Hardening off is the process of taking tender young seedlings and making them rough and tough to survive outside, without losing them to stress and shock. Harsh afternoon sun, pelting rains, wind, dry spells, and temperature swings can all cause stress on any plant, and for a seedling, can be enough to stunt growth or even kill it. Just like we might toughen ourselves up by training through a progression to run a marathon, we can progressively expose indoor-grown seedlings to their new physical environment as well.

When we harden off seedlings, they go through physical changes. Plant growth slows for a bit; meanwhile, more lignin gets added to the plant cell walls (making them stronger), waxy layers develop on the leaves (limiting water loss), more carbohydrates are stored in the plant tissues (more resiliency), and root growth is stimulated (better able to find water and nutrients). 

When to begin the hardening off process

For gardeners in areas with freezing winter temperatures, most of our hardening off is centered around our last spring frost date. Cold-hardy plants like snapdragons or brassicas can be planted out a couple of weeks before the last frost date, and warm-loving plants like peppers and tomatoes need to wait until a couple of weeks after the latest spring frost. Consequently, you may find yourself in a continuous process of hardening off plants for several weeks. 

Ten days before transplant day is normally enough time to harden off seedlings, but two weeks is a bit better. It isn’t an exact number, so don’t worry if you’re a day either way. 

When it is time, set the young plants outside in an area with dappled light and out of the wind (and safe from rabbits). Start with a couple of hours and add time each day, bringing them in overnight. Once they’ve been outdoors for 10 hours or more per day, leave them overnight. They’re now ready to transplant.

Environmental Factors and Hardening Off

A seed planted outside and germinating in your flower bed is exposed to these conditions from the start, but our indoor-grown seedlings have led a sheltered life and need to be gradually exposed to these factors.

Temperature

Most of us keep our homes at a relatively constant temperature, somewhere around 70℉ give or take. But outdoors, temperature swings are a daily occurrence. Depending on your location, it may be common to see a 30℉ (or more) difference between the overnight low and the high for the day. In spring, when we are thinking of moving our seedlings outside, that swing can be even more severe, from just above freezing to warm enough for shorts and a T-shirt by midafternoon.

When I begin hardening off seedlings, I start at the warmer part of the day, which is often still a bit cooler than my house. As the hardening off process progresses, I’ll leave the plants outside for larger temperature variations, but watch to avoid any extremes.

Light

We start seedlings indoors under artificial lighting unless we’re lucky enough to have a greenhouse, and even though some light fixtures can mimic the spectrum of daylight, they don’t mimic the entire light environment. The intensity of sunlight, and its broad spectrum, is exactly what most plants need, but just like we can get a sunburn more easily in spring (when we are not tanned), so can a plant.

Begin by exposing the seedlings to small doses of sunlight, say a couple of hours in the morning on day one, and increasing that time by an hour or two per day until the plants are sitting outside from morning to evening. Don’t bring new seedlings out, plop them on the picnic table, and forget about them. While it may not kill them (or it might), it will cause a large stress and can slow development and weaken the seedlings, making them ripe for other issues like pests or disease.

Wind

Homes aren’t windy on the inside, but gardens are another matter, and wind can always affect plants. From twisted leaves to bent stems, it’s a factor that plants need to be acclimatized to. Some gardeners will place a fan where it can gently blow on their seedlings as they grow indoors, hoping to promote stronger stems and leaves. 

When hardening off, exposure to wind is part of the process, but only in small doses. If there’s a storm coming, or a windy day, place them in a safe location or even leave them inside for a day. Choose a more sheltered location to harden off tender seedlings, like against the house, or create a temporary windblock with an old cardboard box or two. 

Soil Moisture

It’s pretty common for our seedlings to be kept at a fairly even and constant soil moisture level. Whether we grew them in our home in the spare bedroom or purchased them from the greenhouse, they’ve been rather well-watered. The transition to a garden bed and the less frequent rains or watering can be a significant stressor on a young plant. 

When starting to harden off seedlings, begin cutting back on the moisture as well. Let them dry down a bit, but not to the point of wilting, and then provide a deep drink. Seedlings in cell packs can dry quickly, so this may be difficult to achieve on bright, warm, sunny days, but try to reduce watering frequency as you are able.

Do all plants need to be hardened off?

The quick answer is yes. All seedlings that were grown indoors should be hardened off before planting permanently outdoors. The amount of time they’ll need to acclimate properly depends on their indoor growing conditions. Seedlings raised in a greenhouse with only natural sunlight and ventilation have been exposed to many of the conditions they’ll experience in the garden. A tomato grown in the corner of the living room under a grow light will need much more tender care to complete the process. 

Even plants purchased and brought home may need hardening off, again depending on the conditions they were exposed to at the garden center or nursery. Plants displayed outdoors can be pretty safely planted straight away, but those which were under a greenhouse roof, temporary tent, or indoors should still be hardened off appropriately.

Extra care after transplanting seedlings

The gardener’s task isn’t done after the seedlings have been transplanted into their new homes. For a few weeks, they will still be at higher risk of damage and need some extra care. Check on them daily and watch the weather forecast. An unusual threat of late frost can often be dealt with by using frost cloth. 

Light mulching around the new plants will help keep soil moisture more even, reducing swings and how often you need to water. Since young plants have small root systems, they often need watering several times a week until they establish. The mulch will also help to keep weed pressure down. 

Test the soil moisture near the plants by poking down into the soil with your finger. If the soil is dry, it’s time to water. Occasionally, the small plug of soil you transplanted with the seedling in it will dry out even though the surrounding soil is still adequately moist. Until the roots expand outward into new areas, the young plant may use up the water adjacent to it. If the seedlings look droopy even though nearby soil moisture is okay, try a little water right at the base of the plant. In a week or two, the roots will spread out and be more resilient.