How to Harden Off Seedlings to Prevent Transplant Shock

Hardening off seedlings is a process by which gardeners slowly introduce their tender, indoor-grown seedlings to the harsher conditions of living outdoors—by simulating those conditions indoors or by bringing the seedlings outdoors for several stints of time before transplanting.

The hardening-off process encourages young plants to prepare themselves for life outdoors, where they may be battered by wind, rain, and sun.

Plants that are grown indoors (whether on a windowsill or a greenhouse) are used to mild conditions. If you transplant these plants into your outdoor garden directly from their indoor home, they’re likely to develop “transplant shock”—a condition where they stop growing, wilt, or even die in response to being transplanted so suddenly into a new environment. Hardening off allows seedlings to acclimate to outdoor weather prior to transplanting.

The hardening off process takes patience and time, but it is an essential part of outdoor gardening. Here’s a step-by-step guide to help you prepare your seedlings for transplanting:

1. 1. Wait until your seedlings are ready. You don’t want to start hardening off your seedlings before they’re ready or they may wilt or die. You can begin the hardening process once your seedlings have at least two true leaves (as opposed to seed leaves, the smooth, round leaves that many seedlings will initially sprout with).
2. 2. Start with indoor hardening. To prepare your plants ready for outdoor conditions, you’ll need to begin simulating some outdoor conditions. If you’re using heating mats to help your seedlings germinate, begin dialing down the temperature by around 5°F every week to help them learn to handle the cold weather outside. Consider setting up an oscillating fan next to your seedlings on the lowest setting to simulate wind. Once your seedlings feel the breeze, they will begin diverting nutrients to strengthen their stems. If your seedlings are receiving more than 16 hours of light per day from grow lights, adjust the settings to more closely mimic daytime sunlight hours.
3. 3. Introduce your seedlings to the elements. Introduce the seedlings to the great outdoors about a week prior to transplanting. For the initial outing, pick a day when the weather is mild (a cloudy day works great), and move your seedlings to a shady, sheltered location for a couple of hours. After a few hours have passed, bring your seedlings back inside.
4. 4. Continue outdoor hardening off. Every day for a week, bring your seedlings outdoors for increasing lengths of time and slowly expose them to the conditions they’ll be experiencing in your garden. For example, after they’re used to the shade, bring them into partial sun, then to full sun in the morning, then finally to full direct sunlight in the afternoon. If you have a cold frame—an enclosure with a transparent roof that protects plants from the weather—you can move your plants inside of one for hardening-off sessions. Remember to bring your seedlings back inside every evening.
5. 5. Transplant. After a week of hardening off, your seedlings should be ready to transplant. Carefully remove each seedling from its container and plant it into its designated home in your garden bed, spacing the plants out according to each plant’s recommendations. Before transplanting, you’ll need to check the weather for adverse weather conditions like frost, hail, or high winds that can negatively affect your seedlings. If the weather’s not ideal for planting, simply extend your hardening-off process for another few days as you await the optimal transplanting conditions.

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