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How to Harden Off Your Seedlings

Knowing how to harden off seedlings is vital if you want them to survive transplanting. After nurturing your seedlings all winter, you’ll want to make sure you know how to do this last vital step of starting seedlings!

How to Harden Off Seedlings

When you start your own seedlings inside you must prepare them for being outside – gently.  This is called the hardening off process.  It allows your seedlings to gradually get used to being in full sun and wind so they don’t suffer from transplant shock and die.  

Each day you will put the seedlings in a shaded area for an hour or two and gradually increase the number of hours per day they are outside by one to two hours.  If it is a cloudy day you will still only want to leave them for their amount of time. Do not rush the process. 

Give the seedlings time to adjust to being outside in the sun all day.  Once they have done well for a week move them for an hour or two into direct sun.  After a week they should now be able to handle the sun all day.    


For the third week begin leaving them out at night with a frost blanket or in an unheated greenhouse/low tunnel.  Plant them out after that.  You can plant cold hardy plants directly in the garden after the 2 weeks just make sure to cover them nightly with a frost blanket and remove it each morning.

When do I start the hardening off process?

The hardening off process can take a full three weeks – so make sure to plan accordingly based on when you want to plant your seedlings in the garden. In my Zone 5 garden with a frost-free date of May 15, I need to start the hardening off process by the last week of April if I intend to plant them on May 15.

If I want to plant them earlier because the weather looks good, I will need to start the process even earlier. Make sure you know you frost-free date so you can plan! I recommend plugging your zip code into the frost-free calculator here.

Ultimately, frost tolerance, type of plant, and type of protection you have will determine when to plant. Using a tunnel, a small greenhouse or a walk in greenhouse will help you get a jump start on your garden, enabling you to eat yummy home-grown food sooner!

Have you checked your frost-free date yet? 🙂

You can also build your own low tunnel to act as a cold frame and help you get a jump on starting, growing and harvesting food from your garden.


Hardening off Hardy, Frost Tolerant Plants

Very hardy plants like kale, lettuce, peas, and spinach will be set out about 6 weeks before the last frost date.  They handle cold weather better than any other plants so they are very good to practice with.  With these plants even if you forget and leave them outside one night you can start over pretty quickly.  


Frost tolerant plants can be set out 2-3 weeks before your last frost date.  Plants like broccoli, brussel sprouts, cabbage, cauliflower, onions from seed, and parsnips can handle a light frost but even then it will stunt the future fruits dramatically.

A lot of people don’t know this, but corn can also tolerate a light frost as long as they aren’t very tall. If you watch the weather closely and don’t anticipate a cold snap, you can plant your corn seeds directly into the ground two weeks BEFORE your last frost date. Be sure to check soil temperature before planting. Corn germinates best when the soil is at least 55F.


However, tender plants like peppers and tomatoes will be killed by the frost so do not set them out until a week or two AFTER the last frost date.   

Warm plants like beans, cucumbers, eggplant, herbs, melons, peppers, pumpkins and squashes all need the soil to be 70F a good week before planting so check when your soil will be warm enough and start one to two weeks before hardening them off. 

The frost will kill warm loving plants.  Do not start hardening them off unless you have been successful with the very hardy and frost tolerant plants.  Warm plants take longer to grow.  Practice on quick growers in the very hardy list.

What should I not do before taking my seedlings outside?

Only water your seedlings from the bottom and do it the night before sitting them out.  Water droplets can act like a magnifying glass to the sun and burn the leaves so no misting or overhead watering.  


Do not fertilize your seedlings during this time as you want to help learn to cope a bit on their own.  I do use a lite compost tea when I bring them in to help with the stress of hardening off if they ever look wilted.  

Do not close them up in an enclosed plastic containers.  Your plants can cook.  Always give venting space.    

What can go wrong during the hardening off process?


Being left outside too long can cause your plant to wilt even if it is cloudy or they are under shade.  The wind can be too harsh and dry them out or even break them so watch windy days. 

Plants can become sunburnt even if they were grown under grow lights.  The sun is simply brighter than any grow light.  So set an alarm clock to remind you to take them in. 

Rabbits, deer, cats and dogs can actually eat your plants so put a fence or set them in a safe space.  Ask me how I know all of this. 😉

What can go right?


Everything can go right so take a chance and try it.  Every gardener kills their plants while hardening off their seedlings at some point. Life and weather happens. Just buy a few extra packs of seed, divide your plants in half and start one half later or buy seedlings.  If anyone asks, tell them you are succession planting for an extended harvest. 

Let us know if you have ANY questions about the hardening off process and good luck!

If you liked this post on how to harden off seedlings, you’ll like these too:

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Printable Garden Planting Guide Zone 5

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