How to Harden Off Seedlings to Reduce Transplant Shock

Starting seeds indoors is a great way to get a jump on the growing season, but if you plant those tender seedlings directly into the garden without preparation, they’ll likely struggle or even die. So it’s important to harden off your seedlings before they go out to the harsh conditions. So read along to learn more about how to harden off seedlings.

How to Harden Off Seedlings

If you’ve ever had seedlings wilt, turn yellow, or fail after transplanting, they probably weren’t hardened off properly.

This causes transplant shock, which is what causes these issues.

But the truth is that your seedlings will always be shocked when they’re transplanted; it’s natural. However, we can avoid most of the shock by setting them up for success.

And because everybody’s climate is different, everybody’s hardening off process will be different as well.

So let’s get right into how to harden off seedlings to reduce transplant shock.

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What is Transplant Shock?

Transplant shock is basically the stress that a plant undergoes when it is transplanted from one area to a new area. Every seedling will experience some level of transplant shock when you transplant it.

This happens because the plants become used to the growing conditions of the original area, which makes it difficult to grow in a new area.

When this happens, plants may start to wilt, lose a few leaves, the leaves may turn a bit yellow, or some roots can die. They may also be stunted in growth for a little bit before they get growing again.

Transplant shock can be serious, causing the seedlings to fully die. However, the plants usually recover from it, provided that you do everything you can to reduce it.

What Does Hardening Off Plants Mean?

Hardening off is the process of slowly exposing your indoor-grown seedlings to the harsh conditions of the outdoors.

You see, the conditions inside your house are way more comfortable for your plants than outdoors. This doesn’t mean that they can’t grow outdoors, because they obviously can.

But your house is so nice and cozy for the plants; no wind, constant temperature, and constant humidity. Sounds great!

But outside there is so much wind, fluctuating temperatures, and not to mention a burning hot star in the middle of the sky. It would be difficult for your plants to become instantly used to these conditions after they have been in your house for 6-8 weeks.

So in short, hardening off prepares the seedling for transplanting.

When to Start Hardening Off Seedlings

The ideal time to begin hardening off depends on your local climate and the plant type.

For example, some seedlings can be planted outside before the last frost date, while others need to be planted after the last frost date.

Here’s what temperature to hardening off seedlings by type:

• Cold-hardy plants (broccoli, kale, onions, lettuce): Start hardening off when daytime temperatures are above 45°F (7°C).
• Tender crops (tomatoes, peppers, cucumbers): Begin when temperatures are consistently above 50-55°F (10-13°C).
• Frost-sensitive plants (melons, squash, basil): Harden off only when all risk of frost has passed.

For the long-season summer crops, start hardening them off about 1 week before the last frost date, so that they are ready to plant after that date.

So my last frost date is around May 15, so I would start hardening off seedlings around May 8. This gives them enough time to be hardened off.

How to Harden Off Seedlings

The whole hardening off process takes about 7-10 days. Some people like to push it to 14 days, but I think the benefits level out at about 10 days.

So let’s get into the hardening off schedule for plants.

Day 1-2: Introduce to Shade and Shelter

On the first 2 days, start by keeping your seedlings in a sheltered and shady spot for about 2 hours.

Make sure to keep them out of direct sunlight to prevent the leaves from burning. You should also bring them back inside before it gets too cold in the evening.

So for a general rule, bring them outside at about 8 am and bring them back in 10 am. Of course you don’t have to follow these exact times, but it’s when the conditions outside are fairly mild.

Day 3-4: Gradually Increase Outdoor Time

Gradually increase the time that your seedlings spend outdoors by 2 hours each day.

Start exposing them to some morning sunlight as well, but keep them away from the harsh afternoon sun. You can also reduce the amount of water that you are giving the seedlings, which helps to prepare the root systems.

Remember that the areas of shade move throughout the day as the sun moves in the sky. So if you won’t be at home all day, then pay close attention to the shade patterns.

Day 5-6: Increase Sunlight Exposure

On these days, allow your seedlings to spend half the day outdoors, ensuring they get more sun exposure. Morning and afternoon sun is ideal for this time.

At this time, watch for signs of wilting, yellowing, and burning leaves, and adjust the plants’ exposure if needed.

Day 7-8: Full Day Exposure

Leave the seedlings out for all of the daylight hours, bringing them inside at night.

Ensure that the seedlings are getting at least 4-6 hours of direct sunlight at this time.

Day 9-10: Night Exposure

Now you can leave the seedlings out for a full day and a full night. This gives them time to get used to the cooler conditions after the sun has passed.

For the first time, keep them in a sheltered spot when it gets dark, and then move them out in the open the next night.

Extra Tips to Harden Off Seedlings

Here are some more tips for hardening off seedlings that will give you a better result:

1. Don’t put them on the ground: The ground is generally colder than the air, so plants may be shocked by this. Additionally, it puts them closer to pests like birds, squirrels, mice, and slugs, which will destroy your young seedlings.
2. Grow extra seedlings: It is only natural that some plants will die during the hardening off and transplanting process, so grow extra seedlings to ensure that your garden is filled. If you don’t end up using them, then you can give them away to your neighbours.
3. Nursery-bought plants: Seedlings that are bought from a nursery or greenhouse should be hardened off as well, especially if they were grown in sheltered conditions. Use the same process for these plants.

Common Hardening Off Mistakes to Avoid

Avoid these common mistakes that gardeners make when hardening off their seedlings:

• Skipping the process: Some gardeners just don’t have the patience to spend 1 week hardening off their seedlings. It’s only 7-10 days; there are no excuses. Seedlings need time to adjust.
• Exposing too fast: Too much sunlight or wind early on in the process can cause some plants to wilt, burn, and even die altogether. Again, be patient and expose them gradually.

Transplanting Seedlings

Once your seedlings are ready, it’s time to transplant them into their forever home in the garden. Whether you are using raised beds or containers, the process is the same:

• Choose a cloudy day: Transplant on a cloudy day to reduce transplant shock. If you transplant the seedlings right in the middle of a sunny day, it is almost guaranteed that they will wilt and their growth will be stunted. If there isn’t a cloudy day soon, then wait to transplant in the evening, after the sun has passed.
• Water the seedlings: Make sure the seedlings’ soil is moist before and after transplanting. This reduces transplant shock and wilting.
• Fertilize: Add an organic fertilizer to the soil before transplanting to increase new root development and leaf growth.
• Add mulch: To conserve moisture, add a thick layer of organic mulch around the base of your seedlings. Avoid piling it up right next to the stems to prevent them from rotting.
• Provide shade: If you notice that your plants are wilting too much, cover them with a shade cloth to protect from the harsh sun.

Wilting is a Natural Response

Don’t freak out when your plants wilt a little bit; it’s actually a natural response that helps to keep them alive.

When it gets hot, dry, or windy, the leaves of plants will naturally wilt to conserve water. Don’t immediately dump water on them at the slightest sign of wilting.

You only need to give them more water if the leaves don’t plump up again after the conditions stabilize.

Frequently Asked Questions

Even More Gardening Ideas

Here are a few more posts to get the ball rolling in your garden!

• How to Grow Carrots from Seed to Harvest
• How to Make Biochar as a Soil Amendment
• How to Deter Slugs in the Garden Naturally

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