How To: Harden Off Plants

As with spoiled children, your pampered seedlings need to toughen up a bit before you send them out into the big, bad world. Expose them to life’s realities gradually!

By Audrey Stallsmith | Published Apr 23, 2021 4:38 PM

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If you start seedlings indoors under grow lights, you’ll need to give some thought to how to harden off plants. That involves exposing them to real sunlight and wind gradually enough to avoid burning or fraying their still-tender foliage. The hardening process actually thickens their “skin,” but causes them to grow more slowly for a while.

If you intend to get your houseplants out of the house over the summer, you’ll want to expose them to outdoor conditions gradually, too. Recommended time periods for hardening vary from 1 to 3 weeks, so 2 weeks should be about right.

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You can make the process of moving plants in and out easier by carrying seedlings in daisy trays (flats with drainage holes) and placing houseplants in a garden cart or children’s wagon.

STEP 1: Prepare your plants for their first trip outside.

Start the hardening process 2 weeks before the last frost date for your region, as long as you protect your plants when necessary. Stop fertilizing houseplants several days before you take them outdoors and refrain from feeding plants during the transition period. However, make sure their soil remains damp enough that they don’t dry out.

Keep in mind that many houseplants prefer bright, indirect light and should rest under a tree for their summer vacation rather than out in the open where they will sunburn. Those shouldn’t require as much hardening as sun-lovers do.

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STEP 2: Make sure your plants do not sit in full sun right away. 

On their first day outdoors, place the plants for a couple hours during the morning in a location where they will receive no direct sun and where they receive some shielding from wind. You might choose a covered porch open toward the south or a site protected on most sides by shrubs. After 2 hours, carry or cart them back indoors again.

If you won’t have time to move plants every day, place them in a sunny location, but cover them with two shading layers of row cover. Tuck the sides securely beneath the flats so it won’t blow off.

STEP 3: Increase the time of sun exposure by 1 hour on a daily basis. 

Every day, add another hour to the amount of time your plants spend outdoors, edging them gradually into a brighter and more exposed location where they receive morning sun, but continuing to take them indoors every night. Once they have adapted to that light, begin exposing them to midday rays as well.

If you are covering the plants instead of moving them, pull off the top layer of their row cover after about 5 days, but leave the other one in place. After another 5 days, you can remove that second layer.

STEP 4: Prepare your plants for cool nights gradually.

On days when the weather is excessively windy or daytime temperatures don’t rise above 45 degrees, you should keep your plants indoors. On about day 10, after they have been out for 12 hours in full sun, you can let them stay out overnight too, provided that there is no frost in the forecast. Continue to leave them out full-time for 4 more days—until all possibility of frost has passed—before you transplant the seedlings into your garden or move your houseplants to their permanent summer location.

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STEP 5: Make sure the soil does not dry out if the weather turns warmer. 

Since the weather likely will be warming toward summer during this transition period and seedlings tend to live in small pots, the plants might begin to dry out more, and more quickly.  Breezes can desiccate moisture as well, so you might need to water seedlings more than once per day—and your houseplants more often than you watered them indoors—to ensure that they don’t wilt and wither.

Make sure that all of the pots have drainage holes. Houseplants that did OK indoors without such holes can rot outdoors if their soil becomes too soggy after heavy rains.

Despite your best efforts, some of your plants still might suffer from sunburn or windburn, causing scorch marks that can vary in color from brown to tan to white. But, if you have been careful about following the gradual exposure methods detailed here, that burn likely will be a mild rather than a severe one.

Once the seedlings adjust to the new conditions, they should recover quickly—especially in late spring and early summer when they are raring to grow!

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