What does it mean to “harden off” a plant?

Plants that get their start indoors, whether in a basement or greenhouse, need extra care before being planted outside. Because seedlings are weaklings, they need time to toughen up before transplanting in the garden.

Hardening off is a simple process of gradually exposing tender plants to sunlight, wind and uneven temperatures. When plants are almost ready for the garden, take them outside for short amounts of time each day.

Place plants in a protected area, like the north side of a structure or a cold frame for a few hours daily to help them get acclimated. Be sure to protect the tender leaves from direct sunlight and wind for the first few days. On chilly nights, move plants into a shed, garage, or back indoors.

Expose plants to a bit more sunshine each day, starting with morning sun and afternoon shade. Keep soil moist and don’t let it dry out. Hardening off takes a week or more before stems are sturdy and leaves thicken. Then plant in garden beds, raised beds or containers.

To reduce transplant shock, choose an overcast morning for planting.

A simple trick to help plants bounce back quickly is to cover each with a 1-gallon plastic milk jug with the bottom cut off.

Place these mini-greenhouses over plants with the spout end up (minus the cap) for about 3 days.

A stick through the spout into the ground holds jugs in place.