Hardening Off Tomato Plants: Your Key to Robust Outdoor Tomato Growth

Imagine nurturing a tomato seedling indoors, watching it sprout and grow, and then the day arrives when it’s time for its grand adventure outside. This key transition phase is dubbed ‘hardening off,‘ and doing it correctly is as critical as the nurturing phase itself.

This article explores the fundamentals of hardening off tomato plants, arming you with essential tips and hacks to protect your verdant charges from damage.

We’ll uncover the ideal timing to kickstart the hardening-off process, share knowledge on averting sun scald, discuss successful strategies for pest deterrents, and so much more.

Buckle up for this exciting journey from the coziness of indoors to the exhilarating world outside!

What Is Hardening Off?

So what is hardening off exactly? We go through this process with many plant types that are started from seed indoors, including tomatoes, peppers, eggplants, and others.

Hardening off is gradually adjusting an indoor plant to the outdoors before transplanting permanently outside. This slow acclimation to direct sunlight, wind, rain, and temperature fluctuations is crucial to avoid damaging your tender tomato plants.

Why Bother Hardening Off?

Putting your tomatoes outside in direct sunlight may be tempting, but it is not recommended, especially if you started your plants from seed.

The result is sun scald, sunburn on your plant’s leaves. These leaves will become hardened and eventually die and drop off, potentially causing a setback in your plant’s growth by several weeks.

Indeed, the process of hardening off is crucial for tomato plants. Fortunately, the procedure is straightforward, albeit a little time-consuming. It necessitates patience and extra effort to reposition the plants for several weeks.

When To Start Hardening Off Tomatoes

After understanding the concept of hardening off, the next step is determining the appropriate timing to start the process with your tomatoes. This timing will differ depending on your location, primarily influenced by temperature.


If temperature were not a factor, there would be no need to begin tomato seedlings indoors. Starting early gives the plants a head start on the season, resulting in more room for growth and productivity.

Outdoor temperatures determine the timing of when to begin hardening off tomato plants. If you plant your seeds properly, usually 4-6 weeks before the last frost, you can start setting the plants when they reach approximately 1 month in age.

During this time, outdoor temperatures typically reach the high 60s and low 70s (°F), making it ideal for growing tomatoes. Once the outdoor temperatures warm up, you can start gradually introducing your young tomato plants to the outdoors during the day.

It is recommended to gradually expose plants to outdoor conditions for approximately 2-3 weeks before nighttime temperatures consistently reach above 55°F (13°C). This allows the plants sufficient time to acclimate to the various outdoor elements.

If the temperatures are forecasted to drop below 55°F overnight, it is advisable to bring your tomatoes indoors. Once the temperatures rise again in the morning, you can resume hardening them off.


Direct sun exposure is an important factor to consider during the hardening-off process for tomatoes, as it exposes plants to radiation that indoor grow lights and filtered sunlight do not.

Choosing a cloudy day for the first few days of hardening off is recommended to reduce the chances of sun scalding on tomato leaves. Filtered sunlight is gentler in comparison.

Placing plants in shaded areas with less cloud cover is recommended. This allows tomatoes to receive sufficient light while acclimating to outdoor conditions such as wind and temperature fluctuations.

Other Tips

In addition to temperature and sunlight, there are other factors to consider during hardening.

Animals are known to eat young tomato foliage, so keeping your tomatoes elevated on a bench or table as they acclimate is recommended. This precaution will help protect your vulnerable plants from rodents and rabbits.

Wind can pose a challenge for young tomato plants as they lack strength. It is advisable to avoid particularly windy days during the initial week of hardening off. Another option is to use a small stake to support and keep the tomatoes upright during their early stages.

How to Harden Off Tomato Plants?

How to Harden Off Tomato Plants

Hardening off requires patience and careful consideration of the weather. It is important to properly acclimate tender tomato seedlings before exposing them to extended periods of sunlight.

Hardening Off Tomatoes (Steps):

  • Choosing a cloudy, warm day is recommended to start the hardening-off process. During the first 2-3 days, selecting cloudy days or providing shade for the plants to avoid direct sunlight is advisable. Once this initial period is over, the plants can be gradually exposed to more sunlight.

During the initial hardening-off period, selecting overcast days or providing shade for the plants to protect them from direct sunlight is recommended. Following this period, you can gradually increase their exposure to sunlight.

  • It is recommended to avoid cold temperatures when hardening off tomato plants. Typically, tomato plants should be started to be hardened off around 2-3 weeks before outdoor temperatures consistently reach above 55°F. During the day, temperatures should ideally be around 70-80°F for the plants to adjust.

Generally, tomato plants should gradually acclimate to outdoor conditions about 2-3 weeks before temperatures consistently exceed 55°F. It is recommended to expose them to daytime temperatures of around 70-80°F for optimal adjustment.

  • Increase sun exposure gradually by moving the plants into the sun for 20-30 minutes. If the leaves wilt, move them back to the shade until they recover. The following day, increase sun exposure by another 20 minutes, monitoring for any signs of stress.

Shade the plants for a few days. Then, move them into the sun for 20-30 minutes. After that, put them back in the shade. If the leaves wilt, move them into the shade until they recover. The next day, increase sun exposure by another 20 minutes and monitor for signs of stress.

  • After 2-3 weeks, gradually increase the sunlight your plants receive daily. Once this adjustment period is complete, you can safely relocate your tomatoes to their permanent outdoor location, as long as the nighttime temperatures stay above 55°F.

The plants can be permanently placed outside after gradually increasing sun exposure over 2-3 weeks. As long as the nighttime temperatures remain above 55°F, moving the tomatoes outdoors for the season is safe.

Tips For Hardening Off Tomato Plants!

Harden Off Tomato Plants

During the hardening-off phase, some additional tips can be useful and potentially save a struggling tomato plant.

Use Shade Cloth

When moving your plants to a full-sun location, hardening them off in a full-sun location is important. However, if your garden is in a different place than where you live, it may be challenging to fully harden off the plants before transplanting them to their final spot.

I use fabric to help the plants adjust slowly in full sun.

I used to live in an apartment, but my in-ground beds were in a community garden plot across town. As a result, my plants had to transition from partial sun to full sun, which could cause sun-scald.

As a solution, row-cover fabric provided partial shade for the plants after transplantation, extending the hardening-off period. Once the plants have been in the ground for one to two weeks, the shade cloth can be removed to expose the tomatoes to full sunlight.

Shade cloth can protect tomatoes from animals such as rabbits and mice and cold weather overnight.


Staking can be beneficial in protecting your tomatoes from the wind. It is recommended to stake the plants when they are placed outside to prevent any potential damage to stems or branches.

If your plants are still small, prop them up with sticks or food skewers. These will work until the plants are in the ground, where they can be supported with a larger stake.

A useful tip is to use garden velcro, which is easy to use and gentle on plant stems.


When your tomato plants transition to an outdoor environment, they consume more water than their indoor needs. This increased water use is largely due to wind, accelerating water loss via leaf transpiration and soil evaporation. Additionally, elevated temperatures and direct sunlight outdoors can exacerbate this condition.

Be vigilant for signs of dehydration, such as wilting leaves, and frequently assess the weight of the pots to determine if the plants need watering. Strive to maintain consistent watering and ensure they do not completely dry out.

Leave a Comment