How To Harden Off Tomato Plants | Best Way In 7 Days

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Time to Read 7 Minutes.

It is essential to transplant tomato seedlings at the right time. Before moving tomatoes outdoors, we have to harden off tomato plants, which will produce healthy plants and many tomatoes to harvest. Generally, the best time for transplanting tomato seedlings is when it shows a couple of true leaves, and it typically falls 3 to 4 weeks after germination.

When transplanting tomato seedlings, you have to consider soil temperature, night temperature, and many more things. If the soil doesn’t have enough temperature, roots will not develop. Tomato plants are not hard enough to tolerate sudden climate changes, resulting in stunt growth.

In this article, I’ll discuss How to harden off tomato plants. When should I start hardening tomato seedlings? And what happens if you don’t harden off tomato plants.

Harden off tomato plants on windowsill
Young fresh seedlings on the windowsill for hardening.

What does it mean to harden a tomato plant?

Typically, tomato seeds germinate indoors and are exposed to a controlled indoor climate (protected from bright sunlight, wind, rain, cold, and extreme heat). Seedlings hardening off is the process that gradually introduces outdoor climate conditions to indoor plants without stressing the plants. This hardening will help the plant withstand harsh weather conditions like full sun, rain, intense heat, strong winds, and cool night temperatures.

When to harden off seedlings depends on plant maturity and minimum outdoor temperature. This hardening process is a vital step in moving tomatoes outdoors, and this will prevent tomato leaves from sunburn and stunt growth.



Why seedlings should be harden before transplanting?

All tomato seedlings should be hardened before transplanting in pots, garden beds, raised beds, or unheated greenhouses. Generally, tomato seeds germinate indoors in late winter, and the indoor room has a controlled temperature, and there is no sunlight or strong winds. Typically these seeds germinate under growing lights, and it is not intense as sunlight.

When new tomato seedlings are taken outdoors, it is exposed to intensive temperatures, strong winds, and direct sunlight. The seedlings harden process stimulating the plants’ natural defenses (leaf layers and wax layers) and allowing the seedlings to adapt to the new harsh environment.

These harsh environments can result in tomato leaves sunburn, a shock of cold wind, and broken stems and branches. This physical damage may result in stunt development and reduce the harvest. Furthermore, stunt growth plants are susceptible to many tomato pests and other diseases.

 Small tomato seedlings on a windowsill

Small tomato seedlings on a windowsill.


When to harden off tomato seedlings.

Hardening off seedlings depends on the plant maturity and the outdoor temperature (including daytime and night). When you see the first two or three true tomato leaves, plants grow 3 or 4 inches taller, and the outdoor temperature increases steadily over 60°F (15°C) in the daytime, and 55°F (13°C) at night is the best time to harden off tomatoes.

See also  When To Transplant Tomato Seedlings | Explain Everything

We can see these physical changes around 3 or 4 weeks after the seeds germinate. If you plant tomato seeds at the right time, you can start the process around one month. This warmer temperature is the best time for tomato seedlings to transplant.


Outdoor temperature.

The ideal temperature for tomatoes is over 55°F (13°C). If the temperature is below the ideal range, you may have to wait a few days for it to warm up. You can take your seedlings outdoors when the temperature rises above 55°F (13°C) (day and night) for several consecutive days.

If you notice a night temperature below 55°F (13°C), move your plant inside to protect them. The best temperature range for tomato plants is between 55°F and 85°F (13 – 30°C).


Sunlight is another critical factor when hardening tomato plants. In the first few days, make sure that your plants do not expose to direct sun. The filtered sunlight is best for the first couple of days, and it will prevent new seedlings from sun-scalding. Generally, I begin hardening off tomato plants on cloudy days. It is the best weather condition for new plants to accustomed to the intense sunlight outdoors.


Choose a day with a slight breeze. If the wind blows fast, do not expose the seedlings to the wind. Use proper wind protection like a mini greenhouse. Intensive winds can damage the newly grown seedlings’ stems.



How to harden off tomato plants.

The best way to harden tomato seedlings is to gradually keep them outdoors for a few minutes during the first week and bring them in. Gradually increase the time the plants are outdoors. From the third week, they can be left out overnight.

The steps mentioned below are the best way to harden off tomato plants.


Hardening Off Tomatoes (Steps):

  1. First day: Move the new tomato seedlings to a full shade area. Plants should not be exposed to direct sunlight. Verandas, balconies, and windowsills are ideal locations. Never leave seedlings outside at night. There is no need to move if the night temperatures are over 55°F (13°C).
  2. Second day: let the seedlings expose to the filtered sun for 20 to 30 minutes and move to a warmer place like a veranda, balcony,  heated garage, or basement. If the leaves start wilting, immediately move seedlings to warmer and full shade places.
  3. Third to fifth days: Gradually increase the filtered sun exposure day by day. Repeat it for five days.
  4. Seventh day: In the morning, introduce the seedlings to full sun for 30 minutes and move to the filtered sun area. Keep the plant outdoors overnight.
  5. Second week: Gradually increase exposure to full sun from day to day. Here it is advisable to increase it to one hour per day.
See also  Growing Tomatoes In Pots | How Big Of A Pot For Tomatoes?

