How to Harden Off Seedlings

How to Harden Off Seedlings

Last Updated on October 31, 2021 by Phil

If you’re growing plants from seeds yourself at home learning how to harden off seedlings is a very important step to make sure that young plants are ready for the outdoor elements, be it sunlight, wind, rain, heat, cold and anything else.

Otherwise, they will experience shock and stress that will harm their health and ability to grow properly.

In this article, I’ll go through what hardening off is, why you need to do it and step by step processes you can easily follow to ensure your seedlings are well-adapted before you move them outside.

What Does It Mean to Harden Off Seedlings?

Hardening off seedlings is the process of acclimating them to the outdoor environment. This ensures that seedlings that have been started indoors survive when they are moved outside to the garden.

Because seedlings started indoors are used to being protected from the elements, they need to be gradually introduced to outdoor conditions in order to prevent sudden shock or stress that can hinder their growth or health.

By hardening them off, you help them gradually toughen up, so they are able to cope and get used to the harsher sunlight, heat, cold, rain, winds and other elements.




Why Harden Off Seedlings?

How to Harden Off Seedlings

Hardening off seedlings is important because plants that have grown in indoors have lived under controlled conditions. Here, you make sure they get the right amount of sunlight, temperature, humidity and moisture.

As a result, they are not as strong as plants that have had to go through the growing pains of developing their defenses against the elements. Similarly, they’ve never needed to fully develop their natural defenses.

Plants naturally develop a protective coating on their leaves. This is called the cuticle. It plays an important role in keeping water out as well as protect the plant from the sun’s ultraviolet rays while keeping moisture in to prevent dehydration.

Because they have been coddled, seedlings started indoors never fully developed this outer protective layer since the plant never need the extra protection then.

Unfortunately, once moved outside, it will need to build its defenses to survive and grow optimally amongst the elements.

Hardening off gradually exposes the seedlings to the elements. In doing so, it helps the plant build its resistance and develop its natural defenses in order to adapt to the harsher outdoor conditions.


What Happens If You Don’t Harden Off Plants?

Without proper hardening off, seedlings started indoors that are moved outside run the risk of experiencing transplant shock and stress.

Their leaves will likely get scorched since they’re not used to bearing the brunt of the sun’s rays for the entire day. If left there, you’ll eventually see the plant lose its leaves.

Similarly, these plants will have weak stems. As such, they’ll find it difficult to withstand strong winds.

Also, the bigger temperature fluctuations will place further stress on the plant. In fact, larger than normal sudden temperature drops not only can damage the plant but also kill it.

All these stressors will eventually weaken the plant making it susceptible to pests and diseases. As such, it will have a hard time growing and staying healthy.


Do You Need to Harden Off Plants Purchased from A Nursery?

In general, you don’t need to harden off plants you’ve purchased from the nursery. These plants are typically grown in greenhouses and moved outside.

In most cases, you’ll see the plants displayed outdoors or a semi-outdoor conditions. Thus, they’ve already adjusted to outdoor environment.

Additionally, most plants will go through some kind of moving process before they are sold commercially. This often includes propagation, transplanting, shipping, packaging and a few other steps. As such, somewhere along the way, they’ve been kept outside or exposed to the elements.

That said, it is always a good idea to check with the nursery.

In some cases, a visual inspection of where the put the plants will quickly give you the answer. Many garden centers and nurseries will display plants in full or part sun because there just isn’t enough space indoors. If this is the case, you’re sure that the plants have already been hardened.

However, if they are kept inside or under shaded areas, it is always worthwhile to ask. If in doubt, always harden them yourself. This is especially true for small seedlings.

The good news is, even if you do need to harden them, it won’t take as long to do so because they’ve adjusted somewhat to outdoor conditions. As such, it may only take 3 or 4 days to harden them off.


How Long Do You Harden Off Seedlings?

How to Harden Off Seedlings

On the other hand, if you’re starting from young seedlings what have been started indoors, you can expect to harden them off from between 1 to 2 weeks depending on the climate conditions.

The more extreme the weather is, the longer it will take because you want to let them adjust gradually. As such, there’s more timing involved if you live in areas with four seasons and where winters get frosty.

Ideally, you’ll want to start your seeds around spring so that they’ll be ready to start testing the outdoor environment when the weather starts getting warmer (after last frost date).

