Hardening Plants Before Transplanting

Now that some of my seedlings have several sets of “true” leaves, it’s time for the hardening process. This just means that I will be getting my very pampered plants used to being outdoors.

cabbage broccoli and mustard seedlings in trays
Some of my cabbage broccoli, and mustard seedlings

Up until now, these guys have been sitting at a comfortable 70(ish) degrees F (21 C), under constant light, with no fluctuation of weather or wind. If I were to plant them straight into my raised beds without acclimating them to being out in the elements and direct sunshine, they’d go into shock, shrivel up, and die.

Hardening plants is a gradual process. It takes several days of placing your plants outside and letting them adjust to their new environment a little at a time.

You have to be careful not to leave them too long, though, or all the work you’ve put into growing your plants could be lost in a single hour.

There are books that give fairly vague instructions for hardening plants. They say something like, put your plants outside for an hour on the first day, then two hours the next day, then three, and so on until your plants have withstood an overnighter. But I’ve found it to be a little more involved than that.

You really have to watch your plants, watch what they are telling you. An hour on the first day might be too long, especially if the sun is particularly harsh. Or, your plants might be fine pushing two hours right away. There really is no hard and fast rule.

Transplanting plants can be a tricky process. You want to make sure the plant is healthy and strong before you put it in the ground.

By following these tips for hardening off your plants, you can increase your chances of success and enjoy your plants for years to come!

Why Do You Need to Harden Off Plants?

When you grow plants from seed, they are accustomed to the relatively stable conditions found inside your home.

However, once they are ready to be transplanted into your garden, they will need to adjust to the more variable conditions outdoors. This process is known as hardening off, and it helps to ensure that your plants will be strong enough to withstand the rigors of life in the great outdoors.

Hardening off involves slowly exposing your plants to increasingly intense sunlight and wind. You will also need to reduce watering, as outdoor conditions tend to be much drier than indoor conditions.

By taking these steps, you can help your plants to adapt to their new environment and become much better equipped to thrive.

How to Harden Off Plants: General Guidelines

Here’s what I have found to work best when hardening plants…

When Should You Harden Off Plants?

Once your seedlings have at least two sets of true leaves (though 3-4 sets is better, if you aren’t pressed for time), they are big enough to be hardened.

I like to prepare my plants by making sure their soil is very moist before putting them outdoors. They can dry up very quickly in the sunshine.

Once your seedlings have at least two sets of true leaves (though 3-4 sets is better, if you aren’t pressed for time), they are big enough to be hardened.

I like to prepare my plants by making sure their soil is very moist before putting them outdoors. They can dry up very quickly in the sunshine.

1 month old mustard seedlings in trays
Mustard seedlings about 4 weeks old

Pick a fair weather day to take your plants outside. Not too windy, not rainy, not too hot. An overcast day works well as a starting point. I like to take my seedlings out either in the morning, or in the late afternoon, when the sun is weaker than it would be in the middle of the day.

Day 1

After placing the trays of seedlings out near the garden, I set a timer for 30 min. so I don’t get busy and forget all about my seedlings. (Been there, done that.) After a half an hour is up, I’ll go out and see how the plants are doing.

If they’re begging for mercy (wilting, droopy), I will bring them back inside and call it a day. If they look like they can hang a little longer, I leave them for another half an hour. I usually give my plants one hour max that first day.

Day 2

On the second day I do the same thing, but I set my timer for an hour. If after an hour they’re wilting, I bring them back inside again. If they look okay, I push it to two hours. It really all depends on how much your plants can handle.

Day 3 and Beyond

Gradually increase the plants exposure day by day in this manner. Don’t ever let it get to the point that the plants have completely fallen over.

If the stem is no longer erect, your plant isn’t going to survive the shock. Judge by the wilting of the leaves, and err on the side of caution. After weeks of babying these plants, you’d hate to push your luck and fry them in an afternoon.

5 weeks old broccoli seedlings
Broccoli seedlings about 5 weeks old

Steps to Transplanting the Seedlings

Ready to transplant? Besides hardening off your plants, there are a few other things you’ll need to do.

Amend the Soil and Prepare the Beds

Prior to transplanting seedlings, it is essential to amend the soil and prepare the beds. This will help to ensure that the seedlings have the best possible chance of taking root and thriving. The first step is to loosen the soil with a spade or tiller.

