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What You Need to Know About Hardening Off Plants

If you sowed seeds indoors this winter, it is time to start hardening off plants to prepare them for outdoor life in the garden.

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

If you’ve been following along with my seed starting journey, I’m almost done sowing seeds and getting ready to plant them in the garden.

Before planting them in the garden, there’s a process seedlings need to go through in order to successfully grow outdoors.

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 plants 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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close up of caramel creme coreopsis

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.

However, cold-hardy plants, such as sweet peas, snapdragons and iceland poppies, can be planted outdoors two to four weeks before the last frost date.

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

In the last few weeks, I started hardening off some cold-hardy plants that include sweet peas, larkspur, snapdragons, and iceland poppies.

Because I am growing SO many flowers in my basement greenhouse, I need to start the hardening-off process just to get the extra real estate for other seedlings I started.

The struggle is real!

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 to ensure sure I can keep them protected should we get hit with a frost.

Because where I live, that can happen so you gotta be ready!

Since I live in New Jersey, zone 6a, it’s really important to keep an eye on the weather. Anything dipping below 45 degrees, I either cover seedlings or bring them inside.

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.

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.

How Do I Harden Off Seedlings?

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

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 14 days.

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 should harden off your seedlings.

They will not do well if you just take them outside and plant them without acclimating them first.

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 planting.

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!

I am really excited to get mine outside and established in the gardens.

How to Start Seeds Indoors Without a Greenhouse

Have You Been Following Along My Seed Starting Journey?

If you’ve been following along, this is Part 6 of my journey 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 the series, you can check them out below.

Close up of snapdragons in front yard cottage garden

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.

And don’t miss joining my Gardening DIY and Decorating Community on Facebook for more chatter. And follow along there and on Instagram as well. There are behind-the-scenes daily things that I share on Instagram that don’t make it to the blog. Would love to see you there too!

If you prefer to binge-watch Bricks ’n Blooms on TV, we go more in-depth with tours and posts on my YouTube channel. Would love to hang out with you there!

And… If you’re catching up on blog posts you may have missed, be sure to sign-up to get my newest posts via email to stay up to date with everything that’s happening here on the blog and more.

Garden Supplies I Use

I’m often asked about the garden supplies and tools that I use most. From pruners to deer repellents, here are some of my favorites in no particular order.

Click here to shop my vintage farmhouse with close up of the front porch with flowers

Looking for More Flower Garden Ideas?

If you love flowers and want to grow more in your garden, here are some posts that will get you on your way.

From tucking in flowering plants that are deer-resistant or ones that attract more butterflies and hummingbirds, to shade-loving flowers like the lenten rose, these posts will get you on your way to growing a garden that will bring joy for years to come.

Here are more cut flower and cottage garden growing tips, tricks, and design inspiration.

view of the front porch cottage garden with sugar pumpkins, sedum autumn joy, rudbeckia, celosia and snapdragons

My friend Kim over at Shiplap and Shells has been hardening her seedlings off as well.

She’s got a gorgeous garden in the pacific northwest and it’s so neat to see how different our timetables are!

seed starts hardening off with shiplap and shells
Photo by Kim from Shiplap and Shells

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close up of snapdragon seedlings hardening off outside on porch

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Starting Sweet Pea Seeds under grow lights - 6 inch seedlings
These are my Sweet Pea seed starts before starting the hardening-off process.
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.

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9 Comments

  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