You also want to monitor nighttime temperatures because it is when the weather gets colder. It is a good idea to wait until the nighttime temperatures get up to the low to mid 50s (Fahrenheit) before you begin hardening cool-weather plants.

For warm weather plants, allow nighttime temperature to reach the high 50s or just around 60 degrees Fahrenheit before taking the plants outside.

Another option is to use cloches, cold frames or covers. This will allow you move the plants outside earlier despite the colder conditions because they are protected from the elements.


How Do You Harden Off Seedlings (The Easy Way)

Below are two different ways to harden off seedlings.

The first one is the traditional way where you move them outside for short periods of time and gradually increase the duration to get them acclimated to the outdoor environment.

The second method uses a cold frame if you happen to have one. This lets you start the hardening off process earlier since the cold frame can keep the plants warmer.

I go through both methods step by step in the sections below.

But, if you prefer watching a tutorial to reading it, check out this video.


How to Harden Off Seedlings by Gradually Exposing Them to Outdoor Conditions

It will take between 7 to 14 days to harden off seedlings. As such, it is a good idea to plan out the timeline if you have a target day by which you want to have the plants outdoors.

If you plan to transplant them during the spring, you’ll want to wait until the after the last frost date to do so. Of course, you don’t need to worry about this if you live in California, Florida or other places where it does not snow.

It is likewise important to know how cold hardy your plant is. The more it can tolerate the cold, the early you can take them outside. This can be as early as 2 to 4 weeks before the last frost date.

  1. Once the last frost date has passed., choose a warm day where the temperature it as least 50 degrees Fahrenheit. On this day, you begin the hardening off process by placing the seedling outside. Make sure to keep it somewhere protected from direct sunlight and wind. After a 1 to 2 outdoors, bring them back inside and keep them in a warm area.
  2. Repeat the same process, gradually increasing the time they spend outside each day. Go in increments of between 1 to 1.5 hours daily. This will help them get acclimated to the outdoor conditions slowly.
  3. Similarly, on each day, slowly increase the amount of sunlight they receive. Start from keeping them under shade when slowly exposing them until you get to dappled sun. Then gradually increase the amount of sunlight and wind as you go along.
  4. On days when the temperature suddenly drops or it gets very windy, keep them indoors. Or, place them outside protected from these harsher conditions.
  5. Keep in mind that this is a slow, gradual process where patience is key. Don’t get tempted to make sudden jumps like suddenly placing them under direct sunlight too soon. Otherwise, this will cause their leaves to get scorched.
  6. Once the nighttime temperature gets to at least 50 to 55 degrees Fahrenheit, you can leave the plants outdoors as well. By this time, they would have already been exposed to the outdoors for a few days.
  7. After 7 to 14 days of hardening off, the plants will be ready to be transplanted to the garden or container outdoors. Ideally, you want to choose a cloudy to do so and make sure that they get enough water.

A nice tip I learned from one of my gardening buddies is to keep the plants on a rolling cart, wagon or wheelbarrow. This makes it easy to move the plants back and forth, especially if there are quite a few of them you want to harden off at the same time.


How to Harden Off Seedlings in a Cold Frame

Another options it so harden off your seedlings in a cold frame. If you happen to have a cold frame, this will give you the luxury of starting the hardening off process earlier.

Once again, you’ll be starting about 7 to 14 days before the target transplant date.

  • Make sure to check the temperature in your cold frame. Ideally it should be between 50 and 55 degrees Fahrenheit as least. You do not want to start too early when the conditions are too cold. Similarly, avoid temperatures that are too hot for the seedlings. This comes in at around 80 degrees Fahrenheit.
  • In addition to temperature, check soil moisture. In warm, closed environments you don’t want the soil to dry out.
  • To allow the seedlings to adjust to the outdoor environment, open the cold frame for about 2 to 3 hours a day. Then close it again. This will allow the plants to slowly get acclimated to the conditions outside.
  • As each day passes, increase the time you keep the cold frame open by 1 to 1.5 hours.
  • Make sure to keep the cover closed at night. If nighttime temperature drops under 50 degrees, turn on heating.
  • After 7 to 14 days of gradually increasing exposure to the outside conditions, the plants will be ready to be transplanted. Again, choose a cloudy day and make sure the plants get enough moisture.


Last Thoughts

Learning how to harden off seedlings before you transplant them outside is a crucial step in making sure that young plants are well-prepared for the rigors of the outdoors. This allows them to stay healthy and grow optimally.

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