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Hardening off Tomatoes: Prepare Plants to Move Outdoors

Gardening enthusiasts often look forward to the tomato season with great anticipation. However, before these vibrant and delicious fruits can grace our tables, they must first undergo a crucial process known as “hardening off.” This process, often overlooked by new gardeners, is key to ensuring a healthy and productive tomato crop.

What is Hardening Off?

Hardening off is the process of gradually acclimatizing indoor-grown plants to outdoor conditions. It involves exposing young tomato plants, which are often started indoors or in greenhouses, to the elements outside. This transition is essential because plants raised indoors are used to stable temperatures, consistent humidity, and limited wind exposure. Without hardening off, these tender plants can suffer from shock when exposed abruptly to fluctuating outdoor conditions, leading to stunted growth or even death.

The Importance of Hardening Off Tomato Plants?

The primary reason for hardening off tomato plants is to increase their resilience. By gradually exposing them to sun, wind, and temperature fluctuations, they develop a stronger, more robust structure. This process helps in preventing sunscald, wind damage, and temperature stress, which are common issues when transitioning plants outdoors.

Hardening off tomato plants is a critical step in the gardening process, especially for those started indoors or in a greenhouse. This process is important for several reasons:

1. Preventing Shock

Tomato plants raised indoors are accustomed to stable, controlled conditions. The sudden change in environment when moved outdoors can shock these tender plants, leading to wilting, leaf burn, halted growth, or even death. Hardening off allows them to adjust gradually to the harsher outdoor conditions.

2. Building Resilience

Gradual exposure to the elements, such as wind and varying temperatures, strengthens the plant’s stem and overall structure. This resilience is crucial for the plant to withstand natural conditions like wind and rain once fully transplanted.

3. Enhancing Photosynthesis Efficiency

Indoor lighting, even grow lights, is less intense than direct sunlight. Hardening off helps plants gradually adjust to the intensity of full sunlight, which is vital for efficient photosynthesis and healthy growth.

4. Adjusting to Temperature Fluctuations

Indoor environments often lack the temperature fluctuations of the outdoors. Exposing plants to these changes gradually helps them develop the ability to regulate their processes, such as water intake and transpiration, effectively in varying temperatures.

5. Reducing the Risk of Diseases and Pests

Plants that are suddenly exposed to the outdoors without proper hardening off can become stressed, making them more susceptible to diseases and pest infestations. A gradual transition helps maintain plant health and immunity.

6. Preparing for Transplanting

Hardened plants are better prepared for the transplanting process. They are less likely to suffer from transplant shock, which can significantly set back growth and fruit production.

In summary, hardening off is a crucial process that significantly impacts the overall health, resilience, and productivity of tomato plants. It ensures that the time and effort invested in growing these plants from seed result in robust, flourishing plants capable of producing a bountiful harvest.

Is Hardening Off Needed for Nursery-Bought Tomato Plants?

Yes, even tomato plants purchased from a nursery should be hardened off. These plants, although often more mature, are still accustomed to the controlled environment of a nursery and need time to adjust to your specific outdoor conditions.

When to Harden Off Tomato Plants

The ideal time to start hardening off tomato plants is about 7 to 10 days before you plan to transplant them outdoors permanently. This timing largely depends on your local climate, particularly the last expected frost date in your area. Tomatoes are warm-weather plants and are sensitive to frost. Therefore, it’s crucial to ensure that the risk of frost has passed before they are moved outside for good.

Here are some guidelines to help you determine the right time:

1. Check the Last Frost Date

Find out the average date of the last frost in your region. This information is often available through local gardening clubs, agricultural extension offices, or online gardening resources.

2. Monitor Weather Forecasts

Even after the last expected frost date, it’s important to keep an eye on the weather. Unexpected cold snaps can occur, so be prepared to protect your plants or delay hardening off if necessary.

3. Consider the Temperature

Tomato plants generally prefer night-time temperatures consistently above 50°F (10°C). If temperatures are expected to dip below this, it might be best to delay the hardening off process.

4. Start Gradually

Begin hardening off by exposing the plants to a few hours of outdoor conditions, gradually increasing their time outside over a period of 7 to 10 days. This helps them adjust to the changes in temperature, sunlight, and wind.

Remember, the exact timing can vary depending on your specific climate and the current weather conditions, so it’s always a good idea to stay flexible and attentive to your local weather patterns.

