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Secret Guide to Hardening off Plants

Wind, sun, and rain can easily destroy the tender or young plants; if not, some external forces, including pests, do not have mercy on them. That’s how the hardening plants come over!

Generally, hardening off plants is a transition period for the seedlings to adapt to the natural environment from indoors to outdoors. You can start hardening off the plants two weeks before the last frost date under optimum care and transplant them outdoors. 

Houseplant seedlings always find it hard to harden off when winter approaches. Thus, you need to make an additional effort when transplanting the plants in an optimum environment.

So, there will be no later for plants to die when hardening off fails. If you do not want it to happen, do not skip the article until you finish it!

Hardening off Plants Meaning

Hardening off might be new for some, while some might already do it to their seedlings and plants.

To be clear, hardening off is the treatment of the seedlings with a daily dose of the external environment for at least 7 to 14 days before transplanting them completely outdoors.

Ten to twelve clay pot with the seedling is lying over a table beside a window.
Hardening plants can even complete in 7 days to transplant the seedlings.

Usually, hardening off acclimatizes the plants to survive intense light, wind, rain, cold and hot.

This practice provides the seedlings a period to build a shield for themselves and helps trigger the plant’s defense mechanism.

Also, the process applies to vegetables, flowers, or herbs growing in a greenhouse or agriculture. 

How Important is Hardening off Plants?

Plants are grown indoors or in a greenhouse with extra support in a controlled environment compared to the outdoor environment, where the plants survive naturally. 

And the more the plants are away from the natural environment, the more important becomes the hardening-off process.

Shifting the plants directly into the rash outdoor environment makes your efforts go in vain as the plants may weaken and die on the spot.

Further details on the importance of hardening off are as follows.

  • Hardening off provides a gap for the weak and underdeveloped seedling to build a strong cuticle layer.
  • The process reduces the plant’s stress as it is gradual and slow.
  • It helps prevent the transplant shock to the seedling and establish the plant successfully outdoors.
  • As hardening protects the wilting and breakage of the plant, it allows the plant to attain full growth after transplanting.
  • Hardening off allows you to enjoy the green leaves as the plants do not suffer from sunburn after the transition period.

When to Start  Hardening off Plants?

Hardening off is inevitable for protecting the plants from the seedling stage.

In general, seedlings are to harden off two weeks before the last frost date or early spring, depending on the crop you grow.

If I am to share my experience, I started hardening off Broccoli on the 20th of February, nearly two weeks before the last frost date.

Initially, on the very first day, I saw my seedlings droop when it was just 2 hours outdoors, which took the hell out of me.

But later, I found it was normal, and my plant was trying to adapt to the outdoors in the given period.

Here I have listed some of the plants with their starting date of the hardening-off.

VegetablesBest Time to Harden Off

Asparagus, Collards

Kale, Lettuce, Mustard

Can tolerate hard frosts for a certain period

Start hardening off 4-6 weeks before the last frost date.

Beet, Broccoli

Cabbage, Carrot, Cauliflower

Can withstand light frosts

Start hardening off 2-3 weeks before the last frost date
Snap Pea, Sweet Corn

Get easily killed by frost and do not germinate in cold soil.

Start hardening on or after the last frost date.

Lima Beans, Cucumber

Eggplant, Muskmelon, Okra


Cannot withstand cold temperatures

Need warm soil for germination

Start hardening off 1-2 weeks before the last frost date

How to Harden off the Plants?

Hardening off is a transition period for the plants to become familiar with the natural conditions. If you want your plant to harden off successfully, consider the following steps. 

