Flying the Nest: How to Harden-off Seedlings

You’ve pampered them and cared for them every moment since birth. You’ve clucked and fretted over their every need. If they were thirsty, hungry, too cold or too warm, you were there to water, repot, turn up the heat or provide shade. And now they’ve grown into strong, sturdy little plants lush with health and vigor, and are more than ready to tackle the world—a credit to your care.

Now, if they were your kids, would you just sling them out the front door with a cheery wave and tell them to get on with it? I didn’t think so. The transition to being a self-sufficient grown-up is a crucial stage in a human’s development and it’s the same for baby plants.

Plants suffer from shock, just like humans do. Take them out of the warm, cozy environment of a greenhouse and plonk them into chilly soil on a blustery spring day and for them it feels like they are jumping into a freezing-cold swimming pool. Transplanted baby plants spend the first week or two in their new home metaphorically gasping in shock and shivering uncontrollably until they get used to their new surroundings.

Plants that aren’t properly prepared for this transition never do quite as well as those that are, so it’s important to build in a transition period between the seedling stage in a protected environment, and the rough-and-tumble environmental conditions in your outdoor garden. This is known as hardening off, and it should be done gradually so your seedling has time to get used to its new environment. It’s like dipping a toe in, and then a foot, as you gingerly ease yourself into that cold pool.

In a greenhouse or indoor garden, everything is perfectly regulated. Plants receive just the right amount of water, nutrients, humidity, constant temperatures and barely a breeze. Your seedlings get the best of everything. Outdoors is a harsher world. Spring temperatures can yo-yo unexpectedly. Humidity is lower, and when you factor in a brisk wind, moisture can be stripped from a defenseless leaf in minutes. It’s no wonder seedlings need to toughen up before they can face it all.

What’s Going on Beneath the Surface of a Seedling

For greenhouse-grown seedlings, there’s no point in fully developing the waxy cuticle that protects the upper and lower leaf surfaces from drying out in excessive sunshine or wind. These seedlings are perfectly sheltered by glass, and you’re there every day with your watering can. They also never have to deal with the constant mechanical movement—mainly wind—that assails plants outdoors, so they never have to move. This means a baby plant can afford to produce longer, thinner cells, growing faster but with more delicate leaves and longer stems.

There’s less light in a greenhouse, too. Sunlight loses most of its UV spectrum and some of its power as it passes through glass, especially if the windows are a little dirty. So seedlings under cover tend to develop larger leaves than they would outdoors, to make the most of available light for growing. They find this easy because their leaf cells are relatively thin and easily stretched already.

When you plant a youngster outside, you’re changing its environment dramatically, and effectively asking it to become a different plant. It must learn to protect itself from direct sunshine and wind by thickening that waxy leaf cuticle to prevent water loss.

Near-constant buffering from the weather makes a plant grow differently, too, producing cells that are shorter and stockier, with thicker walls that aren’t so easily broken or damaged. Growth slows as the plant is making cells more densely than before, but the resulting leaves and stems will be far stronger, able to soldier on regardless of tough setbacks like droughts, chilly nights or pest damage.

All these adaptations take time. Just as you have to go to the gym for months to build up strong muscles, a plant must gradually build up its strength, too.

Toughening Up Your Seedlings

Luckily, it doesn’t take quite as long as your fitness program to turn a seedling from a pampered, cosseted greenhouse wuss into a seasoned outdoor pro with stems of steel. About two weeks should do it, depending on how warm your greenhouse was, the type of plant you are growing and the current weather conditions.

Tomatoes moving from a fully-heated greenhouse to a frost-free late spring garden are making much more dramatic adaptations than, say, hardy beet seedlings moving outdoors in balmy June sunshine, so they need a full three weeks. The beets go through the same process in about 10 days or so. This is the time frame you allow from the moment you start to physically move your plants out of the greenhouse until you transplant them. But the hardening off process can actually start much earlier.

