Hi Sven,

My partner and I were presented with three onions in small pots to grow as big as we can by the end of August this year, for a fun competition back in the old village. I lean more towards the flower garden with my greenhouse etc, while my partner is a gardening novice. He can cut the grass beautifully, but thinks Virginia Creeper is a horror film! Can you advise on the best way to cultivate these little beauties, and stop the "mould" etc listed in your guides from ruining them please?

Kind regards,

Plodding Prickles

Our Gardens Expert replies:

Hi there,
Ideally with onions you should plant them in soil that has been prepared a good few months ago, which I doubt you have done! Onions don't require too much nitrogen (which promotes leaf growth) and freshly manured soil is full of it.

You can grow onions very easily in pots or containers and this may be the best option for you. You always run the risk in open ground of pests and diseases attacking the plants and in a pot you can provide the ideal growing medium for your plants. You are growing these onions for a competition so I would not want to take any risks!

Onions prefer a well drained, nutrient rich soil, so I would simply use any good compost mixed with a bit of grit for drainage - don't be tempted to use garden soil because this may contain diseases. The pot should be big enough for the root system to develop properly, so go for one that will be a little taller than the foliage will grow in height. You will have to make sure to keep them well watered through dry periods and apply a general purpose fertiliser once a fortnight through the growing season.

Many people swear by using compost tea, once when you plant the sets and again later in the season. Compost tea is when you extract the nutrients from compost or well rotted manure by soaking it in water for anything from a few days to a few weeks and using the water as a fertiliser.

The 'mould' you refer to is probably 'white rot', which is a persistent disease that appears at first, like a white fluffy mould. By growing them in a container you should not suffer this as a problem....in the garden if you find a case of white rot you are in trouble! The disease will linger in the soil for up to 15 years and there is no chemical control. There is another disease called onion neck rot that is not as bad, but if you become infested you will need to clear the bed and not plant onions for at least another 2 years.

Basically, if you grow them in new, well drained compost, give them plenty of water (not too much though) and feed them regularly they should go on to produce some nice plump onions! This all depends on which variety you have of course, but that is another whole subject!

Good luck!
Sven