  • Choose a day with no wind and rain based on the weather the respected plant demands (consider the table above).
  • Keep the seedling outdoors, away from the reach of direct sunlight, preferably in a shaded area for 2 hours.
  • Later, bring the plants back indoors and place them in a place providing warmth as a refrigerator or heated garage.
  • For the next day, keep the plant under the filtered sun for an hour and in the shaded area for two hours.
  • On the third day, increase the time of the filtered sun by an hour. And keep the plants under the sun for at least two hours and two to three hours in the shade.
  • Keep young and weak seedlings indoors if the day is windy or rain pours like cat and dog.
  • Following the process, you can keep the healthy seedlings in the direct sun for at least six to seven hours on the fourth day.
  • However, do not keep young seedlings in direct light to prevent burning and scorching of the leaves and avoid frost conditions.
  • And if the temperature stays constant at 50ºF and does not fluctuate during the day and night, you can leave the seedlings overnight outdoors.
  • Meanwhile, you can continue the same shifting process for at least ten days and keep the seedlings outdoors for at least 8 to 10 hours.
  • Now, on the eleventh day, you can keep your seedlings outdoors for 24 hours without any movement.
  • Lastly, transplant the seedlings into the ground directly, where the plants will continue their life. 

Alternative Ways for Hardening off Plants

Hardening off gets easy if you have free time to tend the plants at the desired time, but that might not be possible for everyone.

We might be away from home during the day or might not have a suitable window shelf or area for hardening plants.

However, plants need hardening only to handle the stresses.

And for that, you can mimic the situation by trying something different to trigger the defense of seedlings.

  • Create water stress by letting the seedlings dry out by maintaining certain intervals between watering but do not let the sands have cracks and become dusty.
  • Induce a windy condition by using a fan but do not give a direct hit to the seedling. You can use a medium oscillating fan to mimic the natural breezes and winds.
  • Rub the top of the plants using your hands to strengthen the stems of the seedlings.
  • If the temperature fluctuates, try moving the pot to warm spots.
  • You can also try to keep them in a sunny window for a certain time and move to a shaded patio in the house.
  • Using a cold frame as a greenhouse can also work wonders for hardening off as it does not require regular movement.

However, learn that you might need to move your seedling outdoors once in their growing phase.

For that, you can keep the pot outdoors in the natural environment during weekends or after office hours once in a while.

If the outdoor condition does not meet the care requirements, you can give the seedlings a day or two off. So you do not need to start over from the beginning.

Tips for Hardening off Plants 

Nursery and bought-in plants may also need hardening off, in case they are from the sheltered condition. To ensure safe and proper hardening plants, leverage the tips below.

  • Keep a wheelbarrow or a flat tray to shift the seedling from indoors to outdoors and vice-versa.
  • Use a frost blanket to protect the seedlings during the night if you want to harden off the plant faster than expected.
  • A plant protection blanket can also be an alternative to protect your seedling during hot sunny days.
  • Keep the seedlings in the area away from the reach of pests and birds, as they can knock them off and nibble on the leaves.

Tips to Care Plants after Transplanting

You might have guessed that transplanting and hardening off are different phenomena, but one follows the other.

Look at the tips below to care for the seedlings after completing the transplantation.

  • Depending on the variety, the plant requires at least six to eight hours of direct light.
  • Although most can thrive in low humidity, keeping it above 50% may work.
  • The difference between the day and night temperature should be 10ºF, similar to a greenhouse, with the minimum daily temperature above 50ºF.
  • Use room temperature water to hydrate the soil thoroughly. Provide the next watering only by checking 2 inches of soil from the above.
  • It is better to use organic fish emulsion at the time of transplanting and once every week, as stated in the news from Iowa State University.
  • You can also use all-purpose fertilizer after 4-6 weeks of transplanting, but try to avoid nitrogen pollution.
  • The soil needs to have the capacity to retain moisture but also be well-draining with a pH between 6.0-7.0.
  • Avoid pruning the plant’s top growth after transplanting to allow the full development of the plant.
  • You do not need repotting for the first growing season as you have transplanted the seedlings in their permanent home.
  • Young seedlings are more prone to pests such as fleas, beetles, cutworms, and aphids. So you need to control them with insecticidal soap and neem oil spray.

Learn more about transplanting Daffodils, Iris, Daylilies, Hyacinth Bulbs, Ferns.

From Editorial Team

Avoid the Damage to Plants from Cold Temperatures!

Cold temperatures, even at the late frost, can kill the plants. So, you need to prune the damaged parts of the plant and resume hardening. 

Also, common pests, including greenflies, populate unnecessarily, needing infestation control.  

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