by CE Wynn

Created on: May 10, 2010   Last Updated: May 10, 2012

To harden off plants it is necessary to gradually accustom plants to the likely conditions they will face in an outdoor environment.  This process is particularly important for fragile seedlings grown either indoors, in a poly tunnel or cold frame, were the environment has been kept artificially warmer and changes in conditions may result in a significant amount of shock and even death to the fragile plant.

Progressive transfer is the key

The general rule to successfully harden off plants is to progressively over 5-10 days move the plants into the new environment for a longer period of time until they are used to the new conditions. The length to harden off plants is dependent on its original location, and how different the new conditions are to the old. 

Ideally the process starts with moving the plants or seedling outdoors for a few hours.  If a few hours are not possible focus on the weather, choosing a few days where the weather forecast is mild. Move the plants outside each day and back in overnight. Continue this for a minimum of five days.

Additional Protection

It is possible to successfully harden off plants or seedlings in extreme weather by providing additional protection from the elements. The protection used will depend on the conditions outside. If hot days are a concern, shade cloth will continue to let warm air through, and protect the plant from the hot sun that may burn fragile leaves. 

If cold is the problem a plastic bag will act like a mini green house, keeping heat from any available sun close to the plant. A similar technique is used with a cold frame, the glass lid is the protection and is opened more each day until the lid is removed and the plants are exposed fully to the elements.

When using additional protection continue to keep in mind the key to hardening off plants successfully.  Keep the protection on for a few days, and progressively remove the protection for longer periods until no longer required. 

Overnight

Once a few days have past and there is no sign of stress, it is time to take the plunge and leave the plants outside overnight. If the weather is hot, and assuming the nights are mild, protection is not required.

If nights are cold continue to provide protection, especially if frosts are of concern. Use the same method for the first five days, slowly removing any protection from the plants until the plants seem to be coping well with the new conditions. By this time the plants should not

• 1
• 2
• Next Page >>