How to Harden Off Your Seedlings in 10 Days

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Seedlings are getting so expensive. Knowing how to harden off seedlings is vital if you want them to survive transplanting from the indoors. After nurturing your seedlings all winter, make sure you know how to do this last vital step of starting seedlings!

woman carrying tomato seedlings to harden off outdoors

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.  

If you leave the seedlings indoors they will get weak and leggy. It’s essential for their long-term survival to have temperature and weather exposure.

Each day you 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, stick the schedule and only 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 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.

zucchini seedling emerging from dirt

10 Day Guide to Harden Seedlings

It’s important to give your seedlings time to adjust. Don’t rush it. Here is a step-by-step plan to harden off plants in 10 days.

10 Days Before Planting:

  • Day 1-3: Start Slowly
    • Place seedlings outdoors in a sheltered, shaded spot for 1-2 hours.
    • Gradually increase exposure by 1-2 hours each day.
  • Day 4-5: Assess Response
    • Check seedlings for any signs of stress (wilting, yellowing).
    • If they look healthy, increase exposure to dappled sunlight for 3-4 hours.

3 Days Before Planting:

  • Day 6: Morning Sunlight
    • Place seedlings in a spot with morning sunlight for 4-5 hours.
    • Ensure they are protected from strong winds.
  • Day 7: Midday Sunlight
    • Expose seedlings to midday sunlight for 5-6 hours.
    • Keep an eye out for any signs of sunburn or wilting.

2 Days Before Planting:

  • Day 8: Afternoon Sunlight
    • Extend exposure to afternoon sunlight for 6-7 hours.
    • Water seedlings as needed, ensuring soil stays moist but not soggy.
  • Day 9: Introduce Wind
    • Place seedlings in a spot with gentle, indirect wind for 1-2 hours.
    • Wind helps strengthen stems, but avoid strong gusts.

1 Day Before Planting:

  • Day 10: Full Day Outdoors
    • Allow seedlings to spend the entire day outdoors.
    • Ensure they have protection from harsh midday sun and strong winds.
  • Night Before Planting: Bring Indoors
    • Bring seedlings indoors for the night to protect them from cold temperatures.

Planting Day:

  • Day 11: Ready for the Garden
    • Plant seedlings in the garden in the morning or late afternoon on a warm day.
    • Water gently after planting to settle soil around roots.


  • Continue to monitor seedlings for any signs of stress after planting.
  • Provide shade if there’s a sudden heatwave or excessive sunlight.
  • Water regularly, especially during the initial days in the garden.

This schedule helps seedlings adjust gradually, reducing the risk of transplant shock and ensuring they thrive in their new outdoor environment.

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tomato plant in dirt sitting outside in dappled lot to start hardening off

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!

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

seedlings emerging from dirt in rows indoors
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black containers of young green seedlings in sunshine to be hardened off

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.

tomato plants getting leggy before hardening off in black containers

When to Start Warm Weather Plants

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.

young seedlings growing in toilet paper rolls

Should I Water My Seedlings Outside?

Only water your seedlings from the bottom and do it the night before sitting them out.  Water droplets act like a magnifying glass to the sun and can burn the leaves. Don’t mist or water your seedling overhead during the hardening off process.

Should I Fertilize Seedlings?

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.  

Should I put Milk Jugs on My Seedlings?

Do not close them up in an enclosed plastic containers.  Your plants can cook.  Always give venting space.    You can use milk jugs for wind protection if needed, but take off the cap so the plants can breathe.

lettuce growing in black containers on wire shelf

Why Are My Seedlings Wilting Outside?

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. 

If you notice your baby plants wilting, go ahead and bring them back inside. This is their way of telling you they didn’t like something.

Why Do My Seedlings Have White Spots on the Leaves?

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

Another way to prevent sunburn in the hardening off process is to keep your seedlings in a shaded area. Don’t expose them to direct sun until the very end of the process.

What Should I do if My Seedlings are Stressed?

Signs of stress in seedlings include wilting or yellowing leaves. Provide temporary shade or protection from wind, ensure adequate watering, and avoid further stress by adjusting the hardening off schedule. Slow is better than stressed!

woman wearing gloves transplanting hardened off seedling into soil

Will Deer Eat My Seedlings?

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. 😉

How Do I Know if My Seedlings are Ready to be Hardened Off?

Seedlings are ready to be hardened off when they have sturdy stems and several sets of true leaves.

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!

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About Michelle Marine

Michelle Marine is the author of How to Raise Chickens for Meat, a long-time green-living enthusiast, and rural Iowa mom of four. She empowers families to grow and eat seasonal, local foods; to reduce their ecological footprint; and to come together through impactful travel.

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