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It's a hardening off life: Prepping your plants for the great outdoors

 Person puts hands in soil of raised bed with transplants.
Madeleine Charis King
Once it’s safe to put your plants outside, you’ll want to plan for a week of hardening off before planting them in the ground.

As winter fades into spring and summer, the seeds you carefully planted inside all those months ago have sprouted into capable young plants. Around the time of the last freeze of the season, your seedlings will be all grown up and ready to take root outside! Every plant parent wants to ensure their seedlings are successful outside, but how? The answer is hardening off.

Hardening off is the process of exposing baby plants to increasing amounts of sunlight, wind and weather, and it’s a vital step in acclimating them to life outdoors. This is true whether you’ve been carefully nurturing them from seed inside or you’re buying transplants from a greenhouse.

To help you make a successful transition, we’ve put together a short guide for transplanting your seedlings.

When to start hardening off

Seedlings put outside when temperatures are still dipping into the low thirties are at higher risk of freezing and dying. It’s best to wait until after the last frost to start transitioning your plants outside. In Iowa, the average frost-free date falls in early May around Mother’s Day.

Once it’s safe to put your plants outside, you’ll want to plan for a week of hardening off before planting them in the ground.

How to harden off

Put your plants outside in a protected, shady location that won’t get too much wind or direct sun. This might mean putting them on a covered deck or by the side of your house to start. Over the course of the week, gradually put them out into more light and more exposure.

If it looks like the temperature is going to dip down to around 40 degrees at night, pull the plants back inside and put them out again in the morning, or bring them closer to your house where it’s a few degrees warmer. You can also protect the plants from too much sun by covering them with a piece of newspaper, or protect them from wind and animals by putting a milk jug or similar barrier around them.

Why hardening off is key to success

It may be tempting to plant your seedlings in your garden right away, especially after waiting all winter, but patience is the name of the game. Your little seedlings have never experienced sun, wind or weather and they may get burned, tattered and torn if you don’t harden them off. Plants from outdoor garden centers will have already gone through the process, but anything grown inside will need a helping hand as it transitions outdoors.


Think of hardening off as training. If you put in the time, effort and patience to do it right, you’ll be setting yourself up for success. Don’t skip this crucial first step in the transplant process!

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Sumner Wallace is an intern for IPR’s digital team. Sumner grew up in Iowa City, but now attends Oberlin College in Ohio, where she is pursuing a bachelor’s degree in Rhetoric and Media Studies with a minor in Chemistry. She has also worked for Little Village Magazine and The Oberlin Review.