Hardening Off Plants

Hardening OffWhy you need to harden off plants:

Seeds sown indoors or plants grown at home from cuttings will have become accustomed to the conditions of your home or your greenhouse. In order to survive when planted out they will need to be gradually acclimatised to the harsher growing conditions outside, where temperatures are cooler, humidity is lower and there is greater air movement. If the shock is too great then plants will not thrive and growth can be severely stunted. Hardening off will allow your plants to make the changes and adaptations needed to do well in their new outdoors growing position. Hardening off will thicken and alter the plants leaves and increase their waxiness. You should note, however, that hardening off can only prepare plants to a degree and frost-sensitive plants will not become hardy.

When to start the process:

Hardening off usually takes around two or three weeks. The warmer the conditions that the plants have become used to, the longer it will take the plants to adapt for outdoors growing. Hardy plants can be hardened off much quicker than half-hardy or tender plants. To know when to begin the hardening off process, especially for tender plants, it is essential to know the last frost date for your area. This is of course earlier in the south than it is in the north of England and Scotland. For a lot of us, the last frost date falls somewhere in April, so this is the month to consider hardening off many of your plants.

How to harden off plants:

By Michael CorneliusHardening off plants is a process that goes in stages. What exactly those stages are will depend on the facilities that are available to you at home. If possible, plants grown inside should first be moved, ideally on a dull, overcast day, to an unheated glasshouse and should remain there for a couple of weeks before being moved to a well-ventilated cold frame and kept there for at least a few days before they are moved to their final growing position. If you do not have a greenhouse then you can move plants to a cold frame right away, first leaving the lid very slightly open during the day and closing it at night, then gradually moving over the next couple of weeks to leaving it open all the time. Cloches can also help to protect plants while they harden off, but it should be noted that these do not offer as much protection as a cold frame. Horticultural fleece can also be used to give extra protection to plants, especially if there is a sudden late frost.

If plants are damaged by cold temperatures, you can sometimes cut out the damaged areas and continue the hardening off process. Pay attention to plants and keep on top of any pests such as greenfly that can multiply as the weather begins to warm.