companion planting: friends with benefits

Most of us love the idea of companion planting, the underlying principle being that by uniting one species with another we’ll free a plant from the scourge of some particular pest or have a miraculous effect on productivity.

Certain plants have a negative effect on the growth of others, a fact that we can use to our advantage – to control perennial weeds, for example. We need to be aware of this in the veg garden, where some things love each other, but others really don’t.

I’ve experimented in my veg patch and am pretty convinced that several combinations are worth repeating, but to glean a bit more expertise I consulted three vegetable growers with decades of organic gardening under their belts. I talked to Chris Smith of Pennard Plants, who uses lots of companion planting at his nursery; the no-dig expert Charles Dowding; and David Blake from Worton Organic Garden in Oxfordshire, who has been growing veg for more than 30 years.

Off the scent

First to the plants with strong smells – these can be used to camouflage the odour of precious crops that a pest is after. Under the impression that the smell of onions deters carrot fly, I have always interplanted spring onions with my carrots, but apparently this well-known companion planting is rubbish. Geoff Hamilton did controlled experiments and found no evidence that the onion gave any protection. This was backed up more recently by trials Charles carried out in his Somerset organic veg and salad garden. To deter carrot fly, it’s best to use Enviromesh or fleece around the edges of the carrot bed, stretched as a screen three-feet high. The pest is a ground flyer and won’t make it over the screen.

There are more positive reports of summer savory – a strong and delicious herb – used to protect broad beans from black bean aphid. The aromatic herb prevents the aphid from smelling the beans. This, combined with pinching out the tips of the broad bean plants when they’re just starting to form beans at the base, has kept us aphid-free at Perch Hill for the past five years. David also tells me that broad beans and potatoes planted near each other inhibit the pests that attack the other. He also finds that all beans grow well near carrots, cucumber, cabbage, lettuce, peas, parsley and cauliflowers, but less well near onions, garlic, leeks and fennel.

We’ve also had success here with Nicotiana tabacum, the true smokers’ tobacco, which protects brassicas from cabbage white butterflies. The tobacco grows huge to form a natural brassica cage, masking cabbages and kales with its strong, acrid smell. This looks much nicer than a netted cage, but you need a whopper garden to fit it all in. For a smaller space, Chris Smith recommends hyssop. He says you don’t need much of it to have the repellent effect – only three or four plants in a 4 x 15ft brassica bed. Artemisia plays the same role: “You’ll see the cabbage whites coming in to land,” he tells me, “but they then get a whiff, have second thoughts and go on somewhere else.” This works best on a sheltered site, so the wind doesn’t carry the strong smell away.

Chris also finds that tomatoes are brilliant with asparagus. The tomato exudes a chemical that repels the asparagus beetle, yet the plants are not big early enough in the year to overshadow the asparagus. The asparagus thrives, pest-free.

My greenhouse is already full of whitefly so I’ve just sown a packet of marigolds (tagetes). I remember visiting Simpson’s Seeds – the tomato and chilli company – at its nursery in the walled garden of Longleat and seeing all its tunnels of tomatoes packed with tagetes. Plants were growing at ground level and in baskets hanging from the roof, so that the tops and bottoms of the tomatoes were protected. Simpson’s likes the French marigolds 'Dainty Marietta’ and 'Red Safari’; I love the taller 'Linnaeus’. They all play the same role.

Then there’s mint, brilliantly effective if you have an ant infestation. Chris uses this successfully time and again on benches in the greenhouse. If the ants get bad, just tear up a bunch of mint and scatter it and replace every few days. The ants disappear.

Fatal attraction

The next category of companions are the sacrificial plants, so tempting to pests that they draw them away from more precious crops. At Chatsworth I’ve seen lettuce allowed to flower and go to seed around the veg beds, a sacrificial crop for slugs. They eat the lettuce rather than the choice crops in the middle.

And there’s the famous nasturtium, which secretes a mustard oil that insects love. They seek out nasturtiums in preference to any brassica, and nasturtiums in the greenhouse protect tomatoes and cucumbers against whitefly.

