THE longest day falls on Sunday, and this is the time to stop using high nitrogen fertilisers to boost growth.

This is especially important with onions, according to the National Allotment Society.

After the longest day the plants will stop leaf production and divert all of their energies into swelling the bulbs in preparation for lifting and storing. Any additional nitrogen used now will only encourage the development of soft tissue that won’t be able to ripen and will cause any onions in long term storage to rot during the winter.

From June onwards the garlic and shallots become ready for harvesting. Wait until about 50 per cent of the top growth has turned to a straw colour before lifting.

Don’t be tempted to cut the foliage down. Pick a sunny drying day and lay the lifted plants flat on crates to complete the ripening of the bulbs. Once the foliage has completely dried the plants can be strung together and hung up somewhere dry and well lit. Allow the skins to dry and become papery before splitting away individual cloves before using them.

For harvesting onions grown from sets, wait until the tops begin to fall over naturally. Don’t be tempted to bend them over this will only damage the neck of the bulb and cause a soft rot to set in. As the tops fall over lay them neatly between the rows a bit like ‘sardines’.

The next step is to carefully push a garden fork under each onion bulb and gently lever it upwards to snap the roots. This will stop the bulb growing any further and tell it to start the ripening process. After a week or two the entire onion plant is ready to be dug up. Don‘t pull the onion out of the soil. If it is warm and the soil is dry lay the plants on the surface of the soil with the base of the onion facing south to dry off. If the soil is wet lay the plants in a greenhouse or frame to protect them.

The leaves don’t have to be crisp at this stage; they can be allowed to dry off over the coming weeks. Once the leaves have dried out completely they can be pulled off the plants. Remove the loose dry skins, but don’t peel the tight layers of skin off the bulbs. The traditional way to store onions is to tie them to a string and hang them up. It is far more practical to store them in net sacks or trays.

Store the onions somewhere that is cool, frost free and light to keep them dormant over the winter months. If onions are stored in warmth they will break their dormancy and start to grow new leaves.