Growing Tomatoes: Hardening Off Tomato Plants

Hardening offtomato plants, or gradually exposing seedlings to the outside environment, strengthens stalks and prepares tomato plants for the transition to the garden. Without hardening off, young seedlings may snap in the wind and foliage may scald and sunburn damaging the plants. Exposing tender stems and foliage gradually to the effects of the wind builds strong stems that stand up to the weather when planted in the garden.

Choose a Sheltered Location to Harden Off Plants

Choose a sheltered location for hardening off your tomatoes. Look for an area that receives filtered or morning light and provides a windbreak. The object is to expose young plants to the outside without causing stress or damaging foliage. Areas near the foundation of the house (out of direct sunlight) or along a fence make the ideal place for introducing tomato seedlings to the outside.

Be Prepared to Bring Tomato Plants Inside at Night

Bringing your tomato plants back inside at night during the hardening off process protects them from chilly nights. Leaving them outside poses the risk of losing the plants. Although it may seem like a bit of a chore, moving plants inside is a necessary part of the hardening off process and will reward you with lush green plants with abundant fruit in late summer.

Hardening Off Tomato Plants: Day 1 through 3

Place tomato seedlings in a sheltered location when weather is warm and mild. Two to three hours of exposure for the first day is sufficient. Place the seedlings in the same location for 2 to 3 days. Watch tomato plants carefully and provide additional water as outside conditions can dry the soil quickly. Tomato seedlings typically require daily watering.

Hardening Off Tomato Plants: Days 4 through 7

On the fourth day, move the seedlings a foot or two away from the building or fence, ideally into an area that receives some direct sun. This allows seedlings to experience natural winds, but still provides some protection. Gradually move the plants further from the fence or building over the next 3 days. Check plants often for any signs of damage or stress. Move stressed or wilted plants inside for the rest of the day and set them out again the next morning.

Hardening Off Tomato Plants: Day 8 through 10

Once plants have acclimated to the outside, begin moving them to direct sunlight. An hour or two of direct morning sun for the first day is adequate. Gradually increase the amount of sunlight until plants tolerate several hours of direct afternoon sun without signs of wilting.

Hardening Off Tomato Plants: Days 11 through 14

Set the plants out in direct sunlight daily for 3 or 4 days. Continue to bring plants inside at night and to provide supplemental watering when necessary.

Many gardeners neglect to properly harden off tomato plants or in are in a hurry and do not allow plants enough time to sufficiently acclimate to outside conditions. Although plants may survive in the garden after a day or two of hardening off, the stress associated with drastic changes in the growing environment may set a tomato plant back for a month or more. Taking the time to harden off your tomato plants gets them off to a good start and produces healthy robust plants.

For more information on growing the best tomatoes see my other articles in this series.

Growing Tomatoes: Seed Starting
Growing Tomatoes: Caring for Tomato Seedlings
Growing Tomatoes: Preparing the Soil

Sources:
University of New Hampshire Extension: Growing Tomatoes
Clemson University Extension: Tomato