As David says: “Nasturtiums are irritatingly rampant growers, but before you succumb to the impulse to rip them out, consider the good they do.”

The same is true of basil. If you ever have basil in a greenhouse, it will be crawling with whitefly long before the tomatoes succumb. Plant basil in pots so you can move them outside every few days and so keep the whitefly at bay.

Some types of companion planting work by drawing in good insects, rather than repelling the bad. It’s an excellent idea to attract as many hoverflies to your veg garden as you can, as they’re the best natural predators of aphids. At Perch Hill, we intersow kale with Calendula officinalis 'Indian Prince’ for just this reason. The strong colour of calendulas attracts hoverflies, lacewings and ladybirds. The insect females feed on the calendulas’ protein-rich pollen before laying eggs on colonies of aphids, which provide a ready food source when the larvae hatch.

Sow marigolds in succession to keep flowers coming, or follow them with any of the umbellifers, the poached egg plant or tagetes. It’s the orange or yellow colour range that hoverflies love best; this also draws in other pollinators, thus increasing your harvest of many important and popular crops, such as runner beans, tomatoes, cucurbits and fruit.

For the same reason, David allows lots of dandelions to flower in his grass, particularly in the Worton orchards. Bees love the combination of pollen and nectar that dandelions provide and, as they flower in April and May when all the fruit blossom is out, they are invaluable for fruit pollination.

I’ve noticed that garlic chives are a fantastic attractant to pollinators, so much so that I’ve wondered whether the honey from our local bee hives is tainted with the taste of garlic. And chives, as Chris reports, have other uses. He makes a tea from their leaves, brilliantly powerful against downy mildew. The high sulphur content of their leaves makes a quick and effective treatment.

Lots of people have told me they had a poor crop of tomatoes last year in their greenhouses, almost certainly due to the lack of pollinators.

David has a tip for this and earlier greenhouse crops, such as courgettes. The early flowering salvias, such as S. algeriensis and S. fruticosa, are what you need to attract bumblebees to your glasshouse. Bumblebees, as David says, are the veg gardener’s best friend.

“They fly out at about 5-6C, while honeybees stay housebound until around 9C. Besides, bumbles are thorough; they pollinate every flower. Honeybees are fastidious and so do less good.”

Soulmates

There are certain plants that are simply good friends. Planted together, they give you a better crop than they would in isolation. I remember discovering this about aubergines and basil in Crete.

There, every veg patch is packed with basil, yet they hardly eat it; gardeners there are convinced that basil ups the productivity of aubergines and peppers, so they leave it where it is. In a similar vein, David swears by dwarf beans, beetroot and potatoes planted in alternate rows. They help each other to stay healthy and make a good yield; he also reports that cucumbers luxuriate in the shade of sunflowers or sweetcorn.

Chris recommends growing cucumbers with climbing beans – French or runners – and has noted an increase in cropping of both by planting them close by. Charles agrees that veg plants like company. He tells me he once planted Brussels sprouts at the standard spacing of 18in on two beds, and on one of the beds planted lettuce in between. After a month, he says: “The Brussels with lettuce were noticeably larger than the Brussels alone, suggesting that small plants at wide spacings do feel lonely!”

Lettuce crops quickly and can be pulled out when the cabbage, kale or Brussels need more space.

Finally, on to the plants with roots that exude chemicals repellent to other plants and insects. Chris tells me that tagetes roots work against soil pests, such as eelworm, so they’re brilliant near potatoes. And Tagetes minuta is effective against perennial weeds such as bindweed, couch grass and ground elder as it gives out a chemical from its roots that is toxic to them. It sounds far-fetched, but I can vouch for its efficacy: it cleared ground elder from my rose garden and yet had no effect on my roses. They’ve gone from strength to strength, yet the ground beneath them is now clean. With vegetable seed sowing now at full tilt, it could be time to take these symbiotic relationships seriously and introduce more companion plants to your own garden.

This article first appeared in The Telegraph 27th May 2013.