How To Harden Off Seedlings Before Transplanting

Hardening off seedlings is a critical step of successfully growing seeds indoors, and it’s one that many new gardeners miss. In this post, I’ll explain what it means and why it’s important. I’ll also tell you when to put seedlings outside, and show you exactly how to harden off seedlings, step-by-step.

Growing seeds inside is fun, and it allows us to get our hands dirty several weeks earlier than we can start digging in the garden.

After spending all that time caring for seedlings indoors, it’s exciting when the time finally comes to plant them!

But you can’t simply take them from their cozy indoor environment and plant them directly into the garden. You must first harden off indoor seedlings before planting outdoors, and this is a crucial step.

Don’t worry, I’ve got you covered. I will walk you through step-by-step to show you exactly how to harden seedlings below.

First, let’s answer some common questions, like what is hardening off, why is it important, and when can seedlings go outside.

What Is Hardening Of Seedlings?

Hardening off is the process of slowly preparing seedlings for the transition to the garden. When you harden off seedlings, you are toughening them up by acclimating them to life outdoors over a period of several days.

Transitioning seedlings from growing inside to outside

Why Do Seedlings Need To Be Hardened Off?

Why can’t we just plant our seedlings directly into the garden? Well, your seedlings aren’t used to the harsh outdoor environment.

Think about it. Seedlings live a very protected life inside the house. They’re used to perfectly warm temperatures, mild lighting, gentle waterings, and consistently moist soil.

If you put them directly into the harsh sun, wind, rain, and fluctuating temperatures outdoors, they would probably just shrivel and die. Eek!

It’s important to take the time to slowly acclimate your seedlings to the outdoors, so that they will be strong enough to survive being planted into the garden.

How Important Is Hardening Off Seedlings?

Hardening off seedlings is the most important step that you must take before planting indoor seedlings into the garden. Many new gardeners miss this step, and it’s one of most common causes of seedling death.

When you don’t properly harden seedlings, full sun can burn their tender leaves, strong winds can break their weak stems, and rain or hail can crush them.

Do I Need To Harden Off Seedlings?

YES, absolutely! Hardening off your seedlings is a crucial step, and one you definitely do NOT want to skip.

Sometimes we get busy and feel so rushed in the spring that it’s tempting to shorten or even skip this step.

But don’t! Always be sure to properly harden your seedlings, otherwise all that time you’ve spent babying them could be for nothing.

Hardening off plants and seedlings on my deck

When Should I Start Hardening Seedlings?

You can begin to harden off seedlings once the daytime temperatures are consistently above 50F degrees.

Start the process 7-10 days before you plan to plant your seedlings into your garden. Find out when to transplant seedlings into the garden here.

How To Harden Seedlings Step-By-Step

Before moving seedlings outside, remove the plastic cover from the seed tray. Be sure the seedlings have had several days to get used to living without the dome lids before moving them out.

To speed things up, you can use an oscillating fan to help to strengthen seedlings. Simply plug the fan into the same outlet timer as your grow lights, and allow it to gently blow over the seedlings during the day.

Also, if you recently potted up your seedlings, give them at least a week to recover before beginning the hardening off process.

Seedlings moved outside to a shady spot

Steps For Hardening Off Seedlings

Check the forecast before hardening off seedlings, and plan to start on a mild weather day. Also, it’s easier to start this on a weekend when you’re home during the day.

If at any time your seedlings start to fade, or turn brown, white or gray, then move them back into the shade right away. That means they’re getting too much sun, and you need to slow down the process.

• Step 1: Put seedlings outside in a shady spot – Begin by moving your seedlings to a shady, protected location. Be sure to also protect your seedlings from animals that may disturb or eat them. A porch or a front step with an overhang is perfect for this.

• Step 2: Protect seedlings from wind, rain and sun – Protect your seedlings from sun, wind and rain at all times for the first few days. So don’t put them out on a windy day or if there’s a storm in the forecast.

• Step 3: Bring them back indoors – Plan to leave your seedlings outside for only a few hours the first couple of days, then bring them back inside. Be sure to put them back under the lights while they’re inside.

• Step 4: Gradually introduce your seedling to sunlight – After a few days, begin slowly exposing your seedlings to sunlight (keep the shade-loving plants in the shade though). It’s best to start by exposing them to morning or early evening sun. Avoid the strong afternoon sun, or the leaves could get sunburn.

Gradually introducing seedlings to sunlight

• Step 5: Check soil moisture a few times a day – The soil will dry out much faster once the seedlings are outside, so check on them often. You may need to water them more than once a day. Allowing them to dry out slightly is fine. But never let them dry completely, especially to the point where they start wilting.

• Step 6: Repeat steps to harden off seedlings for 5-7 days – Each day you can leave them out a little longer, exposing them to more sunlight every day. So eventually, your seedlings will be outside all day long, and acclimated to the full sun.

• Step 7: Expose seedlings to the (gentle) elements – Light wind and rain are great for your seedlings at this point. So if it’s breezy, or there’s a light sprinkle, leave them outside so they can get used to the elements. Just be sure to remove the bottom trays while it’s raining so your seedlings don’t drown.

Seedlings drowning in a tray after heavy rain

• Step 8: Leave them outside overnight – Once your seedlings are out all day long, and the nighttime temperatures are consistently above 50F, you can leave them out overnight. But you’ll still want to protect them from strong winds, heavy rain, and hail. So be sure to keep an eye on the forecast.

• Step 9: Always protect seedlings from frost – Once they’re hardened off, cold hardy seedlings (like root crops, salad greens, and brassicas) can handle a light frost. However, if a hard frost is in the forecast, then it’s best to move them back inside rather than take a chance on losing them.

How Long Do You Harden Off Seedlings?

Allow 7-10 days to fully harden seedling, and don’t rush it. Once your seedlings are outside 24 hours a day for a few days in a row, they are hardened off ready to be planted in the garden!

My seedlings hardened and ready for transplant

Hardening off seedlings can be a bit of work. Moving them in and out every day is a chore, especially if you have a lot of seedlings. But taking the time to harden off seedlings will ensure that they will survive being transplanted to your garden.

If you’re ready to learn everything you need to know about how to grow your favorite plants from seed, then enroll in my online Seed Starting Course. It’s a fun, in-depth, self-paced training that will teach you how to grow any plant you want from seed! Enroll and get started today!

Otherwise, if you just need a refresher, then my Starting Seeds Indoors eBook is perfect! It’s a quick-start guide that will get you up and running in no time.

More Seedling Care Posts

• How To Fix Common Seedling Problems
• How & When To Thin Out Seedlings (Everything You Need To Know)
• How To Repot Seedlings Into Larger Containers

Share your tips or questions about hardening off seedlings in the comments section below.