Jim Buckle points out frost damage on a seedling on Monday at his farm in Unity. Buckle said they lost most of their tomato, pepper and egplant plugs in late April. Credit: Gabor Degre | BDN

You planted your seeds indoors. They’ve been growing for some time now, and with the weather turning you are ready to get them outside. But when you do so, you’ll need to perform a crucial step: hardening off.

Hardening off is the process of gradually bringing seedlings outside in order to adapt them to outdoor conditions — colder temperatures, as well as increased light and wind exposure — before you transplant them into your garden.

“It’s really worth doing,” said said Kate Garland, horticultural specialist at the University of Maine Cooperative Extension. “That supple tissue is developing thicker cell walls. It’s important for the cell walls to thicken up and get ready for what’s to come.”

Seedlings that aren’t hardened off before planting will suffer from transplant shock, displaying wilting or discolored leaves and generally failing to thrive, or will die altogether. Warm season crops like tomatoes, peppers, cucumbers, zucchini and squash are especially susceptible to transplant shock.

Cold season crops, Garland said, are more forgiving — onions and leeks are most tolerant to quick transitions, but brassicas will also survive the exposure — but should ideally be hardened off nevertheless.

Even if you are buying seedlings from a greenhouse, you should ask if they have been hardened off.

“If they’re being sold from a table outside and you can tell that the vendor isn’t moving them in and out every day and night, those are probably hardened off,” Garland explained. “You could ask the garden center employee to see if they have already hardened them off and they’ll be able to tell you pretty quickly.”

Garland cautioned that hardening off should not be confused with another common gardening term that crops up around this time of year with seedlings: damping off.

“Damping off [is] something completely different,” Garland said. “It’s a confusing pair of terms. Damping off is a disease that is very common to seedlings, it happens a lot of time when seedlings are overwatered. That’s definitely something you don’t want to have happen.”

How to harden off seedlings

Gardeners should start hardening off seedlings one to two weeks before transplanting them into the garden.

“Preferably the longer end of that spectrum, [but] it’s ok if you need to do that a little bit less time,” Garland said.

Because you will be moving your plants a lot through the process of hardening off, Garland recommended keeping them in a wagon or other wheeled apparatus to make moving easier. The best practice for hardening off is to start by bringing your seedlings out into a moderately sunny spot during the day, perhaps in the shade of a tree that will protect the tender seedlings somewhat from the wind.

“Leave them out for half a day [and then] bring them back inside for the remainder of the day,” Garland said. “Do it again the next day, maybe in a little bit of a sunnier spot. You wait until three or four days before you’re planting to expose them to full sun all day long.”

At the same time, Garland said to decrease the frequency with which you water your seedlings, letting them dry out completely before watering them again.

“It’s shockingly hard to do because you just want to water them and get them growing as fast as you can,” she laughed. “Those roots will be so beautiful and white if you do a great job watering. That’s really what you want in a transplant so they can be more resilient.”

After a few days, you may notice the change in your seedlings.

“You’ll notice a thicker feel to the leaves, especially the upper leaves,” Garland said. “If you had a side-by-side comparison of a plant that was started indoors and not hardened off, you could definitely tell, especially with the tomato seedlings or the pepper seedlings.”

However, it might be harder to tell the difference in a greenhouse setting if you are shopping for seedlings.

“I wouldn’t say that you could go shopping and know just by feel whether a plant is hardened off or not, so it’s important to ask,” Garland said. “The garden centers are going to be more than happy to offer that advice. They want their seedlings to do well.”