What does it mean to harden off plants?

Hardening off plants

Hardening off plants is the process of gradually exposing plants to harsher conditions, with the end result being a plant that is hardy, healthy, and well-adapted to be planted in a new environment.

Typically this is done with annuals (plants that grow for only one growing season), as they are normally young and not mature in the early spring and need to be acclimated.

You can think of it as training for sports or building up your stamina with running. You’re not going to go out and run a marathon in your first run; you have to gradually work your body up to that point (if that’s the goal).

The same is valid for plants. Significant swings in temperature or its environment can make it difficult to grow, or even cause it to die.

How to harden off plants

One strategy of hardening off plants is to expose them to slightly colder temperatures until they’re strong enough and hardy enough to be planted outside.

Here’s an example:

  • You purchase a plant in April that you plan to plant outside. Since you bought the plant fairly early in the season, the plant is likely to be used to warm, consistent greenhouse temperatures.
  • To acclimate the plant, you can put the plant outside during the daytime and bring it into your garage overnight. This decreases the shock of leaving the warm greenhouse environment and helps the plant to harden off over time.

Do greenhouse businesses harden off their plants?

Many times greenhouses will harden off plants before selling them.

To do this, they may keep the young plants in a warm environment and then transition them to a colder greenhouse as the plant matures.

This helps ensure the transition from the greenhouse to your home isn’t as abrupt. Any changes in the plant’s environment can stunt a young plant’s growth, but hardening off the plant when it’s young helps to build its strength for these changes.

Not all greenhouses harden off their plants, so keep this in mind when purchasing a young plant that you are ready to plant outside right away.

Why is this important?

It’s important to keep hardening off in mind when planting your plants early. It’s likely that most plants will grow better if they aren’t experiencing extreme shock in their new environment. 

Whether you are planting them in landscape, planters, or hanging baskets, it’s always a good idea to make the transition from cozy, warm greenhouses to your home a gradual one.

Plants that are hardened off will be much more likely to withstand an early planting (although, a hard frost or cold snap will normally wipe out even a hardy plant, depending on the plant and when you plant it outside).

Hardening off your plants

The next time you purchase a plant or grow it from seed, keep in mind the principle of hardening the plant off over time.

In all likelihood, you were already hardening off your plants to some degree, maybe without even knowing it.