Next, mix in organic matter such as compost or manure. This will help to improve the drainage and add nutrients.

Finally, level off the bed with a rake and make sure that it is free of debris.

Water the Seedlings Well

Before transplanting your seedlings, make sure to water them well. This will help to minimize stress on the plants and give them a better chance of surviving the transplant.

To water seedlings, simply place them in a container of water and let them soak for a few hours. This will deliver moisture to the root zone only, where the plants need water moist.

After they have been watered, gently remove the seedlings from the container and transplant them into their new home.

Choose a Cloudy, Calm Day

One of the most important factors to consider when transplanting seedlings is the weather. If the forecast calls for a sunny day, it is best to wait until clouds roll in.

Seedlings are delicate and can be easily damaged by direct sunlight. The heat can also cause the soil to dry out quickly, making it difficult for roots to take hold.

However, on a cloudy day, seedlings will be protected from the harsh rays of the sun. The cooler temperatures will also help to keep the soil moist, making it easier for seedlings to establish themselves. As a result, transplanting on a cloudy, calm day is often the best option.

Dig Holes

Before transplanting seedlings, it is important to dig holes that are deep enough to accommodate the roots and provide support for the plant. The width of the hole should be two to three times the diameter of the root ball.

Remove Plant Carefully

Once the holes are dug, gardeners can gently loosen the roots and place the seedlings in the holes, being careful not to damage the root system. Place the plant in the hole, backfill with soil, tamp down gently, then water well.

Using an Unheated Greenhouse to Harden Off Plants

I have an unheated greenhouse, which I’ve been using to harden my plants. During the day, it has been really hot in there (over 80*), so I have to open all of the windows and let it cool down to between 60-70* before I can put my seedlings in there.

Otherwise, the heat itself would kill my plants. Broccoli and Cabbage in particular are cold weather plants, and they cannot withstand high temps as seedlings.

I’m using the greenhouse because we’ve had daytime temps in the 40s, and I’m afraid it would be too much of a shock to go from the warmth of our home to such a low temperature.

You could also use a cold frame in the same manner, just be sure to keep the lid open during the day so you don’t scorch your plants.

Other Tips for Hardening Off Plants

Here are a few more tips to help you harden off your plants.

Avoid Bright Sunlight

It is important to avoid sudden changes in temperature or exposure to bright sunlight, as this can shock the plants and damage their delicate leaves. Instead, harden off plants by placing them in a shady spot for the first few days, and then gradually moving them into sunnier areas.

Choose a Sheltered Location

When transplanting seedlings, it is important to choose a sheltered location. This will help to protect the young plants from wind and other harsh weather conditions. A sheltered location will also provide some protection from pests. In addition, a sheltered location will help to maintain humidity levels, which is essential for seedlings.

Don’t Forget to Water – but Avoid Downpours

During the growing season, it’s important to keep your plants well-watered. But when you’re hardening off seedlings, you need to be careful not to overwater them. Hardening off is the process of acclimating young plants to outdoor conditions, and it’s crucial to do this slowly and carefully.

Watering should be done in the morning so that the plant has time to dry off before nightfall. And avoid giving the plant a soaking – a light misting is all that’s necessary. If the weather looks like heavy rain, you may want to bring your plants inside so they don’t get drenched.

Introduce Them to Cool Nights Gradually

When you’ve finally reached the point in spring where you can set your seedlings outside to enjoy the sunshine and fresh air, it’s tempting to leave them out all day and night.

However, it’s important to take things slowly at first, and introduce your seedlings to cooler temperatures gradually. Otherwise, they may become shocked by the change in temperature and fail to thrive.

Begin by setting your seedlings out for a few hours each day, gradually increasing the amount of time they spend outdoors over the course of a week or two. Once they’ve acclimated to the outdoors, you can leave them out overnight without fear of damaging them.

Avoid Putting Plants on the Ground Where Pests are Problematic

When hardening off seedlings, it is important to avoid putting plants on the ground where pests are problematic.

This can damage the roots and make the plant more susceptible to pests and diseases. Instead, place the plants on a raised platform or in a container that is elevated off the ground. This will help to keep the roots safe and allow the plant to get the necessary airflow that it needs.

Don’t Rush Things

When it comes to gardening, there are a few golden rules that every gardener should follow. One of the most important is to never rush things. This is especially true when hardening off seedlings.

