How to Harden Off Plants for Outside
If you grow your own vegetable plants from seed indoors under lights, you will need to adapt your seedlings before transplanting them into the garden. This simple yet sometimes frustrating process is called “Hardening Off.” Starting your garden from seed can open up a world of new possibilities of things to grow but seed starting is not for the faint of heart. It is one of the hardest things to master for gardeners of all skill levels. In this article I am sharing how to harden off plants for outside.
What is hardening off?
Hardening off is the process of getting your plants ready for the outside conditions. When you start your seedlings indoors, they spend their days in the comfortable environment sheltered from the harsh temperatures, bright light and wind.
The hardening off time lets your seedlings transition from the comfortable growing conditions under lights to the normal conditions they will experience in the garden. Without hardening your plants off – your plants will not make it.
Why is hardening off important?
First, sunlight is a million times stronger than grow lights and can burn foliage if the seedlings are put out into sun without hardening off. Secondly, even a light wind can snap your plants’ stem. Cool temperatures at night may stunt the plant’s growth or even kill your seedlings.
Your plants need time to accustomed to the conditions outside.
How to harden off plants for outside
Step 1: Begin hardening off plants in a sheltered area in your garden
This sheltered area can be a shaded spot next to a fence where the sun and wind exposure is not as strong. I like to use the west side of my house between the house and the privacy fence. This area only gets about 2-3 hours of sun each day because of the structures there.
About a week or two before your last frost date, place your plants outdoors in that protected spot for a few hours on the first day. Allow your plants to remain outside for a few hours, then bring your plants back inside.
Make sure to watch the weather during this hardening off time. Temperatures fluctuate quickly, excessive rain and winds can destroy their leaves, stems and can even knock trays over.
Step 2: Increase the time a little each day
Gradually increase the amount of time that the seedlings spend outside to allow the seedlings to slowly adjust. Continue hardening off seedlings by moving the plants outside while temperatures are warm and then back inside at night when the temperature is cool. I increase the time 1-2 hours each day as long as the weather cooperates.
Use your resources to your advantage by using a shady spot or a place that gets morning sun to get your seedlings what they need. Allow the seedlings to experience some wind and even rain. Make sure to check your seedlings often to see if they need more water as the wind and sun can dry out the containers.
Step 3: Leave them outside overnight
Eventually, let your plants stay in full sun and outside as long as nighttime temperatures do not drop below freezing. If it is going to get below freezing move the plants inside. Continue the hardening off process once temperatures return to normal conditions.
Cool season crops such as broccoli, Brussels sprouts, cabbage, celery, lettuce, onions, parsley, peas, spinach, Swiss chard, and other greens can tolerate low nighttime temperatures of around 45°F once they have had time to adjust. Light frost won’t harm these seedlings after these are hardened off.
Summer crops such as tomatoes, peppers, eggplants, melons, cucumbers, and some herbs prefer warm nights, at least 50°F. They can’t stand below-freezing temperatures, even after the seedlings are hardened off. So continue to bring indoors if nights remain cool.
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Also, check out our article on What are the best herbs to grow? or How to prepare your herb bed for winter!
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