How to Harden Off Seedlings for Healthy Plants

Have you ever put in all the hard work of starting seeds indoors, only to see those seedlings struggle and fail in the garden? If you’ve been gardening for long, you know that you can’t just take seedlings and plant them directly outside. Taking the time to harden off seedlings is imperative to their success.

Whether you’re growing basil, broccoli, cucumber, tomato, or another crop, all seedlings need to go through a hardening off process to thrive in the garden.

I want to alleviate people’s concerns and worries about hardening off their seedlings. That’s why, in this blog, I’m sharing my quick, simple 5-step process for how to harden off seedlings that yields amazing results in a short period of time!

A Passion To Inspire

Here at MIgardener, I am passionate about inspiring people of all skill levels to grow gardens, live healthier lives, and have fun doing it! I hope to empower you to do that by making all the gardening know-how you need available to you.

That is why I have created these comprehensive seed starting guides that anyone can use:

• Crafting the Perfect Starting Mix for Seeds

• When to Start Seeds Indoors

• Cold Stratification

• Soil Blocking

• How to Transplant Seedlings

• When to Transplant Seedlings

• How to Start Seeds Indoors

Successful Seed Starting Indoors

Before you ever need to harden off plants, you must have plants successfully growing indoors. I hope you will use the resources listed above to master indoor seed starting.

A couple of techniques you can implement when starting seeds indoors will make the hardening off process much easier.

First, adding a fan to the grow room where your seedlings are is a great way to strengthen them. A fan makes it easier for your seedlings to adapt to outdoor growing conditions. Pro-Tip: Don't place the fan where the air blows directly on the seedlings. Place it in the corner of the room to circulate the air and add a gentle breeze to your space.

Second, the hardening-off process will be much simpler if endless rays don't spoil your seedlings at the beginning of their lives. It's tempting to leave grow lights on as long as possible to "jump-start" your seedlings. However, grow lights should not be left on all the time!

You want to simulate day and night for your seedlings and give them time to rest at the end of the day. This technique strengthens the plants and perfectly adapts them for life outside. Pro-Tip: I recommend leaving grow lights on for no longer than twelve to sixteen hours a day when starting seeds indoors.

What Is Hardening Off

"Hardening off" describes gradually exposing indoor-grown seedlings or plants to outdoor conditions. A gradual introduction allows plants to adapt to their new environment in stages.

By increasing the amount of exposure to wind, sunlight, and other outdoor conditions daily, you can eliminate transplant shock typically associated with transplanting them into their new home.

The Benefits of Hardening Off Seedlings

Properly hardening off seedlings benefits the overall health of the seedlings. Doing this gives our seedlings the best chance to succeed in their new home.

What Plants Need to Be Hardened Off

All garden plants started indoors need to be hardened off. People will tell you to make exceptions for certain plants when, in fact, the same process should be followed for ALL the plants you move outdoors!

Pro-Tip: Not only do our garden plants need to be hardened off at the beginning of the season, but so do house plants that love to spend the summers outside. Putting these warm-weather-loving house plants outside can be a great way to break them out of a stagnant growth pattern. Harden them off with the same process you would use to harden off garden seedlings. When choosing a location for your house plants, research each plant's preference for sunlight.

What Happens If You Skip Hardening Off

Hardening off is truly a crucial step in the seed starting process. Without the proper preparations, all of your hard work, time, and love can go to waste after a couple of hours in direct sunlight.

Seedlings not hardened off are at a high risk of sunscald/sunburn. You'll notice your plants are sunburned if you see white or silvery leaves on the seedlings.

Once the leaves burn, they will not return. If the burn is severe enough, the plant may die. Even if it survives and grows new leaves over time, you will still lose valuable growing time.

Don't prune away burnt foliage on sunburnt seedlings. Wait until the plant produces new growth. If you've already transplanted your seedlings too early, cover them to shield them from the sunshine until properly adapted.

Other things that can happen when you skip hardening off include transplant shock, snapped stems, bent-over plants, and uprooted seedlings. In other words, don’t skip it, or bad things can happen!

How Long Does Hardening Off Take

Typical gardening advice says to condition plants over a period of 7 to 14 days by bringing them outside for 2 hours on the first day and gradually increase the length of outdoor time by 2 hours every day after that. The problem with that is there are 24 hours a day, so this process takes about 2 weeks.

I just don’t have that kind of time. And honestly, your seedlings don’t either. They want to get out in the garden!

I prefer using the 5-step process I learned from my Grandpa, which takes only 5 days . It has never failed me in the 17 years I have been doing this!

How to Harden Off Seedlings

Here is my simple 5-step process for hardening off plants:

1. Start On a Cloudy Day for 4-6 Hours - Pick an overcast day with minimal wind for the first day of hardening off. Place the seedlings outside for 4-6 hours. As long as there is good cloud cover, your seedlings will not get sunburned in that amount of time. This will instantly jump-start the process.

2. Increase the Time by 4 Hours - On the second day, increase the duration the seedlings are outdoors to 8-10 hours under normal conditions (no extreme wind or sun), placing the seedlings next to a building on the east or west side. The building should allow exposure to the wind and the morning/evening sun but shield the seedlings from the harshest midday sun.

3. Increase the Time by Another 4 Hours - On the third day, leave the seedlings outside on the east or west side of a building for 12-14 hours under normal conditions (no extreme wind or sun).

4. Expose Plants to 1-2 Hours of Peak Sun - On day 4, place the seedlings in 1-2 hours of direct peak midday sunlight (but no more than that) and then move your seedlings into the shade. The plants can be outside for 16-18 hours today.

5. Full-Day Exposure - On day 5, the seedlings are ready for full-day exposure or transplanting into the garden. It’s that simple.

Pro-Tip: If you see extremely low temperatures or a storm in the forecast during your hardening off process, it’s a good idea to skip that day. You do not want to introduce them to that intensity during this time. Come back to your schedule after it passes.

It's also important to note that placing seedlings inside next to a sunny window is NOT a replacement for hardening off your seedlings outside!

When to Harden Off Seedlings & Transplant

It’s time to begin the hardening off process five days before you transplant seedlings into the garden. Successful plant establishment largely depends on knowing when the seedlings are mature enough to be transplanted.

When hardening off is done, avoid transplanting seedlings outdoors in the midday heat and sun. Instead, plant them early in the morning or evening after the day's heat has died down. Here is a step-by-step guide on how to transplant seedlings successfully.

Now that you’ve learned how to harden off seedlings, you can get your plants started strong this growing season. There is nothing more rewarding than looking out over an entire, healthy garden you started from seed!