Before you plant your tomato transplants outside, whether in containers or in the ground, they need to be prepared to live outdoors. The way that you prepare them, is to harden them off.
Hardening off is a simple but crucial step in raising strong, healthy, viable tomato plants that will survive and thrive in your garden.
Jump to:
- What is “Hardening Off”?
- Why Harden Off Tomato Transplants?
- What happens when you don’t harden off tomato plants before you plant them outside?
- How Long Does it Take to Harden Off Tomato Plants?
- How to Harden Off Tomato Plants, Step by Step
- A Few Tomato Hardening Tips:
- Does Hardening Off Make Tomatoes Frost Hardy?
- Are Tomatoes the Only Plants That Need Hardening Off?
- Do Greenhouse Tomatoes Need to Be Hardened Off Before They Are Planted?
- Do You Need to Harden Off Tomato Plants You Buy from a Garden Center or Nursery?
- Hardening Tomato Transplants Pays Off in the First Few Weeks of Growing
What is “Hardening Off”?
Hardening off is the process of acclimatizing tomato seedlings that are started indoors to outdoor conditions.
It is a gradual introduction to life outside.
A simple explanation is that you bring tomato seedlings outdoors for small snippets of time, gradually increasing until they are ready to live outdoors full time.
Why Harden Off Tomato Transplants?
It’s important to harden off tomatoes that were started indoors because the conditions inside are much different and more stable than the conditions your plants will experience outside.
Indoors, the conditions in which your tomatoes live are very stable. They basically do not change. The temperature is always the same; the light is the same heat and intensity; there is always just the right amount of water; and the wind doesn’t blow. Or, if there is wind, it is light and moderate, steady, and constant -- much different than the changeable wind conditions outside!
Once your tomatoes are living outside, they will need to be able to handle a variety of changing conditions. Those conditions can change by the day, or even by the hour!
And, even though your tomatoes were grown in light indoors, that light will never be as hot, strong, or as intense as the sun. Even if your plants are grown in a sunny room, the light is filtered, and it’s not the same.
Tomato plants need a gradual introduction to living outdoors so that the sudden changes and variations in conditions don’t shock or damage them.
Hardening off thickens plants' stems and leaves. It also builds up the waxy coating on leaves, which gives them more protection against strong sunlight and helps reduce the moisture lost through evaporation and drying out.
What happens when you don’t harden off tomato plants before you plant them outside?
Tomatoes that are not properly hardened off can
- Experience shock, which stunts and sometimes stops growth, but will often at least delay it
- May die
- Break stems from wind or strong rains
- Lose moisture too quickly, damaging leaves and stems, and possibly causing death
- Wilt from the higher heat and intensity of the sun
- Burn from the sun’s strong rays
- Lose leaves to burning, which slows and may stop photosynthesis, and can cause the tomato plants to die, because they have no way to feed themselves
How Long Does it Take to Harden Off Tomato Plants?
Hardening off takes about two weeks. These can be the last two weeks of the growing period before you plan to plant, if you build it into your gardening schedule.
You can even start hardening off tomatoes during the day if you are still experiencing frosts at night, or if it is before your expected last frost date.
The plants are not meant to live outside all day and night when you first start hardening them off. So, as long as they are brought in before temperatures fall to freezing, it’s fine for the early days of hardening off to be done while there is still a risk of frost at night.
Just make sure you never put your tomatoes outside if the temperatures fall below 40°F (4.4 °C), and do not leave them out overnight when there is a risk of frost.
How to Harden Off Tomato Plants, Step by Step
Hardening off your tomato transplants is not difficult. It is just a process of moving your plants outside and back in and increasing the amount of time they spend outdoors, along with the amount of direct sunlight they endure, gradually over that two-week period.
- Try to start on a day when the temperature is moderate, and there is no severe weather or high winds predicted. A boring, calm day is ideal!
- If you can find a day that is overcast or cloudy for the first day or two, that is even better. Strong, direct sunlight is not what you’re after for the first few days.
- On day one, move your transplants outside in a protected space and keep them out of direct sunlight. This does not need to be in your garden area (and probably shouldn't be since that should have long, full sun). It is fine to do this on a porch, patio, or picnic table. Placing plants next to an outside wall can protect them from the wind for the first few days.
