How to Harden Off Seedlings for Transplanting


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Hardening off seedlings – What is it, how do you do it and why is it important to do for your garden success?

When I first started gardening I had never heard of hardening off your seedlings. I just direct sowed seeds in my garden or put my purchased seedlings in the ground without really any thought. Those led to various successes over the beginning years. Then I decided to start my seeds indoors which opened a whole new world of possibilities in the garden.

Starting seeds indoors gave me many opportunities to learn new things about plants and about the best ways to get them ready to be planted outside. Hardening off seedlings is just one piece of the puzzle but it is an important step to remember for plant success. Typically you start hardening off your plants after you have potted them up into larger pots.

What is Hardening Off Seedlings?

Hardening off seedlings is the process used to get your indoor plants ready to be planted outside. Think about how you have been growing your plants in ideal conditions inside your house. The plants are used to the temperature in your house and the light coming from the grow lights. They are not used to direct sunlight and fluctuating temperatures from day to night. Maybe even the wind and weather conditions will be too much for them to handle.

Hardening off your seedlings allows your plants to become accustomed to the new environment in small amounts of time over several days. I like to set timers so that I don’t leave the plants outside for too long during this process. This allows them to build up a tolerance to changes and can help to reduce transplant shock.

Broccoli and cabbage Seedlings sitting in trays during the hardening off process.
Broccoli and cabbage seedlings getting some sun during the hardening off process

How to Harden Off Seedlings?

The process of hardening off seedlings is quite easy but can be time-consuming depending on how many plants you need to harden off at once. Begin the hardening-off process seven to fourteen days before you want to plant your plant in the ground outside. Ideally outside temperatures will be above 45 degrees Fahrenheit when starting the process. Your seedlings should have at least 2 true leaves.

My plants sit in 10 in x 20 in trays so that I can move them around more easily. I can get 15 plants in a tray in their 3-inch pots. So I can move 15 plants at a time by carrying the tray. I suggest setting timers as you do this process so you don’t forget about your plants. Trust me I have done it many times.

Day 1

On the first day of the process, you should put your plants outside in a protected area away from wind. Dappled sunlight to part shade is best. You should not expose them to direct sunlight on the first day. Leave your plants outside in this protected shady area for about 1-2 hours and then bring them back inside. I usually start mine out on our covered porch because it gets dappled sunlight and only a little wind during the day.

Days 2-5

Gradually increase the time your plants are outside by an hour or 2 a day. Also gradually move your plants into more direct sunlight and areas where they might get some wind. This will help the plants build up their tolerance to the outside elements. Make sure to watch the weather in case there are any frosts predicted for this time. Do not leave your plants outside if there is frost in the weather report make sure to bring them inside to protect from frost damage. Bring them back outside when the chance of frost passes. I usually harden my plants off on my back deck where I can control the amount of wind they are receiving. They typically receive full sun depending on where on the deck I set them.

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Days 6-7

You should be able to leave your plants outside for most of the day now. Check overnight temperatures to see if you might be able to leave them out overnight. Again be sure to watch the weather report and move your plants inside if there is frost called for.

If you go with the 14-day route just slowly add time they are in the sun. Increasing by an hour a day will work. Hardening off slower than 7 days is not a bad thing, it will ensure that they are ready to be planted.

Mixture of plant seedlings during the hardening off process
Mixture of herbs, flowers, and vegetable seedlings in the process of being hardened off

Why is Hardening Off Seedlings Important to Garden Success?

Hardening off your seedlings is very important to your garden’s success. You don’t want to move your baby plants directly from controlled conditions inside to all the fluctuations of the weather outside. The plant’s leaves need to build up protection from the wind and sun. If you move them outside too quickly without hardening off your plants they can become wind burnt or sunburnt.

You made the effort to baby your plants inside, take the extra few days to baby your plants to ensure greater success. If you move them outdoors too quickly they can go into transplant shock or even fail to thrive when put into the ground or an outdoor container. Those few simple steps to harden off your plants can save you lots of garden heartache if you try to move them too soon.

Ensure while you are hardening off your seedlings that you keep their soil moist. Moving outside and into the sun can dry the seedlings out much more easily. If your plants look stressed move them back into shade and check to see if they need some water. You are now set up for successfully hardening off your seedlings.

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