Hardening Off Seedlings

Hardening off seedlings is one of the most important steps in the seed starting process, but it’s something many new gardeners forget, often with tragic results. If you don’t harden off your seedlings, they could die, wasting the weeks of work you’ve put into them!

What Does Hardening Off Seedlings Mean?

“Hardening off” is a gardening term that means gradually acclimating protected seedlings to the outdoor environment. Indoors, you’ve protected your seedlings from elements like sunlight, wind, rain, and temperature fluctuations. Once outdoors, however, plants bear the brunt of daily weather changes, so your seedlings will need some help adjusting to their new home.

If you don’t harden off seedlings and transplant them straight into your garden immediately, there’s a good chance the shock will be too much for them. They could die (or at the very least, struggle for several weeks, delaying healthy growth). It’s important to give young seedlings time to adjust first.

Thankfully, hardening off seedlings is relatively easy! 🙂 Here’s how:

How to Harden Off Seedlings

Seedlings indoors for hardening off seedlings post

Step 1: Add a Fan Indoors

The first step of the hardening off seedlings process actually begins indoors, before it’s time to set your plants outside. Indoors, your seedlings get been used to warm temperatures, artificial lights, and little to no air movement. To start the hardening off process, put a fan on your young seedlings. (Not only will the moving air help strengthen your plants to withstand the real world’s breezes, but it will also help encourage plant growth in general.)

Step 2: Create a Sheltered Location Outdoors for Your Seedlings

Once the weather has warmed outdoors, you should have a location prepared to begin the outdoor hardening off process. Choose a location that’s sheltered from the wind and direct midday sun, as your seedlings will need time to acclimate to both. Cold frames or even greenhouses work well for this process.

Once a location is prepared, put your seedlings there. You want to keep the seedlings safe and warm in this step, and protect them from any drastic changes in growing conditions.

If you use a cold frame or cover, you may be able to keep the seedlings warm overnight too, as long as you ventilate it during the day, to prevent the plants from getting fried! Don’t put them outdoors overnight immediately, however, as they will need a little time to acclimate. Remember, they’re not used to the outdoors yet!

It’s generally a good idea to start with just an hour or two in a sheltered location, before moving on to the next step in the hardening off seedlings process.

Step 3: Gradually Increase Sun Exposure & Decrease Night Protection

Once the seedlings have been in the cold frame for a few days, gradually start increasing their exposure to the sun, working them up to direct sunlight (like any other garden plant would receive). At the same time, gradually decrease their nighttime protection, until they can withstand normal daily temperature and wind fluctuations.

Note: Make sure your garden is safe from frost before you leave your seedlings completely unprotected at night! You may still need to close up the seedlings for a few nights, if you’re going to get a late frost, so watch the forecast closely. It would be a shame to lose your work to a late frost, when you’re oh, so close to getting them in the ground!

Tray of seedlings for hardening off seedlings post

Step 4: Plant the Seedlings in the Garden

Once you’ve hardened off your seedlings (and the danger of frost has passed for any sensitive crops) it’s finally safe to transplant them directly into the garden. Make sure to provide freshly planted seedlings with adequate fertilizer and moisture, providing whatever the crop prefers.

Common Questions About Hardening Off Seedlings

How Big Should Seedlings Be Before Hardening Them Off?

Seedlings should be well established before you start hardening them off, so they can withstand the shock of being moved outdoors and transplanted into the garden.

When Should You Start Hardening Off Seedlings?

You should start hardening off seedlings approximately one or two weeks before your last frost date, or whenever it is safe to plant your crop in the garden.

Will Leggy Seedlings Harden Off Well?

Possibly, but they may be susceptible to damage, so be especially careful while hardening off leggy seedlings. Using a fan early in the process (see step 1) should help strengthen the plants’ stems, however, alleviating this problem.

How Long Does It Take to Harden Off Seedlings?

Hardening off seedlings takes a week or two from start to finish, though the exact amount of time will depend on your local climate, the season’s unique challenges, and what plants you’re growing in your garden.

Another tray of seedlings for hardening off seedlings post

Conclusion

Hardening off seedlings is essential for every gardener, and now you know how to do it yourself. Have a tip to share about hardening off seedlings, something that works especially well in your climate? Share it in the comments below, so others can enjoy it too.