Every year, British gardeners buy around 10,000 tons of niger (Guizotia abyssinica) seed to feed to garden birds. It’s a particular favourite of goldfinches. Niger seed is normally at least 99 per cent pure (that is, almost all of it actually is niger), but out of the huge quantity imported, even a low level of contamination adds up to quite a few other seeds. So what are they?
It isn’t that easy to find out, since just tipping a bagful on the garden and seeing what comes up doesn’t get you very far. As it’s nearly all niger, seedlings of other species tend to get swamped. Niger is also subtropical, and so are most of the contaminants, which means they often don’t do well in a normal British summer.
However, cometh the hour, cometh the man, and amateur botanist Gordon Hanson has been sorting through bags of niger seed for years now.
He’s got pretty good at identifying the most frequent contaminants from their seeds. Anything he doesn’t recognise, he grows on, in a greenhouse if necessary. He published his findings in British & Irish Botany, the journal of the Botanical Society of Britain & Ireland.
So far, Mr Hanson has found roughly 90 other species. Almost half of them are grasses, including crops such as oat, rice, millet, teff and sorghum, plus a bewildering variety of weeds. Mr Hanson notes that one of these, Digitaria ciliaris (tropical finger grass), is not only an aggressive weed in Ethiopia (the source of most of our niger), but is now infesting his own garden.
Perhaps not surprisingly, since niger is a daisy, the second most frequent family is a variety of other daisies. Among these is Tagetes minuta (Mexican marigold), sometimes sold on the strength of helping to control the growth of perennial weeds such as bindweed, although I think the jury is out on that claim.
Another unattractive daisy is Ambrosia artemisiifolia (ragweed), an American native that’s now a serious weed throughout the warmer parts of the world, and also a major cause of hay fever. Ragweed has popped up once or twice in my garden, presumably from bird seed.
Other crops, or at least wild crop relatives, include lentil, flax, white lupin, Chinese mustard, radish, turnip, various amaranths and sesame. Ornamental plants – or at any rate, weeds sometimes grown for aesthetic reasons – include Nicandra physalodes (apple of Peru, or shoo-fly plant), Cleome rutidosperma (fringed spider plant), Oenothera stricta (fragrant evening primrose), Salvia tiliifolia (lindenleaf sage) and Euphorbia heterophylla (desert or Japanese poinsettia – not a patch on real poinsettia).
Arguably the most curious plant to turn up in niger seed is Cuscuta campestris (yellow dodder). This American is closely related to our own dodder (Cuscuta epithymum), a rootless twining parasite, most frequent on heather and gorse, and often quite common on heathland in the south, especially by the coast. It can sometimes make an area of gorse look like it’s completely covered by a blanket of red string.
Most dodders are harmless, but a few are serious agricultural weeds, including yellow dodder, and all the niger imported into the US is heat-treated to kill the seeds of this species. Yellow dodder doesn’t normally set seed here, so this precaution is unnecessary.
But as Mr Hanson notes, “One of the most intriguing autumnal sights regularly reported under British bird tables is Cuscuta campestris scrambling up the 2m-tall stems of Guizotia abyssinica”.
One thing’s for sure – if you keep your eyes open, you never know what you might find growing under the bird feeder.
Ken Thompson is a plant biologist. His most recent book is Notes from a Sceptical Gardener, the second collection of his Telegraph columns. Visit books.telegraph.co.uk