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How to Harden Off Seedlings

Hardening off seedlings is the last big step before seedlings make their way to the garden. Gradually introducing seedlings to outdoor conditions ensures you’ll have happy transplants.

In this post, I’ll talk about how to harden off seedlings, along with potential problems you may run into. Keep reading to learn more about this important step in gardening.

Trays of seedlings in the garden. Knowing how to harden off seedlings will keep your plants healthy and thriving.

Why Harden Off Seedlings

The term “harden off” may sound extreme, but the process is simple and easy. The purpose is to help your seedlings build a tolerance to living outside.

You start by letting your seedlings stay outside a little longer every day. You can even begin to leave them outside overnight. In turn, they will build strength against sun, wind, and outdoor temperatures.

Without hardening off, your seedlings may suffer from transplant shock. This occurs when your plant’s roots become stressed from being exposed to a new environment all at once.

Hardening off seedlings does require a little patience. Even though you’re ready to plant your garden, it’s best to set aside one to three weeks for this process.

You’ll be glad you did when you see your garden is thriving and healthy.

What You’ll Need

Before I outline how to harden off seedlings, let’s go over a few things you may need.

  • Something to set your seedlings in – This could be a large plastic or cardboard flat. It will make it easier to organize and transport your seedlings every day. You may even find it useful to invest in a rolling cart for this process.
  • Timer – This is an important item. There’s nothing worse than setting your seedlings outdoors only to forget about them a few hours later. Have a timer nearby, or set one on your phone to remind you when to move seedlings indoors.
  • Watering can – I’ll talk about this further down, but it’s important to keep your seedlings from drying out when outdoors.

How to Harden Off Seedlings

Finding the Perfect Spot

Hardening off seedlings is done in slow, simple steps. For the first few days, you’ll want to find a spot that is part sun.

Direct sun at the warmest part of the day can be overwhelming at first. For best results, look for a spot in your yard that receives morning sun and afternoon shade.

You’ll also want to protect your seedlings from high winds that first week. Find a spot that has a protective wall, or you can use a plastic mini-greenhouse.

After those fragile first days, you can start to introduce your seedlings to a spot in full sun. This will likely be more representative of your garden space.

Setting a Schedule

One of the most important factors in hardening off seedlings is sticking to a schedule. If you’re new to gardening, you may have no idea where to start.

I’ll go over two sample schedules for hardening off seedlings.

One-Week Schedule

The first schedule is one week long. This is perfect for fast-growing plants like cucumbers, tomatoes, and pumpkins.

  • Day one – Set outside for one hour in part sun.
  • Day two – Set outside for two hours in part sun.
  • Day three – Set outside for three hours in part sun.
  • Day four – Set outside for four hours in full sun.
  • Day five – Set outside for five to seven hours in full sun.
  • Day six – Set outside from dawn to dusk.
  • Day seven – Set outside all day and overnight.
Pumpkin seedlings sprouted indoors
Pumpkin seedlings.

Three-Week Schedule

The second sample schedule is three weeks long. I would recommend this schedule for slow-growing or finicky seedlings such as onions, artichokes, and cauliflower.

  • Week one – Start with setting your seedlings out for one hour in part sun on the first day. Add a half-hour to one hour every day.
  • Week two – In week two, focus on letting your seedlings get used to full sun. You can start with a few hours and then gradually add time every day.
  • Week three – In week three, your goal is to get your seedlings used to outdoor conditions both day and night. Leave them out all day at the beginning of the week. Then, begin to let them stay out overnight as well.

Caring for Your Seedlings

While hardening off seedlings, it’s important to continue your seedling care routine. You might even say it’s more important during this crucial time.

Because seedlings will be outside, they will eventually be exposed to the sun and wind. Both of these elements cause seedlings to dry out much faster than when they were indoors.

A dried-out seedling is much more likely to go into shock than a hydrated one.

As your seedlings begin to stay out longer, you should check on them every few hours for dryness. Keep a watering can nearby, so you can easily rehydrate your seedlings throughout the day.

What to Avoid

Seedlings being hardened off outdoors.

Let’s talk about what to avoid when mastering how to harden off seedlings. The three main things are extreme weather, bright sun, and fertilizer.

You can do your best to shield your seedlings from harm, but inevitably, they’ll have to endure the elements. That’s not to say you can’t use discernment when hardening off seedlings.

If you see a large storm moving into your area, go ahead and move your seedlings back inside. Their young, fragile leaves won’t stand up to a beating from hard rain.

Similarly, you could have an unseasonably hot, cloudless day in the forecast. Seedlings may need to be moved inside after a shorter time on these days.

Another thing you should avoid when hardening off seedlings is fertilizer. In theory, it may sound like a good idea to make your seedlings stronger. But, you could easily overwhelm your plants during an already stressful process.

Tips for After Transplanting

A row cover to protect newly planted seedlings.

After you’ve completed the hardening-off process, it’s time to transplant your seedlings to the garden. It’s a fun time to celebrate all your hard work so far.

But, it’s still important to use some care techniques to keep your plants happy.

If temperatures are set to be below forty degrees at night, use a row cover to protect your plants from frost. This is especially important for vegetables that aren’t cold hardy such as tomatoes, basil, and squash.

If your garden is spaced out, you can also use a large bucket or pot turned upside down over your seedlings. Just remember to collect them on your morning walk around the garden.

Troubleshooting Common Problems

What happens when hardening off seedlings doesn’t go as it should? Let’s look at some common problems.


Over-watering or under-watering is most often the cause of wilted seedlings. So, which one is it?

Unless you’re overwatering, there’s a good chance that your seedlings have dried out in the bright sun or wind. The only way to know for sure is to inspect the soil or growing medium below your seedlings.

Some vegetables will naturally experience some wilt in the sun. There’s no cause for major concern unless the seedlings still look wilted upon moving them back inside.

Just be sure to keep them hydrated throughout the day, and avoid full sun on hot days while hardening off.

A tomato seedling that is wilting.

Burnt Leaves

If you notice burnt leaves on your seedlings, there could be several culprits.

If the sun is much brighter than your grow light, it could cause leaves to burn. This is helped by sticking to a gradual schedule and keeping seedlings in part sun at first.

Applying too much fertilizer to seedlings can also cause leaf burn and stunted growth. To avoid this, wait to apply fertilizer until your seedlings are established in the garden.

Frequently Asked Questions

1. Should I harden off plants that I purchase from a nursery?

Flower seedlings at a garden center.

Plants purchased from a nursery don’t typically need to be hardened off. Most nursery plants are grown in outdoor greenhouses, where appropriate measures are taken to ensure healthy plants.

Perhaps you have purchased a plant from a nursery with a different climate than yours. You may need to spend a week letting your new plant adjust to your local conditions.

2. Do I have to harden off seedlings grown in my greenhouse?

The answer to this question depends on the temperature of your greenhouse. If you have a temperature-controlled greenhouse, you should still gradually introduce your seedlings to outdoor temperatures. They may not be as sensitive to the sunlight, but wind and rain will be new to them.

If you have a simple greenhouse with no heat, I would recommend following a one-week hardening-off schedule to get your seedlings used to bright sun and wind.

Wrapping Up How to Harden Off Seedlings

Trays of seedlings hardened off in the garden.

Learning how to harden off seedlings is an important skill for every gardener. It’s a crucial transition between growing indoors and starting your outdoor garden. You’ll reduce the chance of transplant shock and stunted growth.

If you found this post on how to harden off seedlings helpful, check out our Seed Starting page on the website. We’ll help you find the best products to use, while learning how to grow your favorite vegetables.