Bolting is the term used for when a vegetable plant starts to seed or flower before it should. Onions and shallots are a biennial crop, meaning that they grow in the first year and flower in the second. When an onion or shallot bolts, it grows a flower in the first year.

Onions and shallots, along with garlic, are all part of the Allium family, a plant that naturally flowers once every two years. But bolting isn't welcome on your patch when you're trying to grow lots of good-quality bulbs to eat.

The best thing you can do to avoid bolting is to plant 'heat treated' sets. Our hardy heat-treated onion sets include:

• Rumba
• New Fen Globe
• Fen Early
• Red Fen
• Red Ray

They are all available to buy between autumn and spring. These special sets have been exposed to high temperatures in a controlled zone, making them extremely resistant to bolting.

Our heat-treated onion sets can be planted from mid-March. They arrive with us after a minimum of 12 weeks of heat treatment, ready for immediate despatch and immediate planting into pre-warmed soil. Prepare your planting bed and warm your soil from mid-February by using fleece or plastic cloches or covering the ground with fleece that’s securely held down with pegs. Plant the onions as described and keep the cloche or fleece on for the first few weeks. Remove once the plant tip has started growing.

There are lots of varieties of onions and shallots available with good resistance to bolting. These include:

• Onion Troy
• Onion Stuttgarter Stanfield
• Onion Autumn Gold
• Onion Sturon
• Shallot Zebrune
• Shallot Biztro
• Shallot Yellow Moon
• Shallot Picasso

If you notice that the onion and shallots that you're growing have started to bolt, there's no need to panic; here are some tips to help:

• As soon as you see onions and shallots develop flower heads you should snip off the flower at the top of the stalk. Or if the stalk is quite thick, snip it all off about 1 inch (2.5cm) above the bulb but not the leaves. Doing this stops the plant from wasting energy on making seeds.
• Once you've snipped the flowers, the bulb can be left in the ground or harvested. Onions or shallots that are left in the ground won't develop any further, so it’s a good way of keeping plants until you want to use them.
• If an onion has bolted, you don’t need to wait until the foliage turns brown before you harvest. Leaving them that long could cause the bulb to start to rot or become woody.
• If some of your onions or shallots have bolted but others haven't, always harvest the bolted ones first. The un-bolted bulbs have a good chance of going on to reach full maturity and can be harvested later. Bolted onions will keep for a week or so in the cupboard, but it's best to use them straight away while they're fresh. If there are too many to use within seven days, they can be chopped and frozen for future dishes.