What Is Hardening Off Seedlings? (And How To Do It Right)


You have probably already put in a lot of time raising plants from seeds indoors, and now it is time to put them outside.  Many seed packets have instructions to harden off seedlings before transplanting them outside – but what does this mean, and how do you do it?

So, what is hardening off seedlings?  Hardening off seedlings helps young plants to make the adjustment to outdoor conditions gradually.  Hardening off seedlings helps them to get used to varying temperature, sunlight, wind, and water outside so that they can survive after transplant.

Of course, hardening off won’t happen overnight.  It takes time – about 7 to 10 days – so you need to be patient while your plants toughen themselves up.

Let’s take a closer look at hardening off seedlings, why to do it, when to do it, and how to do it.

What Is Hardening Off Seedlings?

Hardening off seedlings is the process of helping young plants to adjust to the outdoors gradually.  This avoids damage or death for plants by making a slow transition to outdoor conditions.

tomato seedling
Seedlings raised indoors are used to easy conditions, so they need a gradual transition to the outdoors. This transition is known as hardening off.

Hardening off is necessary for seeds that were started inside and are now seedlings.  The reason is that they are accustomed to living in a controlled environment with the perfect temperature, light, water, and no wind.

When seedlings are transplanted outdoors, they face several potential dangers, including:

  • Temperature fluctuations – nighttime temperatures can vary significantly from daytime temperatures.  This is especially true in early spring, when seedlings are transplanted outdoors.
  • Water loss – both wind and sun cause more evaporation from a plant’s leaves.  This can lead to rapid dehydration, especially if there is not enough water in the soil, or if the plant’s root system is not strong enough to take up water fast enough.
  • Sunlight – when a plant is suddenly exposed to lots of sunlight, it can cause scorching of the leaves.

Any of these can cause transplant shock, which can weaken or kill young plants.  Hardening off helps your seedlings to avoid these dangers by encouraging the following responses:

  • Growth slows – plants stop growing quickly, in response to fluctuations in resources needed for growth (water, nutrients, sunlight).
  • Natural wax on leaf surfaces gets thicker – this helps to reduce water loss due evaporation caused by heat, sunlight, and wind.
  • Cell walls develop more lignin – this makes a plant’s cell walls stronger, which helps them to stand up to wind or attacks by pests.
  • Freeze-prone water in cells is reduced – this makes plants more cold-hardy, and some even become frost-tolerant.
  • More carbohydrates are stored – this storage of sugar allows plants to survive without resources for longer periods of time.
  • More rapid root development begins – a larger and more extensive root system allows plants to take in more water and nutrients from the soil, and gives them a larger area to draw resources from.

For more information, check out this article on hardening off transplants from the University of Nebraska at Lincoln.

At the end of the day, these changes in response to the outdoors help plants to survive in tough conditions.  It also means that they plants require less frequent and less intense attention from you.

Do You Have to Harden Off Seedlings?

No, you do not have to harden off seedlings – they can still survive without hardening off.  However, each seedling has a much better chance of survival if you do harden them all off.

It seems like a waste of time and energy to nurture seeds for weeks until they are ready to transplant, and then let many of them die due to lack of hardening off.

tomato seedling
Seeds planted directly in the garden do not need to be hardened off, but you can still protect them with cloches or row covers if cold, snow, or high wind threatens.

Of course, any seeds that you plant directly into the soil in your garden don’t need to be hardened off.  However, you can still take steps to protect them from unseasonably cold weather.

For instance, plant them according to the seed packet instructions and pay attention to frost dates.  Also, use a cloche or row cover to protect them if late spring frost or snow threatens.

When Should I Harden Off Seedlings?

To find out when to harden off seedlings, first check the seed packet or the website where you bought the seeds.  You should be able to find the indicated transplant date (for example, two weeks before last spring frost).

frost
Pay attention to frost dates when transplanting – even hardened off seedlings may not be able to survive a frost if you transplant too early.

To find the frost date for your area, check out this frost date calculator from the Old Farmer’s Almanac.

