How to Harden Off Seedlings

Whether you sow seeds indoors to start garden seedlings or you buy young, greenhouse-grown plants, it's important to let your seedlings acclimate to outdoor conditions before planting them in a garden. Called "hardening off," this process minimizes stress to tender plants and is simple yet essential to ensure healthy plants that grow well throughout the season.

Hardening off seedlings is to expose them gradually to outdoor conditions, helping toughen them and prepare them for life in the ground. The best time to begin the process depends on the type of plant and the area's climate. For example, cold-resistant plants such as snapdragons (Antirrhinum majus) can tolerate temperatures as low as 35 to 40 degrees Fahrenheit while zinnias (Zinnia spp.) are more tender and suffer when the temperature drops below 66 F. Both of those plants usually are grown as annuals, but snapdragons can survive winter outdoors in U.S. Department of Agriculture plant hardiness zones 7 through 10. For all plants, hardening off takes about one week. So don't begin it until outdoor temperatures in your area are expected to stay above the minimum temperature tolerated by your seedlings' plant type.

When the weather warms to a temperature your plants can tolerate, let the seedlings spend one to two hours outdoors each day for a few days, and then move them back indoors for the rest of each day. During their time outside, they need a semi-shaded area that is well-protected from wind. They also need to be well-watered when moved outdoors and their soil kept evenly moist for the first few days. Test the soil's surface daily with your fingertip; if it feels dry to the touch, water the soil, ensuring that it drains completely to avoid soggy soil.

Increasing seedlings' exposure to outdoor light and weather by adding about one hour of their outside time per day toughens the plants. As part of the hardening-off process, allow the seedlings to be in full, or direct, sunlight rather than partial shade for a bit longer each day, and gradually reduce watering, but don't let their soil dry out completely or else the plants will wilt. If strong storms are predicted, keep the seedlings indoors for that day, and resume the hardening-off process when bad weather passes.

When your seedlings have been hardened off for at least one week and you're ready to transplant them into the garden, do so on a cloudy day so the plants won't be exposed to full, hot sunlight. Plant each seedling in its own hole, spacing plants to allow them room to grow to their expected widths. For example, snapdragon plants are about 1 foot wide when mature so should be spaced at least 1 foot apart. After filling the remainder of each planting hole with soil, tamp the soil well around each plant's stem to provide it support. If unusually cold weather is expected, the newly planted seedlings need protective coverings such as floating row covers or other light sheeting. Keep their soil evenly moist for the first few weeks, allowing seedlings to develop strong root systems and become well-established in their new spot.