I’ve learned after years of gardening to harden off my seedlings before planting them outside in the full sun. So I start my tomato and pepper plants from seeds indoors, where they grow under lights at room temperature. Doing so ensures a smooth transition to the sometimes harsh realities of outdoor growing conditions.
Hardening off seedlings, i.e. gradually exposing them to outdoor conditions, is not hard to do. But it does take a few weeks and a bit of patience.
How To Harden Off Plants
Start with some planning. Here are the steps in the process:
Watch the weather
Only take seedlings outside on days when temperatures are between 60 and 70 degrees. Some seedlings, like peppers and tomatoes, won’t grow when it’s below 60, and may even suffer some chilling damage.
Make sure plants are well-watered
Seedlings, especially those in smaller containers, can dry out quickly outdoors, especially if it’s windy. Water seedlings before putting them outside, then water again when you bring them in.
Choose a shady spot
While LED grow lights many gardeners use indoors seem bright, they aren’t as bright as full outdoor sun. Put seedlings in a shaded spot or under an umbrella to keep the sun off them. Remember, shade moves as the earth rotates. So choose a spot that will remain shaded the whole time your seedlings are outside.
Put the seedlings outside for a few hours initially
On the first day, only leave them outside for a few hours. Then gradually increase the time until you’re leaving them outside all day. Also, gradually transition them from full shade to part-shade.
Bring seedlings in at night or during heavy rain
Because early spring nighttime temperatures often dip down below 60 degrees, bring your seedlings in at night. Also, watch the weather forecast for heavy rain, and bring seedlings indoors before any big storms.
For easy transport, plant seedlings in shallow boxes or plastic trays. Unused plastic storage bins work well for carrying seedlings in and out.
Plant seedlings when you’re sure your area is frost-free
Once you’re past your last frost date and your seedlings have been outside enough, it’s usually safe to plant them in the garden or in containers. But watch nighttime temperatures. Be sure they’re 60 degrees or warmer, especially for tomatoes, peppers and eggplants.
If it works for your schedule, try transplanting seedlings in the evening so they have a chance to recover during the night. That will make them better able to handle the first full day of sun.
What Else Can I Do To Prepare Seedlings To Live Outdoors?
While your seedlings are indoors, take these steps to improve their chances to successfully harden off.
Use bright grow lights placed close to the seedlings so they don’t have to stretch to reach the light. Use a timer to turn the lights on and off. Seedlings need some periods of darkness, too.
Once the seedlings sprout their first sets of true leaves, fertilize them. Use a product labeled for seedlings, or follow instructions on the label for reducing its strength.
Set up a small fan near the seedlings and run on a low speed to simulate a breeze. This helps seedlings grow stronger stems.
Do Houseplants Need To Be Hardened Off?
Yes, if you plan to take some of your houseplants outside for the summer. Follow the same process as you would for seedlings. Remember that even after they’ve become acclimated to outdoor conditions, many houseplants still require a shady spot throughout the summer.