How to Harden Off Seedlings and Young Plants

Hardening Off Seedlings

There’s nothing more satisfying than watching your plants develop from seeds and growing into delicate seedlings. Seedlings do well when they are kept in ideal conditions, but as they grow, we must transition them from ideal conditions to outside conditions. This is called “hardening off”.

A quick transition from a sheltered windowsill or greenhouse isn’t good for young tender plants, they need some help, and the last thing you want to do for plants raised from seed before they are strong enough is move them straight outside. We need to harden them off but what exactly does it mean? Read on to learn how to harden your plants off.

What is Hardening Off?

Seeds Grown On Windowsill

The term refers to the transitioning young plants and seedlings from inside to outside. That doesn’t simply mean physically moving your plants outside and leaving them outside. You don’t want to expose plants to outdoor conditions too quickly as it will likely kill off your seedlings. 

You are in effect moving them from a stable and controlled environment (indoors or in a greenhouse) to an unstable and unpredictable environment that is a lot harsher. This creates stress and stunts young and tender plants.

Hardening off means acclimatizing plants grown indoors or in a greenhouse to outside conditions. This process should be done gradually so you’ll secure your plants a healthy start in their permanent outdoor place.

Why Do Plants Need to Be Hardened Off?

When we talk hardening off and about adaptation to outside conditions, there are three things you must protect your seedlings from.

Direct sunlight – The sun outside is much stronger than any artificial indoor light or any commercial grow light. It can easily burn plant’s leaves or completely burn the seedling. Young plants build up their sun protection over time.

Strong wind – Any kind of stress can be harmful to young plants, including strong wind. The moving air can decrease the soil moisture and evaporate moisture from leaves which can be stressful to young plants too. The transition from inside to outside environment should be done in a sheltered place. Cold frames and cloches are useful as a protection from wind and low temperatures as well.

Low temperature – If you bring your seedlings outside too early, low temperatures or frost can kill them. That’s why you need to gradually expose them to outside temperatures and avoid leaving them outside during the night in the beginning.

When Should Seedlings Be Moved Outside?

When To Harden Off Plants

Seedlings should have a few sets of true leaves before planting out and you may have already potted them on once. Before transitioning your seedlings outside, you should check on the “last frost date” in your region. 

The last frost date will actually determine the time of sowing inside. It’s always a good idea to start your plants 4-6 weeks before the last frost date. That way, your seedlings will be a month old (or more) before you move them outside. Starting them inside on time, in perfect growing conditions, will make them strong and hard enough for transitioning outside.

But even then, you can’t just put them and leave them outside. Instead, introduce them gradually to the outside environment and bring them back inside during nighttime if the temperature gets too low.

Gradually Hardening Off Your Seedlings

As previously said, the process of hardening off seedlings should be done gradually in stages. The idea is to bring them out for just a couple of hours and then gradually increase their time outside until you can leave them out for a whole day and eventually night.

Before adjusting your plants to the outside environment, you can check the weather forecast. Don’t start the transition if the forecast says the weather is going to be overly windy or rainy. 

Day 1 – Bring your plants outside for 2 to 3 hours. Place them in a sheltered location, avoiding the direct sun. 

If manageable, don’t place them directly on the ground. The ground can still be cold, and the temperature is lowest to the ground. Set them on an elevated place, a chair, bench, balcony railing, etc. This will also protect them from slugs and other pests. 

Day 2 – Keep your plants outside, in a shady spot, for several hours longer than the first day. Don’t expose them to the direct sunlight yet.

Day 3 – You can introduce them to the direct sun. Leave them in a sunny place for 2-3 hours, but avoid the hottest part of the day. Set them out for the morning or afternoon sun, and move them back to a sheltered and shady location for another 2-3 hours.

Day 4 to 7 – Over the next 4-5 days, gradually increase the amount of sun exposure. In about 7 days, seedlings should be acclimatized to direct sun, so you can leave them outside for a whole day.

In case of any unexpected weather condition – frost, heavy downpours, strong winds or storms – move your plants inside. If weather conditions are unstable, the process of hardening off may take up to 2 weeks.

Don’t worry about light winds or rains – they can only make your plants stronger. Especially the light wind that helps plants to develop strong stems. You can even use an oscillating fan set on low to produce light wind and strengthen your young plants while still indoors.

Hardening Off Plants On An Overcast Day

How Long To Harden Off Seedlings

If you have a lot of plants and you don’t want to bring them in and out for several days, you can choose another method.

All you need to do is wait for an overcast day or when there is plenty of cloud cover. On an overcast day, the sky is almost completely covered with clouds (95%). However, these clouds don’t block out the sun entirely, but the sunlight is not as intense as during a clear, cloudless day.

The chances of frost on an overcast day are significantly lower.

On an overcast day, you can leave your plants outside for more hours, even on their first day out.

How to Know When Seedlings Are Properly Hardened Off

You will notice the difference in the thickness of the leaves. The seedling that’s still tender and used to inside conditions, has thinner leaves, while the plant that hardened off has noticeably thicker leaves. 

If you don’t have these, you might need to acclimatize your plants for a few days more. However, you will notice that your plants grow much quicker, so thick leaves that hardened off will probably appear very quickly.

Generally, seedlings need around 7 days to adapt to outside conditions. 

Which Plants Need Hardening Off?

Although it seems logical that only tender plants need hardening off, this is a mistake. Every young plant or seedling that has been grown inside needs to be gradually introduced to outside conditions.

Just as you would harden off tomatoes, you will need to do the same for those plants that are considered hardy. For example, cabbage, broccoli, or kale seedlings are equally susceptible to sunburns or frost as any other young plant. They need some time to build up their protective shield and get used to the outside temperature as well.

So, don’t rush to bring your hardy plant seedlings out. Remember, they’re used to much more luxurious conditions inside so treat them with care.

Even if you get your plants from a nursery or a commercial greenhouse, you will need a day or two to harden your plants off. Although there’s a lot of sunlight inside a greenhouse, it’s still considered an indoor space. That’s because you have a controlled environment inside the greenhouse.

Leave a Comment

Your email address will not be published. Required fields are marked *