First, let’s talk about what hardening off is. It’s when you allow your plants that have been grown indoors or under some protection to slowly become accustomed to the outdoors.
This is a necessary process because when plants are grown in an artificial environment, their cells become enlarged, their tissue is full of water, and their surface becomes thin.
Placing one of these plants out into the elements, they are prone to drying out, cold or wind damage and sun scald.
When plants are gradually introduced to the outdoors, we allow them to create new cells that are smaller, thicker walled, and more able to handle harsher conditions.
The easiest way to harden off your plants is to place them in a cold frame ( an unheated enclosure, often made of wood, with a removable glass cover) on a mild day(around 45ºF for hardy and half hardy plants).
You can make a quick, temporary cold frame by taking a storm window and placing it over some hay bales that form the walls. The glazing helps to trap in the days heat to keep the plants warm when the temperatures drop at night.
During the day, raise or remove the glass, depending on the weather. Do this for about 2 weeks at which time your plants should have had enough exposure to become accustomed to the elements.
If you don’t have the space or material to make a cold frame, take your plants outside on any mild spring day, and allow them to become exposed at larger and larger intervals. Start with 2 hours and slowing increase the time they spend outside each day. Do this until they are able to outside for an entire day, spread it out so in 2 weeks they will be ready to be planted outdoors.
Always bring seedlings indoors if you are expecting cold temperatures, hard rain or strong wind. Never expose them to hot direct sun when they are starting the hardening off process.