Skip to content
planting seedlings by greta-hoffman pexels

Hardening-off seedlings

How to harden off seedlings

When you grow vegetables and flowers from seed indoors, there’s one very important step you need to take before transplanting your seedlings into your garden — you need to make sure your plants are hardened off. What does it mean to harden off seedlings? Let’s explain.

Prepare your plants for outdoors by hardening them off

Hardening off is the process of preparing your tomato seedlings, your broccoli seedlings, your cabbage seedlings — all the vegetable plants you’ve been growing and nourishing indoors — for their transition to the great outdoors.

Why harden off plants?

Insert yourself into the “mind” of your happy indoor seedlings. Until now, the moment it’s time to be transplanted into your garden, they’ve flourished in a perfectly moderated growing environment.

In this perfectly moderated environment, your plants have have had constant, controlled light, likely with grow lights. When their soil dries up, or they look thirsty, they’ve been gently misted and watered. At the end of each day, no chilly evening winds have whistled through their leaves or bent their tender stalks. They have been protected.

Acclimate to colder temperatures and wind

Then, the time comes for these tender seedlings to be transplanted from their relatively perfect, protected habitat to the naturally variable conditions of a new outdoor environment, with cooler temperatures, wind, rain, insects — basically, nature.

Hardening off essentially means to “toughen up” seedlings for the shock of the great outdoors. It’s the process of exposing them, little-by-little to the elements so that over time, they become accustomed to their new environment.

Do you need to harden off your seedlings?

If you’ve purchased your seedlings from a garden centre, this process (hopefully) has already been done. (Don’t know what to look for when buying seedlings? Here’s our Buying Seedlings – A Guide for Success.)

But, if, as we suspect, since you’re here, you’ve grown your own seedlings indoors, follow the process outlined below. After a time, you’ll have sturdy, well-hardened plants, with lots of built-up strength, able to withstand nature’s variability when they become residents of your gardens.

The hardening off process

Over the course of about a week, gradually take your seedlings outside for longer and longer stretches of time. There’s no concrete timeline to how long it takes, because life — especially gardening life — doesn’t unfold like that!

That being said, hardening off should be done slowly over two weeks. If you have less time, you can acclimate your plants in a shorter period, but consider reserving a minimum of at least five days for better results. And as always, with gardening, do what works best for you.

Tips for introducing plant seedlings to outdoor conditions

  • Start small, as this acclimation process can be shocking to your tender plants. Find them a spot that’s mostly protected from sun and wind. Don’t put them directly into full sun. (If they’re blasted by too much sun, they can burn.)
  • On the first day, put them outside for just a couple of hours. Or for just one hour, if you’re feeling a bit nervous about how they’ll do outdoors.
  • Bring your plants back inside after their allotted outdoor time. You might place them in a heated garage or basement if they’ve outgrown their indoor growing environment.
  • Day by day, slowly increase the amount of time they’re outside.
  • Bit by bit, introduce more light and wind to your seedlings.
  • Eventually, the time your small plants spend outdoors reach the point at which they’ll be able to spend the night outside, and they’ll be tough and ready to be planted into the ground for sweet summer growing.

Two final tips: hydration and pests

  • On those longer days and when you introduce more and more sun, keep an eye on their thirst levels. Until they’re in the ground, they will dry up more quickly while still in their small containers, so be sure to give them a big drink of water as needed.
  • If you have a busy yard, make sure to protect your seedlings from squirrels, chipmunks and other yard friends, as well as from snails and/or slugs

That’s it! Hardening off plants is all about gradual exposure. And perhaps a little bit of patience.

yellow petaled flower by elias sorey unsplash

hey there

sign up for
our weekly
newsletter

We promise to only share good stuff about plants and people who love plants.