When, Why, and How to Harden Off Seedlings

Spring is in the air! If you’ve got a green thumb and big garden plans that means you’ve probably been growing your seedlings for a while, and it’s almost time to move them outside. But before you do, you need to take the proper steps to harden off seedlings. Hardening off your seedlings is a critical step, especially for those who live in colder or more variable climates.

The process of hardening off your seedlings can be a foreign one if you’ve never done it before. Thankfully, the actual process is less intimidating. Let’s talk about when, why, and how to harden off seedlings so that your garden can thrive this year.

Why do you need to harden off seedlings?

Have you ever met someone extremely sensitive and thought, wow, you are so sheltered! Well, in a way, the same can be said for your seedlings. Whether you’ve been growing them in your windowsill or under a large green light, conditions have been carefully controlled for your seedlings since the start of their life. This may have kept them safe and allowed them to grow, but things are different in the big bad world of your backyard garden.

All plants develop a cuticle as they grow. This waxy coating is a way of protecting their leaves from water, rapid dehydration, and the intense rays of the sun. Your seedlings, however, have not been exposed to these elements, and as a result, indoor seedlings have weak cuticles. To allow them to succeed outside, you need to toughen them up.

Gradually exposing seedlings to the outside elements stimulates their natural defense response, builds their cuticle, and prepares them for their final transition into an outdoor space.

What if I don’t harden off my seedlings?

Now, this probably sounds logical, but what if you don’t want to take the extra time to harden off your seedlings? What happens if you fling your baby plants directly into the deep end of the all-weather pool? Well, there are a number of outcomes that come from this approach, and none of them are ideal.

Among the list of risk factors to plants which are not hardened off properly, your seedlings can experience transplant shock, heat stress, dehydration, or sun damage. Weak stems can also snap in strong wind. The ultimate result of all of these scenarios is death for your unprepared plantings.

Even if your plants do survive the initial transition without a hardening phase, chances are their growth may be stunted by cold or shifting temperatures. We say, why risk it when hardening off your seedlings is easy?

When is the best time to harden off seedlings?

General planting for seedlings is recommended for after your area’s last freeze date. This is different for everyone depending on your climate and region. While gardeners in southern states may be able to move to harden off their plantings in early March, midwestern friends can experience freezes into mid-April.

You can learn more about freeze dates in your state or region by learning your planting zone. This information is readily available by zip code on USDA.gov.

How long will it take to harden off seedlings?

Hardening off of seedlings is not a one-size-fits-all process. Different plants need different lengths of time to make the big adjustment. However, a good rule of thumb is to allow yourself 14 days total before your planting date. Some seedlings may only need a week, while other may need longer. Plants that are about to endure harsher conditions may need more time to adapt.

How to Harden Off Seedlings (Step by Step)

Alright, now that you know exactly why and when you should harden off seedlings, let’s get into the nitty gritty. How you harden off your seedlings will determine their success in your backyard garden. So let’s get to work!

1. Start the Hardening Process Inside

Before you ever bring your plants outside, you can begin the hardening process. Stems growing indoors do not have natural wind to build their strength. There are a couple ways you can begin to the strengthen the stems of your seedlings indoors.

To simulate wind, brush your fingers gently through the stems of your plants a few times a day. Or, set up a gentle fan to blow on your plantings at different intervals. This will make sure that your seedlings are ready for the harsher winds of your garden.

2. Bring Them Outside On A Sunny Day

Your plants first exposure to the outdoor world should be gentle. Choose a sunny day above 50 degrees fahrenheit. Think of this exposure more as a field trip or mini vacation for your seedlings than anything else. On this first trip outside, place your plants on a high place out of reach from children or animals.

Place seedlings in partial afternoon sunlight, and allow them to sit for 2-4 hours, monitoring their moisture levels periodically. Ensure that your plants are not sitting in direct sunlight. Instead, choose a spot under a shady tree, or an eve. Direct sunlight at this stage may shock your plants. 

3. Repeat Exposure

For the next few days, repeat step one, bringing your plants outside into partial sunlight during the warmest part of the day, and monitoring their conditions. Gradually increase time by an hour or so each day. Still be sure to protect your seedlings from direct sunlight or strong winds, and do not set outside on days colder than 45 degrees.

4. Increase Sunlight Exposure and Duration

Once your seedlings have found their sea legs, or sun legs, so to speak, you can begin to increase the amount of sun they are receiving. Over the next two or three days, move your plantings into more direct sunlight. Start with 3-4 hours of direct sunlight, moving towards longer stretches.

After a few days, your plants should be acclimated enough to remain outdoors for a full day’s duration. Be sure to check soil moisture during this time, as young plants will dry out quickly in direct sunlight. 

5. Leave Seedlings Outdoors Overnight

Once your plants can endure a full day of direct sunlight, it’s time for them to go to a sleep away camp. That’s right, your plants can now be left on their own overnight. To ensure this step is successful, pick the night carefully.

Temperatures should not dip below 50 degrees fahrenheit overnight. Keep seedlings off the ground and out of reach of small animals or other damage. Repeat this process of leaving your plants outdoors for a full day and night cycle a few times, keeping your seedlings off of the ground.

6. Congratulations! You can now transplant your seedlings.

The day every plant parent dreams about! Your seedlings are officially ready to leave your home and try their luck in the big outdoors. To give your hardened off seedlings the best shot at an easy transition, plant on a temperate, cloudy day and water well.

Choosing a cloudy day will help ensure that your leaves are not shocked by sunlight and that they are able to maintain their moisture levels more efficiently. Hopefully, you will not have frost in your forecast. If you do, be sure to protect your plantings with a frost cover

Hardening Off Your Seedlings is Not as “Hard” as You Think

If you’re a first-time gardener, then the idea of transitioning your seedlings from the safe, weather-controlled environment of your home to the dangerous outdoor world can be intimidating. Thankfully, this process is easier than you may think.

Hardening off your seedlings is a process that relies on patience and consistency. With just two weeks of careful planning and attentiveness, your tiny seedlings can be ready to make the leap!