How to Harden Off Seedlings Without Losing Them to Shock

What hardening off does to a seedling

To harden off seedlings, move them outdoors for a little longer each day over 1 to 2 weeks before transplanting. Start with a few hours in a sheltered, shady spot during the warmest part of the day. Add time and sunlight daily, keep them in when it is windy or below 45°F, and finish with a day or two outside around the clock. University of Maryland Extension, University of Illinois Extension and South Dakota State University Extension all describe that same pattern.

The reason it works is that the plant rebuilds itself in response to the exposure. Illinois lists the changes: hardening off reduces the growth rate and thickens the cuticle and waxy layers of the leaves. It also stimulates root development, increases the carbohydrates stored in the tissues, reduces the amount of freeze-prone water in the cells and develops lignin in the cell walls. South Dakota State sums up the same process as a thickening of the leaf’s protective surface and an accumulation of carbohydrates.

A seedling raised indoors has none of that yet. Shock is what happens when it meets full sun, wind and cool nights before the changes have had time to occur, and the 1 to 2 week schedule is how long they take.

When to start

Count back from the day you plan to transplant. Maryland says to begin 1 to 2 weeks before setting plants out in the garden. Illinois and Utah State University Extension both say about two weeks, and South Dakota State gives a range of seven to 14 days. Utah State adds that hardening can begin soon after seedlings emerge and should start two weeks ahead at a minimum.

The transplant date itself depends on the crop. Illinois reminds gardeners that warm-season crops should not be transplanted outside until after the last frost for the area. Its examples are tomatoes, peppers, zucchini and cucumber. Our guide to when to start seeds indoors shows how to work back from that frost date. Cool-season crops go out earlier, so their hardening off starts earlier in spring, when cold days are more likely to interrupt it.

A hardening off schedule

The sources agree on the stages and differ slightly on the first day, so the steps below name which is which.

1. Pick the spot. South Dakota State describes the ideal location as a dappled, shady area out of the wind that is protected in the event of a downpour of rain. Utah State also starts seedlings in the shade.
2. On the first day, put the trays out during the warmest part of the day. Illinois suggests about 2 to 3 hours, typically between 12 and 5 PM. South Dakota State suggests three or four hours when temperatures are warmest. Either way the first outing lasts 2 to 4 hours, and then the trays come back in.
3. Each day after that, leave them out longer. South Dakota State adds a few more hours each time. Maryland says to increase the amount of sunlight the transplants receive each day, and Utah State describes gradually increasing time in the sun until the plants tolerate full sun all day.
4. Work up to 10 to 12 hours a day outside and hold there for a few days, as Illinois advises.
5. Finish with full days and nights outdoors. Illinois leaves plants out for 24 hours for a couple of days. South Dakota State does the same for the last one to two days of the process.
6. Transplant.

If you prefer numbered days, our companion guide to hardening off seedlings arranges the same stages across a 10-day plan. Do not rush the middle of the list to make up for a late start. If the weather forces a day indoors, pick up where you left off. Between outings, keep the trays in light. South Dakota State cautions against storing transplants in unlit garages for prolonged periods.

A cold frame can take the place of carrying trays in and out. Maryland calls it an excellent environment for hardening off transplants, and our guide to choosing a cold frame covers the types.

Temperatures that are too cold

Cold and wind are the two conditions that call off a day outside. Maryland’s rule is short: do not put tender seedlings outdoors on windy days or when temperatures are below 45°F. South Dakota State says to bring plants in if daytime temperatures are projected to be below 45°F, or if there is a threat of strong winds or rains.

Warm-season crops need more margin than that. Utah State warns against exposing tomatoes, peppers, squash and melons to temperatures below 45°F to 50°F. For the overnight stage, Maryland says seedlings may be left outside if the temperature will not fall below 50°F.

Freezing, 32°F, is a hazard for every seedling at this stage. Maryland notes that even cold-hardy plants will be hurt if exposed to freezing temperatures before they are hardened. Bring all the trays in on a frost night, whatever stage they have reached.

Cool-season crops have a limit of their own. Utah State reports that excessive exposure of hardy and semi-hardy seedlings to temperatures below 40°F can result in premature flowering. A cabbage or broccoli seedling can survive that cold and still be spoiled by it.

Hardening does buy real tolerance once it is complete. According to Maryland, properly hardened tomato plants can usually tolerate light and unexpected frosts.

Water and fertilizer during the hardening weeks

Water less often, and stop feeding. Maryland says to reduce the frequency of watering to slow plant growth, without letting the plants wilt. Illinois and South Dakota State give the same instruction in nearly the same words. Slower growth is the purpose of the whole exercise, as Maryland puts it, so slightly drier mix helps.

Wilting is the limit. Small cells dry much faster outdoors in sun and moving air than they do on an indoor shelf, so check the trays when you carry them out and again when you bring them in. Water when they need it. The goal is fewer waterings than indoors, with no dry spells.

On fertilizer, Illinois does not recommend feeding plants before or during the hardening off period. Fertilizer pushes the soft growth that hardening is meant to slow.

Do not overdo it

Hardening can go too far. Maryland gives two examples of what happens when plants are hardened too severely. Cauliflower will produce thumb-sized heads and fail to develop further, and cucumbers and melons will stop growing.

In practice that means holding to the limits above and not testing the plants beyond them. Keep the mix from drying to the point of wilting, bring trays in below 45°F, and do not stretch the process far past two weeks while waiting for a perfect planting day.

Transplant day and the first week

Choose the gentlest conditions available. Utah State’s advice is to transplant in the morning or on a cloudy day if possible, so the plants are not set into strong sun at the moment their roots are disturbed.

Set each plant at the depth it grew in its cell, with one exception. Tomatoes can develop roots along the stem, Utah State notes, and may be planted deeper if you wish.

After planting, water is the main job. Utah State says to keep the root ball moist, which may require daily irrigation, because new transplants need water more frequently until they are established. The soil should be kept damp but not perpetually soggy.

Hardening off is the last step of a process that starts on the seed shelf. For next season’s sowing, the indoor setup of light height, hours and germination temperatures is covered in the guide to grow lights and heat mats for seed starting.