French Marigolds (Tagetes)
• discourage and kill vegetarian nematodes (which attack root crops - eg the potato eel-worms) - but only if you plant a bed; a border around your potatoes is not enough;
• repel whitefly, and confuse Cabbage White butterflies, if planted among or as a border around brassicas;
• repel or confuse carrot fly;
• and attract hoverflies, which will eat up your aphids!
Tagetes minuta (Giant Marigold)
is apparently the most effective; as well as killing eel worm, it is supposed to wipe out ground elder, couch grass and bindweed. Worth a try - nothing lost, beyond the cost of a packet of seed.
Pot Marigolds (Calendula)
• A host for vegetarian nematodes - so do take care to choose the right marigold for the job! Calendula are however useful to attract hoverflies.
Nasturtiums
• Lure blackfly away from your broad beans - divert the little bug...s
• Repel aphids from other crops
• And repel whitefly from brassica crops
Sunflower
• A good companion for sweetcorn.
And ....
Sow some annuals, like Borage, simply to attract pollinators (bees and bumble bees), or plants like Phacelia which attract beneficial predator insects - parasitic wasps and hoverflies.
I first came across companion planting when I was working in Yemen, where farmers would plant beans beside their sorghum and millet - they told me the crop of both would be better, because each "fed" the other. It certainly seems to have worked for them.
Even in a bed of 20 plants on my allotment plot, the ones with peas grew bigger and faster than those without.
Crop rotation, green manuring, and sowing two different vegetables together (brassica and broad beans) - all examples of companion planting, and many of us are using these techniques all the time. Even weeds can be used as companion plants - not always very friendly companions, granted, but valuable for all that; they just need a bit of firm discipline!
I'm a little wary of the wonderfully intricate lists of supposedly effective pairings which can be found all over the internet. They are of widely varying quality (and practicality, for that matter). A test of a good list - are the weeds included?
I prefer using something a little more detailed and explanatory - like Bob Flowerdew's "The Companion Garden", or Gunars Ulmanis' article in Kitchen Garden (June 2000).
Enough waffle - I've listed some useful companions I'd happily use widely on my plots. More specific ideas - from experience, or from Bob Flowerdew - are listed on the respective vegetable pages. And if they're not scientifically tested? Who cares - even if they don't work, they certainly look good!
Many are edible - adding colour and/or flavour to salads.
Herbs are, I think, less flexible - I like to have them in the herb bed, where I know where to find and tend them. Let some of them flower - lavender, marjoram, roemary, oregano, sage, thyme in the herb bed are a-buzz with useful insects (pollinators and predators). Dill and fennel particularly attract ladybirds.
And a last thought - why are chrysanthemums such popular flowers on allotments? I wonder if it's because they have natural perst-killing properties? I'm sure I've read somewhere that the organic persticide pyrethrum is an extract from a member of the chrysanthemum family.
I would be VERY interested in your experience of useful companion plantings for allotment vegetable growing. What would you add to my short list - or correct on the basis of your experience? Please drop me an e-mail.