You have successfully sown seeds indoors or under cover. You may now wish to move these to their final growing positions outside in your garden. But you need to harden off seedlings before you do so.

What Does It Mean to Harden Off Seedlings?

There are a number of reasons for the necessity of hardening off seedlings before planting them outside in your garden. For one thing, they are unused to the wider range of temperatures that will be experienced outside. They will also be unused to the buffeting of breezes and to strong sunlight. These factors will also mean that, when moved outside, your plants will experience greater rates of evaporation and could struggle to cope with the greater variation in water supply. 

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 to Harden Off Seedlings

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. The month around this date is the time to consider hardening off many of your plants.

How To Harden Off Seedlings

Hardening off seedlings 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 greenhouse and should remain there for a couple of weeks.
• Next, they should ideally be moved to a well-ventilated cold frame and kept there for at least a few days. A cold frame will usually have the facility to allow for the exposure to be increased gradually over the number of days by opening the lid wider, and for longer each day.
• Finally, the process is complete and seedlings can be moved to their final growing positions outside in your garden.

Make It Easier to Harden Off Seedlings With a Cold Frame

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. 

A cold frame will make the process of acclimatizing seedlings to outdoors conditions far easier. But in order to acquire this useful piece of garden equipment, you do not need to buy one expensively. It is fairly easy to make your own cold frame using inexpensive reclaimed materials which can often be found online or at a local reclamation yard.

Making a Cold Frame

The first stage in constructing a cold frame is to find/buy a piece of glazing to make the lid of your cold frame. This lid will be raised or lowered to expose plants to the breeze while still giving them a little bit of protection. An old window is ideal. The size of this old window will determine the eventual size of your cold frame.

The cold frame will be rectangular in shape, but remember, it should slope down towards the front, ideally facing south, certainly in full sun. Measure your new lid and determine the size of the rectangular base that you will have to make. There are a number of ways to make the base from materials you may have easy access to. Bricks, stones or even logs from your garden can make a very sturdy cold frame but the easiest and cheapest option is probably to construct a simple frame from reclaimed timber. 

Wood is relatively easy to work with and you will need only rudimentary woodworking skills. You will need some wood (e.g. old stud timber) for the framing and some planks to make the sides of your cold frame. You can choose to nail or screw. You will also need some hinges and a stick that will hold up the lid at a couple of different heights when you want it open. Decide on your exact design before you begin.

When you have all the measurements, your design plan and materials to hand, you can start the assembly. Begin by creating the base and sides, then fit the sloping top frame to which your lid will be attached.

When your cold frame base is finished, you can hinge on your lid and you are ready to furnish your cold frame and harden off your plants.

Other Items to Help Harden Off Seedlings

Cloches or row covers 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 glazed 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.

Harden off seedlings correctly and you are far more likely to end up with a garden full of healthy and productive plants.