Young, pampered seedlings that were grown either indoors or in a greenhouse will need an adjustment period to acclimate to outdoor conditions prior to being planted in the garden. This transition period is called "hardening off."
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.
The length of time a seedling requires to harden off depends on the type of plants being grown and the temperature. Be flexible when hardening off your seedlings and prepare to whisk them indoors or cover them if there is a late freeze or snow.
There are three approaches to hardening off plants:
Gradually Expose Plants to the Outdoors
- Begin putting your seedlings outdoors about seven to 10 days before your actual transplant date.
- Place the plants in a sheltered, shady spot outdoors. Under a tree or even on your back porch is fine. Start by leaving them outdoors for three or four hours and gradually increase the time spent outside by one to two hours per day.
- Bring the plants back indoors, or somewhere warm like a heated garage or porch, each night.
- After two or three days, move the plants from their shady spot into the morning sun but return them to the shade in the afternoon. Too much direct sunlight will scorch the leaves.
- If temperatures remain warm both day and night (at least 50 degrees Fahrenheit), the plants should be able to handle the sun all day and stay out at night after about seven days. Make sure the soil doesn't dry in their small pots and bake the plants if the weather should suddenly turn warmer.
- After seven to 10 days your plants are ready to transplant. Try to do so on a cloudy day and water well after planting.
- If you opt to gradually expose your plants to longer periods of time outdoors, the moving in and out process can be made easier by putting your plants on a wagon or wheelbarrow and simply wheeling them into the garage for the night.
- Don't forget to protect your young seedlings from animals. snails, and slugs. Put them on a table or somewhere animals cannot reach them.
Place Plants in a Cold Frame
- Move your plants to the cold frame about seven to ten days before your transplant date.
- Be sure the temperature in the cold frame does not go much below 50 degrees or above 80 degrees Fahrenheit. While they are in this warm, sheltered environment, be sure to check the soil daily to see if the plants need water.
- 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.
- Close the cover and resume heating at night if temperatures dip below about 40 degrees Fahrenheit.
- Plants should be ready to transplant in seven to 10 days. Try to do so on a cloudy day and water well after planting.
Withhold Water for a Short Time
- Starting about two weeks before your transplant date, stop watering your seedlings until they start to wilt. Watch closely. You do not want to leave them dry and wilting for a prolonged period of time. There is not much soil in a seedling pot, so it should not take long for the soil to dry out and the plants to wilt.
- Once they start to wilt, you can water them and then wait for them to wilt again.
- After two weeks of this process, seedlings should be ready to transplant. As always, do so on a cloudy day and water well after planting.
Hardening Off Vegetable Seedlings. University of Maryland Extension