If you sowed seeds indoors this winter, it is time to start hardening off seedlings to prepare them for outdoor life in a thriving cutting garden. Watch your delicate seedlings transform into radiant blooms with this comprehensive guide to hardening off.

But what is hardening off seedlings and why should you do it?

If you started seeds indoors and sowed several cut flower species, it’s time to prepare them for life outdoors. Because they’ve been grown indoors in a controlled setting, we’ve got to spend some time acclimating them to outdoor living.

Before planting them in the garden, there’s a process seedlings need to go through in order to successfully grow outdoors. And that process is called hardening off.

Hardening off plants means we help them transition from growing indoors or in a greenhouse environment to the outdoor elements of fluctuating weather.

The hardening-off process involves a gradual introduction to changes in temperature, wind, and sun exposure that help seedlings transition to a firmer, harder, and sturdier plant without the shock from environmental changes.

Thus, the overall goal of hardening off plants is to slow their growth to help them adjust to outdoor living.

Cause let’s face it, it’s not the same growing indoors sheltered under lights versus outside in the garden where wind, fluctuating temperatures, and sunlight are a factor.

When seedlings are hardened off properly, they will ultimately be able to handle unexpected dips in spring temperatures.

So don’t skimp on this process! Here’s what you need to do.

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When Should I Start Hardening Off Plants?

As a general rule, start hardening off plants about 2 weeks before the last frost date. Most seedlings are typically planted outdoors after the last frost date. This rule applies to all types of plants you start from seed indoors. So while this post chats more about cutting garden seedlings, the same hardening-off rules apply to all plants.

However, cold-hardy plants, such as sweet peas, snapdragons, larkspur, and Iceland poppies, can be planted outdoors two to four weeks before the last frost date. (As an aside, if you want to skip the hardening-off process, you can always winter sow cold hardy annuals too. To learn more about that process, click here.)

So be sure to check your seed packets to determine when that plant can tolerate certain conditions.

Because hardening off seedlings is an easy but lengthy process, I usually start hardening off cold-hardy annuals first, like the ones mentioned above.

seedlings under grow lights that are ready to be hardened off  hardening Off Plants After Starting Seeds Indoors
These seedlings are getting ready to make their way outside.

When you grow a lot of cut flower species from seed, you’ll need to start the hardening-off process just to get the extra real estate for other seedlings you started indoors. But I digress.

The struggle is real y’all!

Since I started hardening off some of my flowers already, they are almost ready to be planted in the garden. However, I want to wait a little bit longer before planting to ensure sure I can keep them protected should we get hit with a frost.

Because where I live, that can happen on the fly so you gotta be ready! Since I live in New Jersey, zone 6b, it’s really important to keep an eye on the weather. Anything dipping below 45 degrees, I either cover the seedlings or bring them inside.

Starting Seeds Snapdragons
My snapdragon Costa Apricot seed starts began the hardening off process almost two weeks ago.

How Long Should I Harden Off Seedlings for?

Hardening off plants is roughly a two-week process. Admittedly it can be a bit of a pain moving plants in and out, particularly if you have a lot of trays.

But it is worth the investment of time. Your seedlings will reward you with healthier growth, harvest, and blooms if you give them the proper start.

Failing to give them sufficient time acclimating and they may not grow, produce, or bloom well during the season.

cut flower patch with calendula, larkspur, snapdragons in potager garden by fountain

How to Harden Off Seedlings

Physical supplies are not needed for hardening off seedlings. Instead, you’ll need lots of patience and energy so you have the will to bring them inside and out during this specified period of time.

Here’s what you need to know!

  • When temperatures are at least 45-50 move plants outdoors to a protected, shady location for two to three hours. When they’ve reached their daily limit, move seedlings indoors and place them in a heated garage or basement.
  • After working them up to two or three days in a somewhat shaded location, locate seedlings to receive morning sun. During the two-week process, gradually increase exposure to direct sunlight. Note: If placed in direct sunlight too soon, the leaves can scorch. With every day, seedlings will be able to tolerate a little more and a little more of the outside elements. To be successful, gradual exposure to outdoor elements is critical. By the end of the two-week process, plants should be able to spend 24 hours outside and withstand the elements. But keep an eye out for any frost or freeze warnings.
  • Don’t bring plants outside on very windy days. Seedlings are not strong enough to handle high winds.
  • If temperatures are expected to fall below 45 bring plants indoors or cover them in a cold frame.
close up of calendula in the cut flower garden
Calendula flowers

How Do You Harden Seedlings Off Quickly?

The process will take two weeks so there’s really no quickening it up. However, you can start the process sooner to get your seedlings outside if the weather is warm enough. Bring them outside in a shaded, protected spot on warm days, then bring them indoors at night.

Gradually increase the amount of sunlight they receive. But don’t put tender seedlings outdoors on windy days or when temperatures are below 45 degrees.

How Big Should Seedlings Be Before Hardening Them Off

As long as the temperatures and weather conditions are optimal, you can start hardening them off after seedlings get their first true set of leaves and are a few inches tall.

Hardening off plants after starting seeds indoors - close up of snapdragon seedlings
Snapdragon seedlings look really happy and healthy. I will thin these out once I plant them.

When Can I Leave Seedlings Out Overnight?

