What is Hardening Off Plants: Everything You Need to Know

Photo of plants hardening off

Are you looking to grow plants indoors this year and need to figure out what is hardening off plants? Hardening off plants is an essential part of starting your seedlings inside with your own indoor seed starting setup. You will eventually come to the point where you need to get your seedlings used to the outdoor environment.

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What is Hardening Off Plants?

If you are new to gardening, you may be asking yourself what is hardening off plants. Hardening off plants is the process of slowly getting your plants that are grown indoors used to the outdoor elements. It is an extremely important step in the process of starting seeds inside. This process will reduce the risk of shocking your plants. It allows your plants to adjust to the sunlight, temperature fluctuations, and wind in a controlled and gradual way.

Understanding Why Hardening Off is Important

Now that you have figured out what is hardening off plants, it’s important to understand why it’s such an important step. Reducing shock in your plants is key to their survival and can be detrimental if you do not prepare them properly. If you do not slowly get them used to the outdoors, they may wither and die from the inability to tolerate the heat and sun. If your plants survive, they may experience stunted growth, burning, or loss of leaves.

The benefits to allowing your plants to properly harden off are abundant. They adapt better to the outdoor environment when slowly introduced to it. This can result in optimal physical changes to the plant. You may see your plant’s stems strengthen and get larger, while the leaves thicken. Plants become hardier and stronger as they respond to the new environment, in order for them to better withstand the sun, wind, and temperatures. Proper hardening off sets a strong foundation for healthy growth of a plant and increases their chances of survival.

When to Harden Plants Off

Choosing when to harden your plants off is important because you will want to time it well with your last frost date. It’s recommended you start to harden off your plants approximately two weeks before transplanting them outdoors. By doing this, you will have plenty of time to slowly get your plants used to being outside for incrementally longer amounts of time. You will gradually do this until your plants can withstand being outside all day. You will need to make sure that your temperatures outside are a suitable temperature for your plants to grow in, as this can vary drastically based on type of plant.

How to Harden Off Plants

Start Slow

One of the most important things about how to harden off plants is that you need to start gradually and slowly. You need to be very careful at first, the plants will not used to the outdoor environment at all. You will want to increase your plant’s exposure to the sun by incremental amounts of time. Typically, you will be able to increase your plant’s exposure by 1 to 3 hours per day, depending on how they handle the intensity of the sun in the beginning.

Increase Time in the Sun

Begin introducing your plants to the outdoors by first putting them in the shade or starting on a cloudy day. Try to do this on a day that doesn’t have extreme temperatures or much wind. Let the plants stay in the shade for a few hours and monitor how they are handling it. If they seem to be doing well, you can keep them outside for longer.

Once the seedlings handle the shade well, the next step is to adjust them to the intensity of the sun. This is one of the harder parts of how to harden off plants for most people. Some plant varieties tend to do better than others during this stage. For example, marigolds tend to withstand this process pretty well, while tomato plants are notorious for being a bit more tricky and taking more time and monitoring. Place your plants into the sun initially and monitor them often. If you see them begin to wilt, place them in the shade to perk back up. Each day try to increase their exposure to the sun by additional amounts, gradually leading up to an extra 2 or 3 hours per day. After a week or two, you should be able to leave your plants outside for the entirety of the 24 hour day period.

Monitor Water Needs

It’s very important that you keep track of how much water your plants are going to need while hardening off. They will often require more water than normal because the outdoor temperatures will be potentially warmer in the sunlight and more humid. The soil may dry out more quickly than it did inside the house. It’s also important to make sure that you don’t overwater your plants and stress them.


Once the plants can handle 24 hours of exposure to the outdoors for a few days, they are ready to be transplanted outside! This means that the plants have completed the hardening off process and are well adjusted to the new environment. Now that you know how to harden off plants, make sure that you check your weather and night time temperatures to check for any chances of frost in the near future. Otherwise, you will be ready to put your plants in their final growing spots.

Plants transplanted into the ground after being hardened off

Check the Old Farmer’s Almanac for the frost dates in your area!