How To Harden Off Plants So Your Garden Can Thrive


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How to harden off your plants and seedlings before planting them into your garden. Taking this simple step can mean the difference between a successful garden or seedlings that die from shock.

Hardening off plants simply means you are getting them used to outdoor growing conditions slowly over a week. This protects seedlings that have been growing indoors or in a greenhouse from a sudden shock that can kill young plants. Let’s take a look at what this means and how you can use it to get your plants off to a great start.

Why You Should Harden Off Seedlings

Think about it, after you have spent a lot of time indoors over the winter then suddenly start spending a lot of time outside in the spring what happens?

You often get sunburned or getting headaches from to much sun because you aren’t used to the strong spring sunshine and are spending more time outside then you have in a long while.

Well, your seedlings are the same way.

You’ve worked so hard to start your own seeds and care for the seedlings to start your garden.

How To Harden Off Your Plants So They Don't Die text overlaid on a photo of pepper seedlings.

Taking a little extra time now to harden off your plants before planting them outside can make all the difference to the success of your garden.

It’s important to remember that plants grown indoors or bought from a greenhouse have had no exposure to the sun or wind. These natural growing conditions are very hard on plants that aren’t used to it yet.

Plants need to be gradually introduced to the outdoors over a week or two before planting them into your garden.

Do All Plants Need To Be Hardened?

Hardening off tomato plants before planting.
Hardening off tomato plants before planting.

Yes, it doesn’t matter if you are dealing with young vegetable seedlings, mature flower plants, or houseplants you want to leave outside for the summer.

They all need to be hardened off so that they can slowly adjust to the outdoor growing conditions.

What Happens If You Don’t?

I know, it can be hard to be patient in the spring when you want to get your garden going. But it really is worth it to slow down a little and do things right.

If you take plants out of your home or greenhouse and plant them into the garden they have a good chance of dying.

They will start by wilting badly and going into shock. Their leaves can turn white from being sunburned.

At best this will slow down their growth at worst the plants you spent all that time growing or money buying will die.

What You Need

  • You’ll need a plant tray or box with low sides to carry your plants outside and back indoors. A strong plant tray works best so that you don’t have to keep moving the plant cells in and out of containers each time.
  • A sheltered place that has shade for at least part of the day and is protected from strong winds.

How To Harden Off Plants

Kale, onions, and cabbage seedlings being hardened off outside.
Kale, onions, and cabbage seedlings being hardened off outside.

7 to 10 days before you want to transplant your seedlings into the garden start is when you want to start the hardening off process.

Start by placing your seedlings in a shady place outside. This could be a covered porch or under a shady tree.

Some gardeners start off by doing this for a few hours right away, but I’ve found better results by leaving the plants outside for just 30 minutes on the first day. Then bring them back indoors.

The next day place them out in the shade again for 1 hour to a 1 1/2 before bringing them back inside.

On the 3rd day place them where the sun is a dabbled shade, for 1 1/2 to 2 1/2 hours.

Continue lengthening the amount of time they spend outside by 30 to 60 minutes a day and increasing the amount of direct sunlight that they get.

Once they can spend the full day and night outside without wilting they are ready to plant into the garden.

This process takes 7 to 10 days, sometimes longer if you get a sudden cold spell.

Once your seedlings have been fully hardened off you can leave the plants outside until you are ready to transplant them into the garden.

Tips For Hardening Off Seedlings

Garden huckleberry seedlings being hardened off before planting.
Garden huckleberry seedlings being hardened off before planting.

Young seedlings will need a little more care than full-grown flowers during the hardening off process.

Wind Protection

Vegetable seedlings grown indoors need to build up their strength to handle the wind.

You can start helping your seedlings with this by brushing your hand gently across the top of them every day while they are still indoors.

Another option is to set up a fan on low and have it oscillate across the plants an hour a day.

This helps to strengthen the stems of the plants to they don’t break in the wind.

When you are hardening off seedlings outside, make sure to shelter your seedlings so the stems don’t snap. You could place them on the side of a shrub or building that blocks the wind.

Water Well

The small cell packs that plants are grown in can dry out quickly in the hot sun. Keep an eye on the moisture levels and water often so the potting mix stays moist.

This will help your plants not to have stress from lack of water on top of their new growing conditions.

Pepper plants being hardened off.
Pepper plants being hardened off.

When To Plant

Once the plants have been hardened off and no longer show signs of wilting or stress after spending full days and nights outside you can safely plant them into your garden.

Remember you still need to follow normal planting guidelines. Frost tender plants like tomatoes can’t be planted into your garden before your last frost date unless you are giving good frost protection.

If possible try to pick an overcast day to transplant seedlings into your garden. This helps to reduce the shock plants feel after being disturbed.

Another tip I’ve found very helpful is to fill the planting hole with water or liquid fertilizer when planting.

Once I started doing this in our garden we never had seedlings wilting or being set back after planting outside.

Taking a little time every day to harden off your plants properly will save you a lot of time and money in the long run by keeping your seedlings happy and healthy.

So don’t think of this as a useless, time-consuming step and try to rush it.

Instead, think of your plants like the baby plants they are that just need a little extra care, in the beginning, to toughen them up and get them set on the right growth path.

Taking Plants In And Out Time
1 hour

Hardening Off Time Over A Week (hands off)
2 days 4 hours

Total Time
2 days 5 hours

Difficulty
Easy

Instructions

    1. Check your last frost dates before starting to harden off your plants. Some vegetables like onions and spinach can be planted out before your last frost date. While others like tomatoes need to wait until after your last frost date.
    2. Select a good place for your seedlings. You’ll want a place that is shaded and has some protection from the wind too. Under a tree, a covered porch, or in the shadow of your home can all work.
    3. On the first day place your plants outside for 30 to 60 minutes. Then bring them back indoors.
    4. Lengthen the time you are hardening off each day. Leave your seedlings outside for an extra 30 to 60 minutes depending on the weather. Gradually move them into a full sun area.
    5. Protect your seedlings from bad weather. If the temperatures drop or you are expecting a storm bring the seedlings back inside or cover with a row cover.
    6. Let them stay outside at night. After 7 days of hardening off during the day, you can start leaving the seedlings outside at night a little later each evening. Until they are handling the change well and can stay outside all night.
    7. Water the plants as needed. The small seed trays can dry out quickly in the sun. Make sure to keep them moist and water if you see the seedlings wilting.

Planting time!

  1. Once your plants have been hardened off and the weather is good, pick an overcast day to plant the seedlings.
  2. Dig a hole as deep as the plants’ root ball, fill it with water or liquid fertilizer and place the plant inside.
  3. Pack the soil down around the plant and gently firm it down.

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