Gardening: Sowing your seeds for a fruitful relationship

Did you know that a border of lavender will protect plants from slug and snail invasions, or that chives help prevent blackspot and increase performance in roses?

For more than 30 years, Bob Flowerdew, a regular on Radio 4's Gardeners' Question Time, has observed useful interactions between different plants, how some will flourish together while others will fail, and with this in mind he has now penned Companion Planting, an invaluable little guide to the best and worst plant partners.

He explains: "We commonly grow around 5000 cultivars in our gardens. It would be very unlikely that not one affected any other one, with a leaf secretion, root secretion, leaving a root channel behind for another to follow, leaving leaves of a higher percentage of phosphate or calcium.

"Over the years I've accumulated interesting snippets of what gardeners have observed, one plant getting on well with another. For example, I was growing crops in a polytunnel and noticed that where radishes were being grown, the peppers did really badly. One reason is competition for scarce resources."

There are ways to put this type of knowledge to your advantage, he says.

"A good example is the leguminous pea and bean family, whose members usually create a fertility surplus. By interplanting legumes with other more demanding plants, we satisfy the latter's needs without continuing effort or expense.

"Sweet peas, lupins, brooms, laburnums and many others help feed other flowers around them, and peas and French beans feed your cabbages and sweetcorn. Simultaneously, this mingling misleads and confuses pests and obstructs the spread of diseases."

However, not all plants get on. Soil types and locations are all important, and you find that plants which have traditionally been grouped together tend to get along, he observes.

"The Mediterranean bed of silver foliage plants is a natural grouping of plants which like dry, well-drained, sunny positions. The silvery colour is the hairs on the leaves which is their adaptation to prevent them losing moisture, and they don't like damp."

Many plants will benefit other plants by hindering pests and diseases, he adds.

"Roses, for instance, are very prone to blackspot and aphids. If you grow a dense bed of ornamental alliums or garlic or chives underneath the roses, anecdotal evidence would suggest that you get much less fungal disease. A bed of lavender under the roses would keep the aphids away."

Aubergines grow well with peas and thymes, cucumbers like legumes, dill, beet and carrots but dislike potatoes, while tomatoes combine well with basil, alliums and nasturtiums, but not with lettuce and radish.

"Planting dwarf French beans and cabbages together works well, and mixing brassicas with clover reduces aphid attacks and maintains yields. Broad beans significantly help main crop potatoes.

"Beans and onions don't get on. The crow-foot buttercup delphinium family seem to have secretions that harm other plants. They are very hungry themselves and need to be grown in blocks so they don't harm other things. Very few things like to be near wormwood or pittosporum."

Most glossy-leaved evergreens have substances on their leaves to prevent weed germination, but we can use these as a barrier, he says. If you are sowing seeds, don't put them near glossy evergreens, but if you are growing plants they are a good barrier because they give off scents which confuse pests and secretions which suppress annual weeds which would otherwise be coming up.