The hardening of the seedlings is not the most fun for me to have in the garden, but it needs to be done. I try to harden them off as soon as I possibly can so I can move beyond carrying the seed trays out and in every day. I try to harden them off in 7 days if the weather is favorable. But sometimes it can take up to 2 weeks if the weather pattern interrupts my plans.
Harden off the seedlings about 1-2 weeks before the date you plan to transplant them in their permanent spot outdoors. The seedlings of the cool-weather crops can be hardened off when the day temperatures reach consistent 45F during the day. On the first day bring the seedlings out and place them in a shady spot for a couple of hours. Then bring them inside for the night. The next day, bring the seedlings outside again but keep the outside an hour or two longer. Increase the hours the seedlings stay outside every day. Gradually expose the seedlings to the sun’s shine so they can adjust to it. After about 7 days, if the weather is favorable and your seedlings spent the whole day outside and out of the shade, your seedlings are ready to be transplanted into their permanent spot in the garden.
When to start hardening off the seedlings?
The seedlings are mature enough to start the hardening-off process when they have two sets of true leaves and the root system is large and strong enough to hold the soil the seedlings are potted in. Start hardening them off about 1-2 weeks before the date you want them planted in their permanent spot in the garden. Start hardening them off when the temperatures are about 45F during the day. The best is to start hardening off when overcast. If not overcast, just place the seedlings in the shade on the first day.
I start to harden off the seedlings when I see the second set of true leaves and the roots coming out of the bottom. I give them enough time to adjust while they are still growing the second set of true leaves. By the time the second pair of true leaves are out, they are hardy and in just a couple of days, they are fully ready to be planted outside. Thus, as soon as they have two sets of true leaves they are out in the garden. I check the weather as well, to ensure there will be 7 days of nice weather when the seedlings can go out and there are no freezing temperatures during the day.
I plant the seedlings outside when there is no frost for at least 7 days after they are planted outside. If there is occasional frost, I cover the seedlings with garden fleece. I am not willing to risk losing the seedlings and all the hard work that went into them after frost. I rather put some effort to keep them covered for the night.
The sooner I can transplant the seedlings outside, the sooner I have the seed trays ready for another crop to be started. As soon as the seedlings are out I keep them protected from pests and wildlife that could devour them. I ensure they are properly watered and fed after they are transplanted.
I have written an article about what hardening is and why harden off the plants before transplanting them outside. Read What is plant hardening, here!
How big should seedlings be before hardening off?
The seedlings are ready to be hardened off when they have two sets of true leaves. If the weather is nice, they can start the hardening process while the second set of true leaves is still developing. The roots should be extensive by now to envelop the majority of the soil the seedlings are planted in. The roots should be extensive and strong so the plants have a way to receive water and nutrition properly once the seedlings are transplanted.
Can you skip hardening off seedlings?
It is not recommended for any seedlings to skip hardening them off. Hardening off the plants is transitioning them from the protected and controlled environment to the unprotected and uncontrolled environment. The tender seedlings can be especially sensitive to direct sun and get sunburned. They would need time to recover from sunburn or sun scolding. If the seedlings are gradually introduced to the outdoor elements including direct sun, they can toughen up and become hardier and more resistant. They will then place their energy into proper growth rather than into recovering from transplanting. By skipping the process of hardening off the plants one might offset the growth of the seedlings for several weeks. Some seedlings might not recover from sun scolding.
Inspect the seedlings during the hardening process. Inspect how they handle the transition. Ensure the soil is moist but not overwatered, and do not let the seedlings dry during the days of hardening. They might need to be watered more often while being outside due to the direct sun. Inspect if they are not wilted, if wilted or the true leaves turn yellow, it might be a sign of sunburn or need to water. If you see the leaves are yellowing, hold back on sun exposure yet and increase sun exposure very slowly. Do not leave your seedlings out when freezing, especially at night, until they are fully hardened off.
Hardening off the plants step by step
- Day 1: Place the seedlings outside at a temperature of at least 45F. Place the seedlings in a shady spot for 3 hours.
- Day 2: Increase the hours to 5 hours, same shady spot.
- Day 3: Increase hours outdoors but combine shade with a dapple, or indirect sun. Keep them in shade for 4 hours then, introduce 1 hour of dapple or indirect sunlight.
- Day 4: Place the seedlings in a shady spot for 4 hours, and increase to 2 hours of dapple or indirect light.
- Day 5: Place the seedlings in shade for 4 hours, then increase 3 hours of dapple or indirect sun. Inspect the seedlings for sunburn. If they got sunburned keep them longer in shade and provide less time in the dapple sun. There is no reason to hurry.
- Day 6: If no sunburn your seedlings are adjusting well. It is time to expose them to less shade and more direct sun. Expose them to the dapple sun for a couple of hours and if they do well expose them to direct sun again but increase the hours. Keep the eye on the seedlings for sunburn.
- Day 7: if your seedlings handled the sun well until now, they can be exposed to even more direct sunlight. If they handle the sun well on the 7th day with the longest sun exposure, they are ready to be planted outdoors the next day. M
If I can, I inspect my seedlings during the day and pull them out of the direct sun as soon as I see the sign of distress, like wilted leaves but the soil is moist, or when I see yellow spots on the leaves. I keep them in the dapple sun and then gradually expose them to the direct sun, especially in the morning when the sun is not as strong as in the afternoon.
I like to harden off my seedlings in the morning hours when the sunlight is not as intense and there is more shade that gradually turns into more and more sun exposure, so it is easy for the seedling to adjust, and for me to spot if anything is not right.
A GOOD READ:
When and how to plant a seedling
Is April too late to start seedlings in zone 6?
What seedlings to plant outdoors in April in zone 6
The best way to start seeds outside
How to start seeds and care for seedlings
What do hardiness zones mean and how to determine yours