You can leave seedlings out overnight as long as temperatures don’t dip below 45 degrees during the hardening-off process. Whenever I do this, I typically start leaving mine out overnight under my covered porch as long as the weather conditions are optimal.

Can You Harden Off Plants in 5 Days?

The hardening-off process should take about 2 weeks. Trying to complete it is 5 days might be setting yourself up for disaster.

You spent all this time starting and caring for seedlings, why rush the final time? For best results, try to follow the process for the full 14 days.

I know it seems laborious but trust me, the time you spend hardening off plants will be well worth it in the long run.

snapdragon seedlings are hardened off an ready for planting in the garden
Snapdragons hardening off on back porch

Should I Harden Off My Seedlings?

Yes! You 100% should harden off your seedlings. They will not do well if you just take them outside and plant them without acclimating them first.

And I would add, that if you don’t want to take the time to harden off seedlings, you should either try winter sowing outdoors or working with seeds that can be directly sowed in the garden in lieu of starting them indoors under grow lights.

Can You Harden Off Seeds Too Early?

While hardening off seedlings is an easy process, it does take some pre-planning to do it correctly. If you start hardening seeds off too early, cold temperatures could zap your plants and ruin all the hard work you put in to get them to this point.

On the flip side, if you wait too long to plant them, seedlings might decline from staying in their cells for too long. For best results, follow the seed packet directions and grower recommendations for sowing and planting.

snapdragon bouquet  from the cut flower garden with view of front porch garden and gomphrena truffala pink

What Happens If Seedlings Are Not Hardened Off?

It could be disastrous if you don’t harden off your seedlings before planting them outside. Direct sun, heavy winds, and rain, or cold temperatures can zap seedlings wrecking all the hard work you did to grow them.

Some plants may rebound, but others may not. Why risk it?

Best of luck with your seedlings! Happy Gardening!

madame butterfly bronze snapdragons in the cut flower garden
‘Madame Butterfly Bronze’ Snapdragons

More About Hardening Off Seedlings

Have you hardened your seedlings off yet? Do you have any tips you’d like to share? I would love to know more in the comments below.

cut flower patch: strawflowers in the potager garden

Have You Been Following Along My Seed Starting Journey?

If you’ve been following along, this is Part 6 of my journey of starting seeds indoors without a greenhouse. It has been so much fun starting seeds indoors and really helped with the winter blahs this year!

I’ve really enjoyed watching these seedlings grow and hope they acclimate well to the gardens this spring. In case you missed any part of this Grow With Me series, you can check them out below.

Close up of snapdragons in front yard cottage garden

Pin It To Remember It Later

snapdragons in a cutting garden
close up of snapdragon seedlings hardening off outside on porch

Thanks for stopping by the blog today!

Enjoy your day! xoxo

Bricksnblooms Stacy Ling logo
Starting Sweet Pea Seeds under grow lights - 6 inch seedlings
These are my Sweet Pea seed starts before starting the hardening-off process.
How to Start Seeds Indoors Without a Greenhouse
Hardening off plants after starting seeds indoors with sweet peas
Here are those same Sweet Pea plants outside. I’ve been hardening them off for 2 weeks now so they are ready to be planted in the ground.
sweet pea seedlings Hardening off outside - plants after starting seeds indoors
These sweet peas are ready to be planted.
Hardening off plants after starting snapdragon seeds indoors on the grill to keep them off the ground
Here are those same snapdragons. They have been acclimating well to the outdoors. I keep them high on the grill so my dogs don’t knock them over. They will get planted in the ground this week.
Starting Seeds under grow lights
Larkspur seed starts
What You Need to Know About Hardening Off Plants - close up of snapdragon seedlings
How to Start Seeds Indoors Without a Greenhouse - waiting for seeds to germinate under grow lights and domes
close up of snapdragon seedlings hardening off outside - The Complete Guide to Hardening off Seedlings
Early Spring in the Cottage Garden
It’s a good thing I plant lots of bulbs in fall so I get lots of early spring flowers. I can’t wait for all these seed starts to get planted and bloom in the gardens.

The bricks \'n Blooms guide to a beautiful and easy-care flower garden book by stacy ling
The Bricks ‘n Blooms Guide to a Beautiful and Easy Care Flower Garden
  • Have you never met a plant you couldn’t kill?
  • Have you dug around in the dirt with nothing to show for it except a sunburn and a sore back?
  • Do you currently enjoy growing flowers, but are looking for more tips and ideas to level up your gardening game?

Then the Bricks ‘n Blooms Guide is for YOU

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  1. I’m so excited that we’re so close to transplanting our seedlings into the garden. Great information Stacy.

  2. Hey there Stacy! This is a great post, I am planning to share it on my blog roundup tomorrow! Happy Weekend!

  3. Pingback: Friday Favorites at the Farmhouse - The Everyday Farmhouse
  4. I made a big mistake. I just took my wildflower seeds and scattered them all around. Needless to say only a few, which are in the shade, have started to grow.
    I have more wildflower seeds so I’m going to do a better job with what is left.
    I’d really like to secretly go to all my neighbors homes and sprinkle them in some nice warm, not hot, places. Would be so much fun to see some grow in all the neighborhood.
    You are such an inspiration to so many people, and I’m on the list!!!!!!

    1. Thank you so much Diana! I truly appreciate hearing that. Hopefully the next set will take for you! xo