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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 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!

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Hardening Off Plants After Starting Seeds Indoors
These seedlings are getting ready to make their way outside.
Starting Seeds Snapdragons
My snapdragon Costa Apricot seed starts began the hardening off process almost two weeks ago.
Hardening off plants after starting snapdragon seeds indoors
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.

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.

Starting Sweet Pea Seeds
These are my Sweet Pea seed starts before starting the hardening off process.

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!

Hardening off plants after starting seeds indoors
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.

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.

Starting Seeds
Larkspur seed starts

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.

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

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.
Hardening off plants after starting seeds indoors
These sweet peas are ready to be planted.

Best of luck with your seedlings!

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

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.

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.

How to Start 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.

How to Start Seeds Indoors Without a Greenhouse

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!

Shiplap and Shells
Photo by Kim from Shiplap and Shells

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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