Hardening off plants is one of my least favorite gardening tasks. It requires a lot of care and attention, and it’s really easy to mess it up. I’ve yet to perfect the shuffle that involves bringing trays of seedling outside then back in. There’s always something that goes wrong. So let me help you avoid mistakes of your own by sharing all of mine, past, and present.
What is hardening off?
Hardening off plants involves acclimating your indoor-grown seedlings to the outdoor climate and conditions. Inside your house, your baby plants are given plenty of tender, love, and care. If you don’t take the time to harden off your seedlings here’s what can happen:
- Seedlings die because it’s too cold.
- Seedlings experience leaf burn because the sun is a whole lot stronger than any indoor lights your plant has previously been exposed to.
- Seedlings die or become sickly because of transplant shock.
- Seedlings topple over and stems break because it’s windy.
How do I harden off my seedlings?
First, you should have an idea of when it’s appropriate to transplant your seedlings. Peppers and eggplants won’t be heading outside anytime soon in my climate because it’s still way too cold at night.
Hardening off plants involves exposing them, little by little to outdoor temperatures, and, of course, the strong rays of the sun. In my experience, it takes a little over a week to complete the process because not every day looks the same weather-wise. Today, for instance, I didn’t bother putting my transplants outside for their second day of hardening off because the weather is stormy. I didn’t want to risk harming my tender starts.
Here’s a handy guide for hardening off seedlings
- Day 1: Set out in the shade for 1-2 hours. Make sure the plants aren’t exposed to direct sunlight on their first day out in the wild.
- Day 2: Increase outdoor plant time slightly, keeping them in shade.
- Day 3: Increase time outdoors again with a little bit of time in the sun—make sure they’re not exposed to the sun at the hottest part of the day. They’re still not ready for that.
- Day 4 and 5: Increase time outside in shade and sun incrementally.
- Day 6 to 7: Keep plants outside almost all day but bring them in at night.
Once that week of hardening off is complete, it’s time to transplant your starts in the garden.
Errors to Avoid
There’s a lot of juggling around when hardening off plants. Set yourself a timer, so you don’t forget to bring plants inside, especially in the first few days. It’s easy to accidentally leave a tray of seedlings out in the hot afternoon sun and oops! You have a tray of burned plants on your hands.
Don’t forget to water your plants! And if it rains, watch that your plants don’t stay sitting in a puddle of water all day. Drain the trays accordingly.
Check the forecast, too. You don’t need to stick to a strict hardening off schedule. Adjust if you need to. Will it be unexpectedly chilly tomorrow? Don’t set out heat-loving starts until a day later when the warmth has returned.
Steph Coelho is a freelance writer gardening in zone 5b. She is a certified Square Foot Gardener and has taught various garden-related workshops. When she’s not digging in the dirt or writing, she’s cooking up fresh produce, running, or listening to her favorite podcasts.