How to harden off seedlings

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Hardening off is the process of adapting seedlings to the outside to get used to sun, wind, rain and more extreme temperatures. If you start seeds indoors or buy young seedlings from a retailer or farmers market, you’ll need to know how to harden off seedlings!

Hardening off is important because without taking that important step, your seedlings can easily get too hot, too cold, or blown over by the wind or rain.

This process helps them strengthen and acclimate to the conditions in your container garden.

Now that you know why the hardening off process is essential for protecting your seedlings from the elements and helping them thrive once they’re transplanted, you’re ready to get going! Follow these steps carefully and your plants will be off to a great start!

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Steps for hardening off your seedlings

The best time to start hardening off your seedlings is about 1-2 weeks before your last expected frost date (check this frost date calculator if you’re not sure!). The goal is to gradually expose them to increasing amounts of sun, wind and rain over a period of time.

Make sure to wait until temperatures are at least 45-50 degrees F or higher before setting your seedlings outside. Even cold-hardy plants can suffer in colder temps while they’re young and tender.

Start by placing them in a shady spot outside for a few hours each day, also protected from wind and rain. Bring them back inside at night. Then gradually increase the amount of time they spend outside over the course of a week.
Once they’re used to spending most of their day in the shade, it’s time to move them to a spot where they’ll get a little bit of sun. Again, increase the amount of time they spend in the sun each day until they’re fully acclimated. This can take about a week, depending on the plant.
Once they’re fully outdoors and in full sun, consider leaving them under a protective covering such as row cover fabric during the hottest part of the day. A few days of this should be enough to finish hardening them off.
If you don’t have any protective covering, be sure to keep an eye on your seedlings once they’re exposed to full sun.

Signs of overexposure in plants

Pale foliage that looks washed out or bleached
Sunburns with white or brown spots
Crinkled, dry leaves despite watering

How long does it take to harden off seedlings?

The time it takes for seedlings to harden varies depending on the type of plant. But generally, 1-2 weeks is long enough. More extreme hot or cold temperatures could require a longer amount of time.

What happens if you don’t harden plants?

If you don’t harden plants, they will be very sensitive to the elements. They can get too cold at night and this can actually kill them or severely stunt their growth. Or they might not have a strong enough root system to withstand rainstorms or wind gusts that could damage or blow over your young seedlings.

Do you need to harden plants purchased from a nursery?

Nursery-bought plants are often hardened off when you buy them. However, it’s a good idea to double check. Ask someone at the garden center or the seller at the farmer’s market.

And if your patio or deck has a lot of direct sun or wind, I recommend giving them a week or so to acclimate, even if they’re already hardened off. You can give them a few days of partial shade to ease the transition.

Tools for hardening off seedlings

While you can harden off seedlings without a fancy setup, there are some tools and materials that help a lot when you have them.

Portable mini greenhouse

Mini greenhouses are so cool! This one is 6′ x 3′, small enough for a large patio and big enough to hold a bunch of containers while you’re acclimating seedlings to the outdoors. It’s made out of mesh and PVC piping, lightweight so you can easily move it around your patio or deck.

Row covers

Similar to a mini portable greenhouse, row covers are a good way to protect tender seedlings while hardening off.

They’re more versatile in terms of shape and size and can be customized just about any way you like.

You’ll need the mesh cover as well as hoops for the frame, as most kits don’t include both. These are lightweight, easy to use options, and best of all, they’re reusable season after season.