According to the Penn State University Extension, you should plan on starting your hardening off process 14 days before the date listed on the seed packet.

This will give your plants time to get used to outdoor conditions, but also ensures that they are transplanted on time.  This is especially important in areas with short growing seasons (such as in the northern U.S.).

As an example, let’s say that you want to transplant tomato seedlings into your garden.  The Old Farmer’s Almanac suggests starting seeds indoors 6 to 8 weeks before transplanting outdoors.

They also suggest transplanting after the last spring frost date.  In my area, the last spring frost date is May 8.

So, I would want to start hardening off my seedlings 2 weeks before this.  Counting back 14 days from May 8, we get April 24.  This is the date I should start hardening off my seedlings.

Finally, there is one more caution about transplanting outdoors.  If you are going on temperature, remember that the air may be warmer than the soil!

How to Harden Off Seedlings

There are several steps you can take to harden off your seedlings properly.  This gradual change in conditions will help them to survive and thrive outdoors in your garden.

First, you will want to make it easy to transport your plants between indoors and outdoors.  So, keep them together for now – either in the seed starting tray or in a larger container for individual pots.

seed tray
Keep your seedlings together in a tray or container for now – that will make it easier to move them between indoors and outdoors.

Next, wait for a calm day without snow or strong wind.  On this day, move the plants outdoors to a shady spot.  You can put them under an umbrella or pergola, or in the shade of a tree.

Leave them outside for an hour in the morning, when the sun is not too strong yet.  After the hour is up, move them back inside the house, a heated garage, or a greenhouse.

On the Day 2, do the same thing, but leave the plants outside in the shade for two hours in the morning.  Continue this for the first week, until the plants are spending 7 hours per day outside in the shade.

During week two, start exposing the plants to more direct sunlight.  Put them out in direct sunlight for an hour on Day 8, and put them in the shade for the rest of the day.

sunlight through forest
After the seedlings have had a week outside to get used to the outdoors, start exposing them to some direct sunlight.

On Day 9, do the same thing, but leave the plants in direct sunlight for 2 hours.  Continue this for the 2nd week, until the plants are spending 7 hours per day outside in direct sunlight.

By the end of Day 14 (2 weeks), the plants should be accustomed to the outdoors.  The transplant date has arrived, and so it is time.

During this two week hardening off period, you should also stop fertilizing and decrease watering frequency for your plants.  You don’t want them to wilt, but you do want them to become accustomed to an occasional lack of water.  That way, they will develop a stronger root system and resistance to drought.

After two weeks of hardening off your seedlings, it is time to transplant them into your garden.  Handle the seedlings carefully, being careful to avoid damaging the roots.

After you transplant the seedlings, give the plants some water.  However, you should wait a few days to use a weak fertilizer, to avoid burning the plants.  They have enough stress to deal with as it is, so don’t over fertilize them now!

For more information, check out my article on over fertilizing your plants.

If frost, snow, or high winds threaten after transplanting your seedlings, then take steps to protect them.  Just because you hardened them off does not mean they can survive unseasonably cold or harsh weather.

water jugs
You can cut the bottoms off of plastic bottles to make cloches to protect your plants from cold while they are still young.

You could use a cloche to protect individual plants, and/or a row cover to protect an entire row against extreme weather events.

For more information, check out my article on how to protect your plants from cold and frost, and my article on how to protect your plants from wind and storms.

Watch the weather – avoid snow or heavy wind, but use an overcast day as an opportunity to get them used to the outdoors without so much sun all at once.

For more information, check out this article from the University of Maryland Extension on hardening off for vegetable seedlings.

Conclusion

By now, you have a good idea of what it means to harden off seedlings.  You also know why to do it, when to do it, and how to do it properly.

I hope you found this article helpful – if so, please share it with someone who can use the information.  If you have any questions about hardening off seedlings, please leave a comment below.

jonathon.david.madore

Hi, I'm Jonathon. I’m the gardening guy (not guru!) who is encouraging everyone to spend more time in the garden. I try to help solve common gardening problems so that you can get the best harvest every year!

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