When & How To Harden Off Seedlings (5 Steps!)

Hardening off seedlings is a critical step, and it’s something that many new gardeners miss. The process takes a little time, but you definitely should not skip it.

Don’t worry, I’m going to make it very easy for you, and show you exactly how to do it the right way.

In this post, I’ll explain what it means and why it’s so important. Then I’ll tell you when to start moving your seedlings outside, and show you the exact process to properly harden them, step-by-step.

What Does “Hardening Off Seedlings” Mean?

Hardening off seedlings is the process of slowly acclimating them from being indoors to living outside, which prepares them for the transition to your garden.

When you don’t do it the right way, full sun can burn their tender leaves, strong winds can break their weak stems, fluctuating temperatures can cause them to wilt, and rain or hail can quickly crush them.

Why Do You Need To Do This?

If you don’t harden off your seedlings, they probably won’t survive the transition to your garden. That’s because they aren’t acclimated 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 and consistent lighting, gentle waterings, and evenly moist soil.

If you take them from their cozy indoor environment and plant them directly into your garden, they would shrivel up and die. Eek!

After spending all that time caring for your seedlings, you certainly wouldn’t want that to happen. That’s why it’s so important to slowly introduce them to life outdoors.

When To Harden Off Seedlings

You can start hardening them off 7-10 days before you plan on transplanting your seedlings into the garden, and once the daytime temperatures are consistently above 60°F.

I recommend starting on a weekend, or during a time that you’ll be home for the first few days. Also, do it on a week when there’s no rain or heavy wind in the forecast.

Preparing Them For Life Outside

A week or so before you plan on moving your seedlings outside, there are a few steps you should take to get them ready for the transition:

• Remove the plastic cover from the seed tray at least 1 week before moving them outside.

• To speed things up, I recommend using an oscillating fan to help to strengthen their stems. Simply run it on the lowest setting, and allow it to gently blow over your trays during the day.

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

• Remember to check the forecast a few weeks before, so you can plan to start putting them outside during a period of mild weather days.

How To Harden Seedlings

Step 1: Find a protected spot – Begin by moving your seedlings to a shady location where they will be shielded from any wind or rain. A porch, covered back patio, or a front step with an overhang are perfect for this.

Schedule For Hardening Off Seedlings

Here’s a sample schedule that you can use as a guide. But keep in mind that some seedlings may take longer to acclimate to the great outdoors than others will.

If at any time the leaves 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. So give them a few more days in the shade.

Tip For The Best Success 

• Protect them from pests – Be sure to protect your seedlings from any animals and bugs that may disturb or eat them. If you have pesky ground-dwelling pests, then put your trays on a table or other raised surface, so they can’t reach them. Otherwise you can put chicken wire or netting around them.

• 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 more than once a day. Allowing them to dry out slightly is fine, and it’s part of the hardening process. But never let them dry completely, especially to the point where they wilt.

• Always protect seedlings from frost – If there’s a chance of freezing temperatures in the forecast, then you should protect your seedlings, even if they are hardened off. It’s best to either move them back inside for the night, or cover them with a blanket or frost cloth, rather than take a chance on losing them.

• Gradually expose them to the (gentle) elements – Light wind and rain are great for your seedlings after the first few days of hardening them. So if it’s breezy, or there’s a sprinkle of rain, leave them outside so they can get used to the elements. Just be sure to remove the bottom trays so your seedlings don’t drown, and bring them inside during harsh weather or heavy rain.

How Long To Harden Off Seedlings

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