Companion planting - protect with plants

Companion planting is one of those soft, fuzzy, feel-good ideas which float around habitually when you’re discussing growing vegetables. It’s a good concept, that by planting one thing next to another, you can cut down your problems with pests. Put garlic with carrots and wave goodbye to the peppering of your roots with lots of tiny holes excavated by the carrot root fly maggot. Under-plant your tomatoes with French marigolds – tagetes – and white fly will never appear. Intersperse lines of the thyme-like annual herb, summer savoury with your broad beans and the black bean aphid doesn’t infest the growth tips. How nice this all sounds, but is it true?

‘It’s mainly folklore not science’ says Andrew Halstead, chief entomologist at RHS Wisley. Pauline Pears at the HDRA, the organic gardening organisation agrees that proper controlled trials have yet to be done on most partnerships, but there are plenty of good anecdotal stories which encourage one to have a go.

There are various mechanisms by which companion planting is thought to work. The three I’ve mentioned use smell, the fragrance, nice or nasty of the helper plant masking that of the vulnerable crop you’re trying to keep clean. The pest doesn’t realise you’ve got the tasty things in your garden and goes to infest your neighbour’s crop.

The tomato and tagetes pairing is a classic example. The tagetes 'Simba' pong – and it has a strong, unusual smell, at its most powerful when the plants are in full flower – is repellent to the whitefly, so it keeps it away.

Under the guidance of HDRA last year, a nearby school in Coventry did an informal trial of this duo. They planted up one greenhouse with tomatoes and no French marigolds. The other identical tomato variety was grown with marigolds interspersed. The first lot were dripping with white fly, and in the second, there wasn’t a sign until they finished flowering.

I use Tagetes linnaeus, a flamboyant brightly-coloured form dotted around tomato, aubergine and chilli plants at a ratio of one, to two. Plant some tagetes in hanging baskets, as well as some at the base of the tomato to protect the plant tops as well as the bottoms. It's important to remember to dead-head to keep the flower going and to sow once in March or April and again in a couple of months to give protection later in the year.

The garlic or onion and carrot partnership uses a similar mechanism and this is one of the few companion plant associations which has been properly assessed. Trials have concluded that you need four times as many onions as carrots for the roots to remain completely clean and that the onions need to be in active growth – before they start bulbing – to properly repel.

I did my own mini trial of these two last summer, mixing up a packet of carrot seed with a packet of my favourite, handsome, deep-red, sweet and tasty spring onion, ‘North Holland Blood Red’. I didn’t count the number of seeds, but chucked in four or five good pinchfulls of each interspersed. They were sown in a series of lines in a panel about ten foot long and five foot wide and harvested from July until November, and were almost, if not quite one hundred per cent hole free.

I also tried growing summer savory with my broad beans. The French are much keener on this partnership than we are and many organic growers I’ve talked to there swear by it. Autumn sown bean crops are less vulnerable to pests and diseases, but winter and early spring sown ones benefit from the herbs protection.

Your broad beans can be sown straight into the ground from about Valentines day on – they’re as hardy a veg as you will find – but the summer savory needs a bit more T.L.C. inside. To get it to the right stage at the right time, you can’t wait until your soil has warmed up enough in April for safe direct sowing, so sow it in early March into modular seed trays and plant it straight outside from them.

I planted the savory in a double line swathe around two double lines of broad beans and it seemed to do the trick. I was late to pinch out the growth tips, another anti black fly move, and even so, my bean plants were clean.

Proper scientific trialling apart, there’s too much positive chat amongst serious and amateur organic growers to ignore this feel-good natural approach to vegetable hungry insects. Try out these three combinations and others and let me know how you get on.