Hardening off must be done over the course of several days or weeks. If you try to rush the process, you run the risk of shocking the plants and damaging their delicate roots.

How to Harden Off Seedlings: Individual Plant Guidelines

Here are some guidelines for different types of commonly grown plants.

How to Harden Off Cabbage

Cabbage is a versatile cool-weather vegetable that grows best in temperatures ranging from 60 to 70 degrees Fahrenheit (16 to 21 Celsius).

Transplanted outdoors, cabbage prefers full sun but will tolerate some shade. If you live in an area with hot summers, it’s best to plant cabbage in the fall or winter so it has a chance to mature before the heat sets in.

Cabbage can be started from seed or transplanted from a nursery. If you start cabbage from seed, sow the seeds indoors about six weeks before the last spring frost date. Sow the seeds ¼ inch deep in flats or pots filled with moistened sterile potting mix.

Once the seeds have germinated, move the flats or plots to a cool location with temperatures between 60 and 65 degrees Fahrenheit until seedlings are ready to transplant outdoors.

Harden them off before transplanting them into your garden. To harden off plants, gradually acclimate them to outdoor conditions over a seven- to 10-day period. Start by placing the plants outdoors in a protected area and leave them out for a few hours the first day, then add more time each day.

How to Harden Off Broccoli Seedlings

Start by placing the broccoli seedlings outdoors in a sheltered spot for a few hours each day. Then, over the course of a week or two, gradually increase the amount of time they spend outdoors and exposed to direct sunlight.

Be sure to water the delicate seedlings regularly during this process. This will ensure adequate plant growth.

One thing that’s important to note when hardening off broccoli (along with cabbage) is that these seedlings can be placed outside a bit earlier than other types of plants.

These hardy plants are cold-weather crops and can tolerate a light frost. That said, seedlings are more fragile, so be sure to provide protection from windy days and freezing temperatures.

Hardening Off Tomato Plants

Tomato plants are delicate warm-season vegetables that require careful treatment when they are first transplanted outdoors.

To harden off tender plants, the first step is to choose a sunny spot in your garden and prepare the soil. Then, you will need to transplant your tomato seedlings into individual pots. Over the course of a week, slowly move the pots further and further from your house.

Each day, leave them outdoors for a few more hours, making sure to bring them inside at night.

Finally, water them well and fertilize them with a high-quality tomato fertilizer. With a little patience and care, you will have strong and healthy tomato plants that are ready to produce an abundance of fruit.

Side note – hardening off other warm-season crops, like cucumbers, peppers, squash, and melons, will follow the same steps as the ones above.

How to Harden Off Herbs

It is important to gradually acclimate your herbs to the outdoors before planting them in their permanent location.

Start by moving the herbs outdoors for a few hours each day, then increase the amount of time they spend outside over the course of a week.

You’ll want to pay close attention to the specific type of herb you are growing to find out how long or when it can be hardened off, since different herbs have different growing requirements.

Oregano, for instance, can be hardened off and transplanted in much cooler conditions than basil can.

Once they have acclimated to being outdoors, you can plant them in their permanent location. With a little care and patience, your herbs will soon be thriving in their new home.

What Happens if You Don’t Harden Off Plants?

If you don’t harden off your plants, they will likely experience shock when you transplant them into the garden. This is because they are acclimated to the relatively stable indoor environment and sudden changes, such as wind, humidity, and temperature fluctuations can be stressful.

In some cases, you might get lucky and see minimal transplant shock. However, this almost never happens. Transplant shock doesn’t always kill a plant, but it can severely diminish and delay your harvest.

The symptoms of plant shock include wilting, leaf scorching, and slowed growth. To avoid this, it’s important to gradually expose your plants to outdoor conditions over the course of 7-10 days before transplanting them.

By taking these steps, you can help your plants adjust to their new home and reduce the risk of transplant shock.

Do You Need to Harden Off Plants That You Bought from a Nursery?

If you’ve ever bought a plant from a nursery, you might have noticed that it’s usually kept in a sheltered environment, out of direct sunlight and strong winds. This is because plants that are grown in nurseries are typically quite delicate, and could be easily damaged by exposure to the elements.

As a result, it’s generally advisable to “harden off” plants before introducing them to their new environment, even if you’re buying nursery plants that may have already been outside for a while.