Where to Harden Off Tomato Plants

Choose a sheltered location outdoors, such as a porch or against a wall, where plants can be protected from strong winds and extreme sunlight initially. As they acclimatize, you can gradually move them to more exposed locations.

Hardening off tomato plants requires a location that offers a balance between exposure to the elements and protection from extremes. Here’s how to choose the right spot:

1. Start in a Sheltered Area

Begin the hardening off process in a location that is sheltered from strong winds and intense direct sunlight. This could be a spot on a porch, under a tree, or beside a building. The goal is to expose the plants to fresh air and natural light without overwhelming them.

2. Consider Partial Shade

Initially, choose a spot that receives partial shade to prevent sunscald, which can occur if the plants are suddenly exposed to too much direct sunlight. A location that gets morning sun and afternoon shade is often ideal.

3. Gradually Increase Exposure

Over the course of the hardening off period, gradually move the plants to areas with more direct sunlight and less protection. This will help them acclimate to the conditions they will face in their permanent garden spot.

4. Be Prepared to Move Plants

Since the weather can be unpredictable, be ready to move your plants back to a protected area in case of sudden strong winds, heavy rain, or a cold snap.

5. Use Cold Frames or Cloches

If you have them, cold frames or cloches can be excellent for hardening off. They offer protection while exposing the plants to outside temperatures and light.

6. Ensure Accessibility

Choose a location that is convenient for you to access. Hardening off requires daily attention, including moving the plants and adjusting their exposure based on weather conditions.

7. Avoid High-Traffic Areas

Place the plants in a spot where they won’t be disturbed by pets or people. This reduces the risk of physical damage to the young plants.

By carefully selecting the right location and gradually increasing their exposure to the elements, your tomato plants will be better prepared for transplanting into the garden, leading to healthier growth and a more productive harvest.

How to Harden Off Tomato Plants: A Step-by-Step Guide

Hardening off tomato plants is a vital step in transitioning them from an indoor or greenhouse environment to outdoor conditions. Here’s a detailed step-by-step guide to effectively harden off your tomato plants:

Day 1:

  • Duration: Place your tomato plants outside for approximately 2-3 hours.
  • Location: Choose a sheltered, partially shaded area to protect them from strong sunlight and wind.
  • Observation: Monitor the plants for signs of stress.

Day 2:

  • Increase Duration: Extend their time outside to about 4 hours.
  • Adjust Location: Move them to a spot with a bit more sunlight.
  • Watering: Ensure the plants are well-watered to avoid drying out.

Day 3:

  • Extend Time: Let the plants stay outside for about 6 hours.
  • Gradual Sun Exposure: Increase their exposure to direct sunlight, preferably in the morning.

Day 4:

  • Outdoor Time: Aim for 8 hours outside, including more direct sunlight.
  • Monitor Weather: Be cautious of extreme weather; bring plants in if necessary.

Day 5:

  • Near Full Day: Leave the plants out for approximately 10 hours.
  • More Sun and Wind: Allow exposure to normal wind conditions to strengthen stems.

Day 6:

  • Full Day Exposure: Keep the plants outside for most of the day, slowly moving them to their eventual garden location.
  • Watch for Stress: Look for wilting or leaf burn and adjust exposure accordingly.

Day 7:

  • Overnight Test: If there’s no risk of frost, leave the plants outside overnight.
  • Acclimation: By now, they should be acclimating to the full range of day and night temperatures.

Post-Hardening Off:

  • Transplanting: After this week of hardening off, your tomato plants should be ready for transplanting into the garden.
  • Gradual Transition: If possible, choose an overcast day for transplanting to ease the transition.

Additional Tips:

  • Be Flexible: Adjust the schedule based on your local weather conditions and the plants’ responses.
  • Watering: Keep the soil moist but not waterlogged.
  • Protection: Be ready to cover the plants or bring them in if a late frost or harsh weather is forecasted.

Following these steps will help ensure your tomato plants transition smoothly from their controlled indoor environment to the varying conditions of outdoor life, leading to healthier growth and a successful harvest.


Hardening off tomato plants is a simple yet crucial step in your gardening journey. By following this gradual process, you help ensure that your plants transition smoothly from their sheltered indoor environment to the more variable conditions outdoors. This preparation not only bolsters their growth and resilience but also sets the stage for a successful and rewarding tomato season. Happy gardening!

John Michael
John Michael is a self-help writer and a hobby gardener. Michael’s passion in writing is to inspire the beginner gardeners to not just “hang in there” or “make it through” but to thrive. He does this through blogging.

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