Hardening Off Demystified

  • Difficulty Rating: Beginner
Hardening Off Demystified

Ever bought a packet of veggie or annual flower starts at an early spring plant sale, taken them home, planted them and found them dead the next morning?

Or, maybe you embarked on starting some crops from seeds indoors only to plant them into your outdoor garden where they croak overnight. If this has happened, the odds are, your plants hadn’t been hardened off before you planted them.

Hardening off refers to a simple process that prepares tender, indoor-grown plants for a harsher life in the outdoor garden. How long it takes, when it is done and how much effort goes into the process can vary from zone to zone. Still, understanding the basics will get every gardener growing in the right direction. So here goes:

Begin by understanding the temperature ranges your flowers and edibles can handle. Cool season crops like kale can grow in temperatures around 40F, and once they are adjusted to their environment, they can even withstand a freeze. Tender summer flowers and edibles like tomatoes may die or become stunted if exposed to temperatures below 60F. Check plant labels or seed packets for your plant’s temperature range.


Once outdoor temperatures are conducive to supporting your young plants and your young plants have grown at least a set or two of true leaves, they’re ready to begin venturing outdoors to acclimate. To do this, watch the weather forecast for a period, likely after your last average frost date, when temperatures should drop no lower than your plant can tolerate. During these days, gather up your precious starts and place them outdoors in a protected location with filtered sunlight. You may do this for just a few hours the first day, a few more the second and so forth. If nighttime temperatures threaten to drop below your plants’ tolerance, bring the plants inside overnight and return them to their outdoor location during the day. If you live in an area where day and night temperatures stay well within your plant’s cold tolerance zone, you may be able to set them out in a protected spot for a few days and then plant – no moving things back and forth as is required in colder climates. How much moving and how long it takes for your plants to harden off can vary widely from year-to-year, plant-to-plant and location-to location. Watch the weather and always err on the side of caution if you’re concerned temperatures may be too harsh for your plants.

While acclimating your young plants to cooler temperatures, be sure to keep them watered and resist the urge to fertilize. Keeping roots moist will help them avoid drying out or freezing. Fertilizing while hardening off may encourage the plants to put energy into additional delicate new growth at a time when your goal is to simply toughen up existing growth ahead of planting in the garden, so hold off on fertilizing for these few days. There’s plenty of time for that after you get them into the ground.

A great tool to help gardeners get plants ready for life outdoors is the cold frame. A cold frame is a smallish, usually unheated, box with windows. Trays of plants can be placed inside of a cold frame, which captures sunlight and creates a warmer, wind-free environment for the enclosed plants. Take care to open your cold frame, releasing heat build up, so plants don’t fry. And, close it to trap some warmth on dark, cold days and chilly nights. And, same rules apply: if the night temperatures in the cold frame will dip below your plant’s tolerance zone, bring them back inside so they don’t freeze.


After your plants have spent a few days getting acclimated in your chosen protected location, and once day and night temperatures are consistently within the plant’s tolerance range, they’re ready for planting in the garden. At the time of planting, adding a bit of fertilizer may help the plant make that transition to growing in the garden soil. And, of course, water each plant well right after putting it in the ground. Consider building additional growth-stimulating warmth and protecting them from surprise freezes, heavy rains or hail, and pest insects by adding a layer of horticultural fleece over your new plants or installing a temporary hoop house.

If you’re buying starts from a plant sale or nursery, don’t assume early spring plants are ready to go into the garden directly from the sale table. Before you buy, ask if your selections have been hardened off. If they haven’t, either complete the hardening off process at home or wait a few more weeks to shop. Eventually the nurseries will harden everything off for you.