- The plants should stay shaded from the sun for the first one or two days.
- Leave your tomato plants outside for about two hours on the first day, then take them back inside.
- On day two, leave your plants out for a longer period of time. Give them a full three hours outside. They should have sun exposure for the first hour, and then be moved to the shade for the rest of the time. (Or, you can choose a spot where the sun will move behind something that shades them.)
- Bring the plants back inside after three hours on day two.
- From day three forward, lengthen the amount of time the plants stay outside. Add about an hour each day.
- From day three forward, extend the time the tomatoes are in the sun versus the shade. Give them an extra half hour to one hour in the sun each day, but then move them into the shade after that time.
- By day four, you can leave your tomatoes in full sun the whole time they are outdoors.
- After day four, place the tomato plants in a place where they will get full sun all day, but increase the amount of time they stay outside by an hour per day.
- By about day 11 or day 12, your tomatoes should be pretty well acclimated. Now it is time to let them experience nightlife outside.
- From day 11 forward, you can leave your plants outside all night long, as long as there is no chance of a frost.
- After about two weeks of hardening, your tomatoes will be hardened off and ready to plant in the ground. Go ahead and start that tomato garden!
A Few Tomato Hardening Tips:
- Be sure to keep your tomato transplants watered
- Stop fertilizing tomatoes when you harden them off; this helps to acclimate them, too, and is another way to harden and prepare your tomato plants for life outside
- Your tomatoes will dry out faster outside in the lower humidity and in the wind and sun
- You may need to increase watering in the pots to twice per day as the time in the sun increases, and as your tomatoes continue to grow and use more water
- If the weather is too severe at any point during the hardening off period, it is okay to skip a day outside; just start again where you left off the next day after the weather moderates
- At no point should you leave your tomatoes outside during a frost, whether it is in the daytime or at night
Does Hardening Off Make Tomatoes Frost Hardy?
No, hardening off does not make tomato plants frost hardy. Tomatoes will never be able to tolerate a frost, even as adult plants.
If a frost should pop up unexpectedly, you need to take measures to protect your tomatoes. If the tomatoes are still in their pots, move them inside.
If the tomatoes have been planted in the ground already, cover them with frost fabric, old sheets, or floating row cover. If you have to use plastic, you can, but it should not touch the plants because plastic can transfer cold.
Are Tomatoes the Only Plants That Need Hardening Off?
Hardening off is not just for tomato plants. Hardening off should be done for all plants that have been started from seed in an indoor, protected, or controlled environment.
It doesn’t matter if the plants are frost-tolerant, either. The point is to acclimate the plants to new and varying conditions. Any plants, whether cold hardy or not, that have been grown indoors under steady conditions need to be hardened off.
Do Greenhouse Tomatoes Need to Be Hardened Off Before They Are Planted?
Yes, even greenhouse-grown tomato plants should be hardened off before they are planted or moved outside permanently.
A greenhouse is still a protected environment, and it is still much steadier and predictable than conditions outside. The plants in a greenhouse, like in a house, have been coddled, and they need to be toughened up before they can stand staying outside.
Do You Need to Harden Off Tomato Plants You Buy from a Garden Center or Nursery?
It is usually a good idea to harden off plants that you buy at a nursery or garden center if you find them in a protected structure or in a greenhouse.
If you know the plants have been moved outside during the day and that they have had extended periods of direct sunlight outside, you can shorten or skip the hardening-off period. (Say, harden for a few days and slowly introduce the plants to staying outside overnight.)
If you are sure that the plants have been left outside all day and all night, you can consider them hardened off, but if not, it’s best to strengthen them with a hardening process.
Hardening Tomato Transplants Pays Off in the First Few Weeks of Growing
It can be tempting to skip hardening off and tempting to save time and effort by just planting your transplants in the ground when you move them out of your home.
That won’t save you growing time in the long run, though. The time that you think you are saving is lost when plants break, struggle, die, or experience shock from the sudden change. Those that do survive will often become stunted, at least for a week or two. That sets them back. It takes longer for the tomatoes to rebound from the shock and longer to catch up in the end.
What you think is giving your tomatoes a two-week headstart by this type of early planting is really just stressing them to the point where they will probably end up further behind than they would have been if they’d been hardened and then planted.
The wise tomato gardener always hardens off their tomato transplants!
