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How to Harden Off Seedlings

This time of year a lot of gardeners are beginning to harden off seedlings. If you’ve never done it before you may be wondering how to harden off plants and why you need to do it. Let’s start with defining hardening-off.


What is Hardening-Off?


If you started plants indoors or you bought some little plants at the store, these plants have been grown in a very sheltered environment. It was neither too warm nor too cold. They had just enough water and plenty of gentle light.


But when you put them outside, everything will change for these small plants. It will get much warmer and much colder outside than it did inside. The sun will shine more brightly and the wind will blow. Rains may pelt down or it may go days with no rain at all.


Plants need a gradual introduction to these extremes and a chance to toughen up. Easing your plants into this new environment is commonly called “hardening-off.”

seedling tomatoes and garden tools on a wooden background

Tomato seedlings need temperatures to be over 65 F before they go outside

When Should you Harden Off Plants?


The timing for hardening off plants depends on 3 things–the type of plants, the size of the seedlings and the local weather.  Gardeners should wait until their plants have at least 3-4 true leaves, but not so long that their roots become cramped and in danger of getting pot bound. Gardeners also need to wait until nighttime temperatures are warm enough for the plants.


  • Cold Hardy Crops, such as broccoli, brussels sprouts, kohlrabi, cabbage, onions, leeks and parsley can be hardened off when nighttime temperatures are above 40℉. Tender
  • Crops, such as cucumber and melon should wait until temperatures are above 50℉.
  • Warm Weather Crops, such as basil, tomatoes, peppers will need to wait until temperatures are consistently above 65℉.


How To Harden Off Plants


The key is to hardening-off is to make all changes gradually. Pick a mild day, without much wind. For a few hours, when the sun is not too intense, put your little plants outside. Whenever the temperature is expected to drop or to increase significantly, bring the plants back indoors. Do the same when strong winds or rain are expected.


During this time avoid fertilizing the seedlings. Especially don’t give them extra nitrogen. They need to put their energy into getting stronger and stockier, so don’t encourage leafy growth by increasing nitrogen.


Over the course of a week or more, gradually increase the amount of time outdoors and the extremes you expose your plants to. After 7-10 days you should be ready to plant out your seedlings.

Transplanting Seedlings


If you can, pick a mild cloudy day, after the heat of the sun is gone from your garden area. After you have planted your seedlings, give them a drink of diluted fertilizer to help them get going after the move. It is common for plants to undergo some degree of “transplant shock” at this time. But a gradual transition and a little extra care can keep this to a minimum and get your garden growing as quickly as possible!

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