Chapter 8

Hardening Your Plants

Tomato plants started indoors have been protected from the outside environment and need a transition period to become accustomed to being totally exposed to the elements. Young plants may be injured by sudden exposure to the sun, the wind, and excessive heat or cold. Hardening off is the process of slowly allowing your young tomato plants to adapt to the outdoor sunlight, wind, and lower temperatures.

At least 7 to 10 days before moving your plants permanently out into the garden, they must be hardened off, slowly adapting them to outside. This must be done over a period of a week or more with slowly increased intervals of exposure. Start by putting the plants outside one hour a day, at first on nice days only, and then work up to 5 to 6 hours a day. If the night temperatures are greater than 50˚, you may leave the plants out overnight. Protect them from wind and make certain they receive enough water, as plants tend to dry out more quickly when outside. On the other hand, do not overwater.

Avoid full sun and wind when you first place your plants outside. Start by putting them in full shade for a few days, then gradually move them to partial shade and then full sun. Cold frames can be used to harden off plants. The covers can be opened on good weather days and closed if the weather is inclement. Be certain to open the cold frame on warm days or you will cook your plants.

The hardening off process actually accustoms the seedlings to ultraviolet light. Temperature is not a big factor in hardening off unless it drops below 45˚. More seedlings are damaged by excessive ultraviolet light or windburn during the hardening off process than by low or high temperature. If the outside temperature goes below 45˚, bring the plants inside. When the temperature increases, take them back outside and start the hardening process over again.

If you harden your plants properly, they will be strong and able to withstand full sun, lower temperatures, and breezes when permanently planted outside.

Grower Comments on Plant Hardening

"I use a plastic mini greenhouse that I bought in a garden shop – just a plastic tray with a clear plastic dome. It works fine for me. I operate it manually. I hate that part of gardening. It would be so much easier to just plant directly outside." Minnie Z.

"For the last 10 to 12 days before transplanting to the garden, I put the plants outside every morning behind a wind shield. If the weather is too bad, I will run home and bring them back inside. By the time they are planted in the garden, the plants should be 16 to 18 inches tall and starting to bush out." Ron W.