Hardening off is a process that acclimates new plants and seedlings to your microclimate.
When a plant finds itself in a new environment, it must make several adjustments to all the changes. This is called ecesis. Sudden changes in temperature, sun exposure, and wind can be fatal to plants raised in a greenhouse. Most plants sold commercially are raised in greenhouses. Greenhouses are generally warm, moist, protected areas that allow plants to get a good start. It does not, however, prepare them for the outside world. Many nurseries use equipment that bends the plants over, back and forth, a few times each day, in an effort to mimic the effects of wind. This strengthens the plants through a process called thigmomorphogenesis. It helps, but you can significantly help new plants become acclimated through hardening off.
When to harden off plants
Since all newly acquired plants have the possibility of carrying pests or disease, it is always a good idea to start them off in a protected quarantine area for 40 days. This gives you time to watch the plant for signs of infection or infestation. It also provides an opportunity to see what conditions best help the plant thrive, and it gives you time to carefully decide where in the garden this new plant will go. Depending upon the climate tolerance of the plant species, you may have to wait until it is a couple of weeks before your last frost date before you begin hardening off.
How to harden off plants
Plants should be brought outside for a few hours each day, at first. They should be in a location protected from wind, with filtered or morning sunlight. Increase the amount of time and sunlight by an hour or two each day until they are outside all day. If temperatures allow, plants can now be left outside overnight. If you are growing plants in a cold frame, you can harden them off by opening the frame a little more each day until the lid is no longer needed. Hardened off plants can now be installed in the landscape or garden, with a significantly higher chance of success.
Which plants need hardening off?
Generally speaking, bare root stock does not need hardening off. There are no leaves to dry out or get sunburned. Young blueberry plants, garden sale seedlings, and seeds you have started yourself indoors will all benefit from hardening off.
By gradually getting plants used to your microclimate, they are more likely to thrive.
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