve been giving them just the right amount of light, moisture and nutrients. Outdoor conditions are more challenging, with fluctuating temperatures and light levels, more variable soil moisture, and wind. About a week before you plan to set the seedlings into the garden, start hardening them off. Place them in a protected spot outdoors (partly shaded, out of the wind) for a few hours, bringing them in at night. Gradually, over the course of a week or 10 days, expose them to more and more sunshine and wind. A cold frame is a great place to harden off plants.