How to harden off plants


If you’re a gardener, you’ve likely hardened off your plants, or at least heard the phrase. But what exactly does that mean?

What does hardening off plants mean?

According to Colorado State University Extension, hardening off plants is the process of adjusting plants to outdoor temperatures after they’ve been started indoors.

Plants that have been started indoors as well as in greenhouses need to be hardened off before being transplanted outdoors. These seedlings are tender and are used to protected indoor environments. Outdoors, plants are exposed to sun, wind and precipitation that can damage stems and leaves. An abrupt change in environment can damage plants and affect their growth.coleus seedlings

University of Nebraska Extension in Lancaster County explains that during the process of hardening off plants, plant wells become thicker and stronger. Plant growth will slow down, cell walls will become stronger and the rate of water loss will be reduced because natural waxes on leaves will be thickened. In addition, root development will be stimulated.

How do I harden off my plants?

Hardening off plants isn’t a difficult or time-consuming process, but it takes a little bit of time over the course of about a week. Here’s what you need to do:

  1. When you decide to harden off plants, choose a mild or cloudy day. Set plants outside in a partly shady area for 2-3 hours, then bring them back inside until the next day.
  2. Continue to set plants outdoors each day, slightly increasing the amount of time they are outside each day, increasing the amount of sunlight they are exposed to each day but decreasing the amount of water each day.
  3. After 7-10 days, when plants have been kept outside for 10-12 hours a day, they should be acclimated to the outdoor environment and ready to be kept outside for 24 hours. You’ll know your plants are ready to be transplanted when they show no signs of stress from environmental factors.

University of Nevada Cooperative Extension says that the hardening off process may take a shorter or longer period of time, depending on the plant’s conditions when it was grown and marketed as well as the conditions of where it is being transplanted.

When should I transplant seedlings?

Be aware of the threat of frost before you transplant seedlings. Consider waiting to transplant after your area’s last spring frost.

Colorado State University Extension recommends transplanting on a cloudy day. Make sure you support plant stems with soil. You can water your transplants with a weak solution of fertilizer.


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