Train your babies to grow like outdoor plants from the start by simulating a breeze to help them grow thicker, stronger cells designed to withstand rougher conditions. Don’t get too enthusiastic, though, as seedlings are still delicate. Waft a stiff piece of cardboard gently over the top of your seedling trays once or twice a day or turn on a fan to generate a gentle breeze.

As they get bigger, start brushing them gently whenever you pass by with your hands or a feather duster. Your neighbors will think you’ve gone crazy, but once you explain why you’re dusting your seedlings, they’ll understand.

A couple of weeks before you begin the hardening off process, reduce watering and let your seedlings dry out a little. Again, don’t overdo it. Just a little less water than usual will encourage plants to toughen up those all-important cuticles that hold water in the leaves as long as possible.

Finally, if you’ve been raising your plants on a windowsill indoors or in a greenhouse heated at 50°F or more above outdoor air temperatures, move them into a cooler environment about a week before hardening off. This can be an unheated greenhouse, an enclosed porch or even the spare room—anywhere there’s light but no extra heat. This acclimatizes your plants to a temperature below what they’re used to without asking them to make the jump all at once.

How to Harden Off Seedlings Using a Cold Frame

When seedlings are prepared for their next big step in life, you can begin to move them outside using this three-stage process:

• Move seedlings into the cold frame and place in a south- or southwest-facing spot. This is basically a box with a window on top, like a mini greenhouse but unheated so plants are still protected but kept at lower temperatures. Leave the cold frame closed for 2-3 days to let your seedlings adjust to their new environment.
• Open the windows on top of the cold frame about halfway to let some light in, and close them again at night. Do this only in good weather, when it’s relatively warm and preferably dry, although a little light rain won’t matter. You’re aiming to keep your seedlings between 40-50°F, where they’ll keep growing while still making vital adaptations to cooler conditions.
• If the weather is colder, you’ll want to close the cold frame up tight. A layer of bubble wrap insulation over the top helps on really chilly days. Never be afraid to pause the hardening off process if the weather suddenly turns. You don’t have to go back to the beginning, just wait out the bad weather and continue once conditions improve.
• Fully open up for the light during warmer days, and leave things half-open the rest of the time. Close up again at night at the start of the week, but if you have a mild evening, leave things open until dark.
• In the second half of the week, start leaving the cold frame open day and night as long as it is reasonably mild. Again, take your cues from the weather. Once you can confidently leave the frame open 24/7, and the weather forecast is holding good for a few days to come, you’re good to plant.

Hardening Off Seedlings without a Cold Frame

If you’re simply moving your plants from a windowsill into an outdoor environment, find a sheltered position and some horticultural fleece. Find your hardening-off spot by walking around your garden on a frosty day. There’s usually somewhere that stays pretty clear of ice such as near a house wall, under a hedge or at the foot of a south-facing fence. Earmark this spot as it’s the perfect halfway point between your windowsill and the open garden.

When it comes time to harden off, move your plants out to your sheltered spot in the morning and cover with a double layer of pegged or weighted-down fleece. Bring them in again at lunchtime for the first few days. For the second half of the week, leave them outside—still under fleece—until the end of the day.

In the second week, take off all but one layer of fleece for the first half of the week, still bringing the plants indoors at night, and in the second half of the week leave them outside—still only during the day—uncovered. Finally, spend three days or so leaving plants outside day and night, but cover them with a double layer of fleece after dark, removing it in the morning. After this, if the weather is holding fine, go ahead and plant.

A Final Helping Hand

Most of the hardiest vegetables like early beets, turnips and cabbages grow happily with hardly a backward glance after such a careful introduction to the open garden. More sensitive crops like tomatoes, peppers, salad greens and spinach appreciate one final helping hand.

Once you’ve planted them out, pop a row cover over the top for another fortnight or so until the weather is properly warm. Leave the ends fully open, closing them only if the weather gets bad, and remove altogether as soon as you feel confident your plants will be warm enough in the open air. It’s just one last things to do, a type of care package to tuck under their arm as they leave the nest.