How to Harden Off Plants

Preparing Seedlings to be Planted Outdoors

By
Marie Iannotti
Garden Writer Marie Iannotti
Marie Iannotti
Marie Iannotti is a life-long gardener and a veteran Master Gardener with nearly three decades of experience. She's also an author of three gardening books, a plant photographer, public speaker, and a former Cornell Cooperative Extension Horticulture Educator. Marie's garden writing has been featured in newspapers and magazines nationwide and she has been interviewed for Martha Stewart Radio, National Public Radio, and numerous articles.
Learn more about The Spruce's Editorial Process
Updated on 09/06/22
Reviewed by
Debra LaGattuta
Debra LaGattuta
Reviewed by Debra LaGattuta
Debra LaGattuta is a gardening expert with three decades of experience in perennial and flowering plants, container gardening, and raised bed vegetable gardening. She is a Master Gardener and lead gardener in a Plant-A-Row, which is a program that offers thousands of pounds of organically-grown vegetables to local food banks. Debra is a member of The Spruce Gardening and Plant Care Review Board.
Learn more about The Spruce's Review Board
Closeup of potted seedlings ready to be planted and hardened off

The Spruce / Randi Rhoades

Project Overview
  • Working Time: 4 - 8 hrs
  • Total Time: 1 - 2 wks
  • Skill Level: Kid-friendly
  • Estimated Cost: $0

Hardening off seedlings gradually exposes the tender plants to the wind, sun, and rain, toughening them up by thickening the cuticle on the leaves so they lose less water when exposed to the elements. This helps prevent transplant shock, the term used for seedlings that languish, become stunted, or die from sudden changes in temperature and exposure to sunlight.

What is hardening off?

Young plant seedlings that were grown either indoors or in a greenhouse will need an adjustment period to acclimate to outdoor conditions before being planted in the garden. This transition period is called "hardening off."

When to Harden Off Your Plants

Begin the hardening-off process seven to 14 days before the date you will plant your seedlings outdoors. In spring, most seedlings are typically planted outdoors after the last frost date. However, cold-hardy plants can be planted outdoors two to four weeks before the last frost date.

Because the length of time a seedling requires to harden off depends on the type of plants being grown as well as the outdoor temperatures, be flexible when hardening off your seedlings and prepare to whisk them indoors or cover them if a late spring freeze or snow is in the forecast.

Choose either of the following methods to harden off seedlings.

What You'll Need

Equipment / Tools

  • Wheelbarrow (optional)
  • Cold frame (optional)

Materials

  • Seedlings grown indoors

Instructions

Materials for hardening off transplants

The Spruce / Steven Merkel

Harden Off Seedlings by Gradually Exposing Them to Outdoor Conditions

  1. Begin Exposure to Outdoor Conditions

    On a warm day when temperatures are above 45 degrees Fahrenheit, place seedlings in an outdoor location that is protected from direct sunlight and wind for a duration of one hour on the first day.

    Exposing transplants to the outdoors

    The Spruce / Steven Merkel

  2. Bring the Plants Back In

    Move the seedlings indoors when they've reached their daily outdoor time limit, and place them somewhere warm like a heated garage or basement.

    Bringing the transplants back indoors

    The Spruce / Steven Merkel

    Tip

    Don't forget to protect young seedlings from animals, snails, and slugs. Place them on a table or somewhere animals cannot reach them.

  3. Increase Exposure

    Increase the amount of outdoor exposure one hour each day to gradually acclimate the seedlings to increasing amounts of dappled sun and wind. The key to success is gradual exposure to the outdoors. Don’t put tender seedlings outdoors on windy days or when temperatures are below 45 degrees Fahrenheit.

    Gradually increasing sun exposure

    The Spruce / Steven Merkel

  4. Increase Amount of Sunlight

    After two or three days in a somewhat shaded location, you can place the seedlings in locations that receive morning sun, and over time, gradually expose them to more direct sun. Exposing very young seedlings to direct sunlight too soon can scorch the leaves. Each day, the seedlings will be able to tolerate more hours of exposure to outdoor conditions.

    Increasing the amount of sunlight

    The Spruce / Steven Merkel

    Tip

    If you decide to gradually acclimate your plants to longer periods of time outdoors, moving them indoors and out is made easier by using a wagon or wheelbarrow.


  5. Leave Plants out Overnight

    If temperatures remain warm both day and night (at least 50 degrees Fahrenheit), the seedlings should be able to handle increasing amounts of sunlight and can be left outdoors overnight. Make sure the soil doesn't dry out if temperatures should suddenly turn warmer.

    Leaving the transplants out overnight

    The Spruce / Steven Merkel

  6. Transplant

    After seven to 14 days of acclimating your plants to the outdoors, they are ready to be transplanted into the garden or container. If possible, choose a cloudy day to do so and water well after planting.

    Planting transplants in the ground

    The Spruce / Steven Merkel

Harden Off Seedlings in a Cold Frame

  1. Move Your Seedlings

    Move your seedlings to an outdoor cold frame about seven to 14 days before your transplant date.

    Adding the transplants to the cold frame

    The Spruce / Steven Merkel

    What is a cold frame?

    At its simplest, a cold frame is a wooden box with no bottom, which is covered in some kind of transparent top that is usually hinged. It's similar to a greenhouse. The biggest difference is the top can be opened or closed as a way of protecting plants and gradually exposing them to the elements.

  2. Monitor the Temperature and Moisture

    Be sure the temperature in the cold frame does not go much below 50 degrees or above 80 degrees Fahrenheit. While the seedlings are in this warm, sheltered environment, be sure to monitor soil moisture; don't let the plants dry out.

    Watering transplants

    The Spruce / Steven Merkel

  3. Begin Exposure Routine

    Turn off heating cables and/or open the cold frame cover for longer periods of time each day. Start with three or four hours and gradually increase the exposure time by one or two hours per day.

    Monitoring transplants

    The Spruce / Steven Merkel

  4. Protect Plants at Night

    Close the cold frame cover and resume heating at night if temperatures dip below 40 degrees Fahrenheit.

    Closing the lid to the cold frame

    The Spruce / Steven Merkel

  5. Transplant

    Seedlings should be ready to transplant in seven to 14 days. If possible, do so on a cloudy day and water well after planting.

    Planting transplants into the ground

    The Spruce / Steven Merkel

What is Direct Seeding or Direct Sowing?