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Seed Starting
Published January 2, 2024 by Nicole Burke

How to Harden Off Seedlings: Step-by-Step Guide

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The Final Step of Seed Starting Indoors Is Hardening Off Your Seedlings

I like to say that starting seeds indoors is an advanced gardener's skill. Sure, there are tons of advantages to starting your own seeds—you can get a head start on your growing season for the cost of a couple packets of seeds, and you can be in charge of the quality of these plants from the very beginning, to name just two.

But there are also some disadvantages. You have to set up a space indoors that tricks your plants into thinking they're outdoors, and you're setting yourself up for a bit of a time commitment as you play the role of Mother Nature. You're looking at one to two months of monitoring the growth of your little plants each day.

And then you face your biggest time commitment yet: hardening your seedlings off. Each tray will need to be transported outside so that those little seedlings can bask in the sun for a bit before being brought back inside. That's every day. For at least a seven-day period.

Let's get into the nitty gritty of hardening your seedlings off.

hardening off kale and swiss chard seedlings

What Does It Mean to Harden Off Plants?

The time has come to move those tiny plants you've nurtured for weeks outdoors so they can experience true nature.  

But you can't move them outdoors by simply planting them in the garden and checking “start plants” off your to-do list. Oh, no.

Think of a baby swaddled in a crib. They’re safe, they’re warm, they have everything they need handed to them, and they don’t have to fight for anything. Life inside the nursery is blissful. Now picture taking that baby and plopping them in the middle of a dirt field. That’s essentially what you’re doing to plants if you move them from their sheltered life inside straight out to the real world of the garden, without taking the time to harden them off first. Talk about transplant shock!

Hardening off is the gradual process of transitioning your plants to the great outdoors so that they’re ready when the time comes. The goal is to gently expose plants to the conditions they’ll experience full-time in the coming days. Direct sunlight, wide-open sky, wind, pests, fluctuations in temperature—all these are new to the plants no matter how well you imitated nature inside. Plants are living things with feelings, and they need time to get used to big changes, just like people do. 

Without being properly hardened off, plants can experience shock and die. So consider this a necessary task. If you're short on time, this is when you can recruit your children to help you!

Now, let's look at the steps to harden off your plant babies.

how to harden off tomatoes

When Are Plants Ready to Be Hardened Off?

Plants should get at least 25 to 30 days to grow indoors before you begin transitioning them outdoors. This matches up with the "4-6 weeks before last frost date" instructions you often see on the back of seed packages.

Ideally, your plants will have created their first true leaves and will be filling out their little cells or pots. If you don't move them out to the garden space soon, you'll need to pot them up (move them to a bigger container).

If you planned your seed starting right, your plants should be ready to go outdoors around the same time as the weather is ideal for them.

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Steps to Harden Off Seedlings

Step One: Toughen Your Plants Up

Before we harden our plants off, we have to toughen them up a bit. 

About a week before you’re ready to move seedlings out into the garden, stop fertilizing them and decrease their water. This will encourage them to produce a thicker mat of roots instead of really long roots. It also slows down their growth a bit. You don’t want your plant to be in the middle of a wild spurt of growth when—boom!—you surprise it with a move. 

If you know the soil in your garden will be ready for your seedlings in six week’s time, you’ll start hardening them off in five weeks and toughening them up in four weeks. 

harden off plants meaning

Step Two: Begin Hardening Off Process

Before you move a seedling outside, make sure the air is close to the plant’s ideal temperature.

If I’m ready to harden off a brassica like broccoli or kale, I’m not going to move the plant outside when it’s 20°F. Even though brassicas can withstand some frost and low temps, these seedlings have never been exposed to a temperature lower than what I set my thermostat to. The lowest I might consider exposing kale seedlings to is 30 degrees, but even then, I’d cover them with frost cloth. The same rule applies to heat waves over the summer.

In addition to not moving seedlings outside if temperatures are dipping into extremes, avoid precipitation: rain, snow, sleet, and hail. Keep your seedlings safe inside if even a little rain is in the forecast. Your goal is nice, mild weather so that you can give your seedlings the best change of success.

Okay, so you’ve waited for a dry day with the right temperature. Now it’s time to move your seedlings into the garden space near where they will be planted. For the first day, pick a shaded spot as close to where the plants will be planted as possible. I like to bring my plants outside during the early morning or late afternoon, but in the spring, you might need to opt for the warmest part of the day (probably midday) if it's still chilly.

Leave your seedlings there for four to five hours before bringing them back inside. Keep an eye on your plants while they're outside. If they start to wilt, bring them back inside.

steps to harden off a plant

Step Three: Increase Exposure 

Bring your plants outside each day for five to seven more days. As you move toward the end of the week, you can stretch the time your seedlings spend outdoors and move them into sunnier and sunnier locations. By day five or so, your plants can be exposed to direct sunlight. Set your trays in the actual beds they’ll soon be planted in if there’s room.

While you’re hardening off your seedlings, maintain a good moisture level. Plants will dry out much faster when moved outside and exposed to sun, wind, and (depending on the time of year) heat. Check on the soil as you bring your plants in each day and add water as needed.  

This is a great time to consider using garden covers, either frost cloth or garden mesh, to protect your plants as they transition to the great outdoors. I’ve found a lot of success using covers, whether I’m adding frost cloth to protect seedlings from colder temps or draping shade cloth to give plants an extra little barrier from sun and wind. These covers are also helpful in protecting young plants from insects when they’re most vulnerable to pest pressure. 

One more thing: I recommend setting a phone reminder each day to bring your plants back inside, where they'll be safe and sound. I can't tell you how many times I've woken up in a panic going, "Oh no! I left my baby Napa cabbages out all night!"

how long to harden off plants

Step Four: Move Plants to the Garden

By day seven of the process, your seedlings will graduate from the school of hardening off. They’re ready to move into the garden full time. 

Before transplanting them, check to make sure you’re not anticipating a big weather event. Make the big move on a calm day—no rain, no hail, no extreme wind. Give your plants time to settle in before they have to handle whatever Mother Nature might throw at them. 

When you're transplanting your seedlings, consider adding some mycorrhizae or earthworm castings to the soil. Dig a nice, wide hole and gently plant each seedling. 

Water your plants in well. Keep an eye on them for the next week. They'll need to be watered daily until they get better established in your garden space. Young plants are particularly vulnerable to pests like slugs and even bunnies, so be on the alert. These plants might have grown up and moved out of your house but they're still not adults, you know?

steps to harden off seedlings
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Time to Harden Off Your Seedlings!

Once your plants have moved to their new home outdoors, you can give yourself a huge pat on the back. You just completed a long and complicated gardening task. It hasn’t always been easy, but you made it this far! 

I hope this guide helps you have more success in starting your own seeds indoors. I know that learning how to transition my seedlings outdoors instead of planting them up the very first day the weather's nice was a game changer for me. After a couple of seasons going through these steps, you'll have a really good idea of the timing that works for you and your little plants!

Thanks for being here and helping to make gardening ordinary again!

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How to Harden Off Seedlings: Step-by-Step Guide