How Long Should You Harden Off Plants For?

Depending on the type of plant, the length of time required for hardening off will vary. In general, however, most plants will need to be slowly acclimated to outdoor conditions over the course of 7 to 10 days.

During this time, it is important to gradually increase the amount of time that the plants are exposed to direct sunlight and wind. The plants should also be protected from extremes of temperature, both hot and cold.

Once the plants have been hardened off, they can be transplanted into their final location outdoors. With proper care, they should then be well-suited to survive and thrive in their new environment.

Final Steps

When my plants are ready to spend the night outdoors, I’ll do it in the unheated greenhouse first (w/ windows closed to keep it somewhat warm still).

If they do well, the next night I will let them stick it out in the garden. As eager as I am to get them into their raised beds, I’m still going to take it slowly.

I’ve killed many seedlings in the past by pushing the hardening off process. It’s a terrible bummer, and I’m hoping to avoid catastrophe at all costs this time around.

I do need to get these guys out of the house now, though. I’ve got trays and trays of new seedlings popping up daily, which will require the limited space I have under grow lights. One of these days I’ll have a heated greenhouse, and I won’t have to use my indoor lights anymore. One day…!

That’s how I harden plants. Do you have a different, or better way of doing things? I’d love to hear what works for you!

9 thoughts on “Hardening Plants Before Transplanting”

  1. Hi there! Sorry to hijack an old post…
    I’m trying to troubleshoot my brassica seedlings…they never seem to grow well after their first true leaves appear. Do you fertilize yours at a certain stage with a certain fertilizer or have any tips for getting the little guys from the two small true leaves to the bushy seedlings? They have sunlight from morning until about 1pm and water to keep moist. I sowed them in seed starter and then moved them up to 4″ pots of half seed starter, half potting soil.
    Any ideas are much appreciated!

    • Hi Allison,

      I usually don’t transplant my seedlings into another pot unless they’re getting very big and the garden isn’t ready to be worked yet. I just let them continue to grow in the seed starting mix until they have several sets of true leaves. They do need about 8 hours of sunlight or grow lights. If they don’t get enough light they’ll start getting very “leggy”, long and scraggly looking, as they reach for the sun. I don’t fertilize the seedlings until they’ve been established in the garden. I hope that helps a little!

  2. I had never heard of this before and after reading the description I’m wondering if its because its not required where I live? I’m in gulf coast texas and by the time my tomato seedlings get true leaves it’ll be warmer outside my house than inside….so is this still something i need to do? It is quite windy down here so I could see getting them used to a breeze. I’d love some advice b/c I would hate to loose my plants….we planted a garden last yr and got nothing! SO far this yr we have watermelon canteloupe zucchini cucumbers carrots green beans basil and dill in the ground but we had a freak hail and thunderstorm 2 days ago, so my carrots got flooded and the garden got beaten badly by the hail..we also lost 2 chickens so I’m wanting to be extra protective of my baby maters!

    • Jaci,

      Definitely get them used to being outdoors for a few days before transplanting them. Your climate doesn’t really matter. They need to get used to the sunshine and the breeze, otherwise they’ll go into shock when transplanted. That doesn’t mean they’ll necessarily die… they might die, they might not… but shock will wilt them, stunt their growth, and can kill the plants. Better safe than sorry 🙂

  3. Before I take the seedlings outdoors, for a few days I run my hands lightly across the tops of the seedlings, first one direction and then the other with my palms down and fingers barely grazing the new plants. I do this several times each day, touching them a bit further down and more roughly each day.

    This process makes the stems toughen up so the plants will be strong from the beginning.

  4. I too have set plants outside only to forget them…….overnight…….in a storm. 🙁 Not a good idea. I will definitely be starting broccoli inside next time, I direct sowed my broccoli last time and it took forever for my plants to grow. I tend to try to harden my plants too quickly and stunt their growth. I get so excited about filling my raised beds. I have found peppers to be especially hard to harden. I guess learning to grow your own food really is a HUGE learning process. 🙂

    • It really is, Michelle! There’s so much to learn, and you can’t learn it all in one season, or one class. People think they can just keep “survival seeds” on hand and plant a garden when they need it in a survival situation, but they’re mistaken if they think it’s gonna be that cut and dry.


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