Gardening Q&A: What does ‘hardened off’ mean? |

Gardening Q&A: What does ‘hardened off’ mean?

Deana Wise
CSU Extension Master Gardener, Larimer County

Q: The plants I bought at the nursery need to be hardened off according to the tag, what does that mean?

A: According to Narjess Zriba, plant pathologist at Iowa State University, “Hardening off is a process by which transplants are toughened up so that they can withstand nature’s caprices as they grow.

It’s done for your plant’s benefit, and hardened plants will grow better in the future. It gives your plants a better chance to survive the transition phase between the sheltered indoor environment and the garden’s less predictable conditions.

Plants can develop hardiness, allowing them to withstand wind, sun, subfreezing temperatures, and fluctuations in moisture. Plants will struggle and may succumb to environmental damages and pests if not hardened off.”

The best way to harden off tender new plants is to put them outside in shade or partial sun, in a location protected from the wind, for a few hours each day and take them in again as the temperature drops.

Each day, increase the time the plants remain outside and expose them to a little more sun and wind.

Do not put them out if the temperature is below 40 degrees or on days with high winds.

Most plants require 10-14 days to become hardened off.

“Transplanting from indoors to outdoors is a profound shock for a plant,” says Zriba. “This method slows their growth and toughens them up. Exposure to sun and wind forces the development of a thicker cuticle layer, thereby reducing water loss. The transplanting procedure shocks plants, but the shock is a mild and controlled one. Transplants are most likely to recuperate quickly if they are smaller rather than larger when transplanting occurs.”

Zriba recommends slightly reducing the water to the transplants before they are taken outside, however, do not let the plants get too dry and become stressed.

She cautions against the application of fertilizer before or during hardening off.

A starter fertilizer can be applied at the time of transplanting.

Recommended Minimum Temperatures

Hardy 40° F Broccoli, Brussels sprouts,

kohlrabi, cabbage, onions,

leeks, parsley

Half-Hardy 45° F Celery, Chinese cabbage,

lettuce, endive

Tender 50° F Squash, pumpkin, sweet corn

60° F Cucumber, muskmelon

65° F Basil, tomatoes, peppers

To determine when to start hardening off your plants, you will need to know if they are hardy, half-hardy, or tender. The terms “hardy” and “tender” relate to whether a crop can withstand frost.

The official last frost day for the Fort Collins/Loveland area is May 15. ❖

For more information on vegetables, visit the Colorado Master Gardener website at

Looking for additional gardening information? Check out the CSU Extension Horticulture Agent blog at for timely updates about gardening around the state. Visit PlantTalk Colorado for fast answers to your gardening questions at

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