How to Harden Off Plants

You have worked hard to start your plants from seed; they all look fantastic, lush, green, and ready for the garden. It’s past your last frost date, and you’re ready to move them outside. Does the next step of hardening them off seem a little intimidating? Throughout this article we will be diving into the art of how to harden off plants, methods of how to harden off plants, and the reason behind why it is a crucial step in transplanting your seedlings. 

​Why is Hardening Off Important?

Hardening off plants is a necessary step to long term success with your plants. All of your little seed starts are tender plants that must be climatized to handle the natural outdoor conditions. They have been protected from most environmental challenges and have never been exposed to wind, heavy rain, direct ultraviolet rays, or fluctuating temperatures. 

Whether started in a greenhouse or in your living quarters by a window, you will still have to harden your young seedlings off. If you do not slowly acclimate your plants to the outdoor weather, they could die easily or have stunted plant growth from shock. Hardening off is such an important step. 

The process of hardening off your plants will help the plant cells develop thicker cell walls, which in return will result in a strong plant stem. Having a strong stem is important for a plant. When your plant starts to grow tall or starts to bear fruit, it will need all the strength it can get to stay upright during a windstorm or during heavy rain. It will also allow the plants to adjust to the UV Light direct from the sun.

How to Harden Off Plants

There are many ways to harden off your plants, but I am going to teach you step by step how I have found to be the most successful for me.

The process of hardening off plants will take between one and two weeks. It is very important to have patience during the hardening process in order to avoid your seedlings being damaged from transplant shock. It’s one of my least favorite garden tasks but one of the most important.

1. On the first day, move your plants outside to an area of your yard that receives roughly an hour of sunlight in the morning and an hour of sunlight in the evening. Then, move them back inside by nightfall. I use the north side of my greenhouse for this. You will repeat this process for the second day, as well. 

2. On the next day, you will move your plants a little farther into the sun so that they receive close to 2 hours of sun in the morning and 2 hours in the evening. Repeat this step for the following day, day 4, as well. Be sure your plants are not receiving sun in the middle of the day. To do this, you might have to move them back into the shade midday.

3. On days five through seven you can start to expose them to the midday, full sun between 12:00 and 3:00 pm. Be sure to slowly acclimate them to the heat of these hours, one hour at a time. 

4. By day seven, you can start to leave your cold-hardy plants outside at night, adding more plant varieties as the days pass. 

5. Depending on your climate and weather forecast, your little seedlings will be ready to be transplanted between days seven and fourteen. 

If you live in a more temperate climate than my zone 3b with frost until the end of May, the amount of time it takes you to harden off may be shorter, but if you follow this basic outline, your plants should be off to a great start. 

Things to Be Aware Of

1. Drying of the Soil: Outside environmental conditions are not controlled like your indoor growing conditions. This can result in your soil drying out quicker and your plants needing more frequent water. 

2. Time of Watering: Watering your healthy seedlings is crucial, but so is the time of day you water them. When watering the young plants, be sure not to do it during the heat of the day. It is best to do it first thing in the morning so the leaves have time to dry. If you place wet leaves in the midday sun, they may burn!

3. Timing: When you start hardening off your plants, it is important to start with morning and evening sun. During this time, the weather should be more temperate and consistently mild. It is also essential to watch the time of day, ensuring your young plants do not endure the heat of the day at the beginning of the hardening-off process. 

4. Extremes: If you have a random day during your hardening-off process where the weather conditions are going to be extreme with high winds or extreme cold temperatures with a danger of frost, I recommend keeping your plants inside. 

5. Climate: If you are at the point where you need to begin hardening off but do not have a shady spot to start the process, you can use a cold frame or row covers to assist you during the hardening off process. 

Commonly Asked Questions

1. I start all my seeds in an unheated greenhouse; do I still need to harden them off?

The answer is yes they need proper hardening off! A greenhouse generally has plastic that can filter the UV rays. It also protects the plants from environmental factors such as strong winds, various outdoor temperatures, and all other factors from the great outdoors. 

2. If I started grow my seedlings in a south facing window in my house, do they need to be hardened off?

The answer is yes! Like plants grown in a greenhouse, Glass is UV-protected to keep your household furnishings from fading, which means your indoor-grown plants are protected from UV rays. They are also protected from all the outdoor elements due to the indoors being a controlled environment. 

3. Do I have to bring my seedlings in every night during the hardening off process?

Yes, you do have to. You will slowly acclimate your young seedlings to the overnight temperatures and the morning dew. You do not want to place them outside and never bring them back inside in the early days of hardening oft. Don’t try to rush the process and ruin all of your hard work. 

4. It is going to be a cloudy day, do I need to shelter my plants from the direct sunlight between 12 and 3?

​Depending on what day in your hardening off cycle you are on, you may need to bring them in. Be sure to monitor them closely and see how the plants appear to be doing. 

5. Is there a way to avoid having to harden off?

Yes! If you have a small number of seedlings and it’s warm enough during the day, you can start putting them outside right when they germinate and bring them in every night. This way they have adapted to the outside elements from the beginning and hardening off won’t be necessary. 

​Mistakes that I have made

When talking about gardening, I speak from experience. I have been gardening for 20+ years, have had successes and failures, and have learned from both. There’s a lot of trial and error involved and a good dose of being able to let go when you make mistakes.

If you were to take a plant from indoors and put it right out into the direct sun without the opportunity to adjust, it would burn the leaves, like my little eggplant in this photo:

Thankfully, I had remembered to move it into the shade, but if I had left it for another hour, the plant would have died.

UV rays are essential to consider when hardening off, but the wind is also something to keep in mind. If it is a windy day or the wind is coming in from the north, you’ll need to offer a little extra protection for your tender seedlings, particularly during the first few days. A few years ago we had a really cold north wind that froze several of my seedlings even though it was a sunny day, so keep an eye on the wind.

Learning how to harden off plants is extra work, but it is always worth it and will ensure your seedlings have the best start possible.

Happy Gardening!