Outdoor harden off tomato plants



How do hardening tomato seedlings weekend gardeners?

If you are not a full-time gardener and have only a little spare time on weekends to care for your plants, you can quickly harden off tomato seedlings. The following steps will help part-time tomato gardeners to harden tomato plants.

  • When the tomato seedlings are ready to harden off, use a fan to stimulate the outside winds. Do this two days before taking out the tomato plants.
  • On the first day of the weekend, move tomato plants outside for a couple of hours. Ideal locations are veranda, balcony, and windowsill. Do not expose new plants to direct sun and move inside at night. You can use a mini greenhouse to protect plants from winds.
  • On the 3rd day, move plants to the northern faces location under filtered sun. Move plants inside at night and let the plant sit for over a week in this location. If the night temp drops below 55°F (13°C), take the plants to a warmer place.
  • The following weekend, move tomato seedlings outdoors, expose them to full sun for 30 minutes to 1 hour, and move to partial shade for the rest of the day. Gradually increase the full sun over the weekend. Closely monitor the tomato leaves’ behavior; if you notice wilting, move to partial shade. If you use a mini greenhouse, remove it these days.
  • Move tomato plants to partial shade. I use the shade of trees where tomato plants can expose to direct sun for a couple of hours. Let the seedlings stay overnight to experience the night’s cold winds.
  • At the end of the second week, the seedlings are well hardened off and ready to transplant tomato seedlings outdoors.


Northern-facing locations generally do not expose to direct sun. Therefore as a part-time weekend gardener, that is the best location to harden off tomato plants.


Moving tomato seedlings outside
Moving tomato seedlings outside.


How long do tomato seedlings take to harden?

Generally, we can harden off tomato plants within 7 to 10 days, and we have to expose the new seedlings day and night for better results. No matter what USDA hardiness zone you live in.



How big should seedlings be before hardening off?

When you see the first two true tomato leaves, it is the best time to start hardening off tomato seedlings. It is generally 3 or 4 weeks after the seeds are germinated, and in this period, seedlings are grown 3 to 4 inches taller. No matter if the plant develops more than 3 or 4 leaves.

See also  Is Wood Ash Good For Tomatoes? | 5 Ways To Use Tomato plants


What happens if you don’t harden off tomato plants?

When the tomato plants are not hardened enough, they can sunburn tomato leaves and turn brown. Furthermore, plant stems need to be strong enough to withstand strong winds, and at night cold winds can damage the leaves. Finally, seedlings can die or are not strong enough to produce tomatoes.


What temperature to harden off tomatoes.

The best temperature for hardening tomato plants is between 55°F and 85°F (13 – 30°C). The optimum daytime temp is 60°F, and the minimum night temperature is 55°F (13°C). Below this temp range can damage the seedlings.


Do tomato plants need hardening off?

Tomato plants need hardening off before they transplant outdoors. Generally, in cold late winter, tomato seeds are germinated with weak indoor lights. When moving tomato plants outdoors, it exposes intense heat and bright sun. These harsh climate changes can damage leaves, like tomato leaves turning white. These physical damages cannot recover, stunt the plant growth, and severe damage can kill the plant.


How do you harden seedlings quickly?

The quick method to harden off tomato plants is to use a mini greenhouse. You can use a mini greenhouse for germinating tomato seeds, letting the plant grow in a warmer environment.

  • Move the plant to a full shade, warmer area on the first day. Temperatures should be over 55°F (13°C). Ideal 60°F (15°C).
  • On the second day, move the plant to 1 hour of full sun. After that, move it to a warmer area.
  • The 3rd day increases the sunlight exposure to 2 hours and a slight breeze. Keep watching for any leaves wilting.
  • On the 4th day, move plants outside early in the morning to expose them to morning cooler temperatures. Let the plant for 3 to 4 hours of direct sun and closely monitor leaves.
  • The 5th and sixth days increase direct sun exposure and keep the plant outside overnight.
  • On the 7th day, the plant is ready to transplant on the outside. You can refer to my how to transplant tomato seedlings article for more detail.



Do greenhouse tomatoes need to harden off?

The heated greenhouse has controlled temperature and breeze. Therefore it is not necessary to harden off greenhouse tomato plants. If you are planning to move the plants out of the greenhouse, you have to harden seedlings.

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