Isolation
Runner beans and the bees: why keeping a variety true is hard
French beans more or less look after themselves; runner beans do not. On a site with a hundred plots and runner beans on most of them, keeping one variety pure takes either luck or effort, and I have tried both.
Packed 24 March 2024Re-checked 17 November 20246 min
Two beans that behave differently
The runner bean and the French bean look like close cousins on the cane, but they are separate species and they set seed in different ways. A French bean flower usually pollinates itself before it has fully opened, so two varieties growing a few metres apart rarely cross. The runner bean needs a visitor. Its flower is built so that a heavy bee landing on it pushes down the keel, the curled lower petals, and presses pollen and stigma against the bee's body. That is the moment crossing happens, because the same bee has just come from someone else's beans.
On our site that someone else is nearly everyone. Walk down the main path in August and you see scarlet flowers on cane wigwams on perhaps four plots in five, with a few whites and a few of the old red-and-white bicolours among them. The bumblebees work them from first light until dusk. Seed saved from my plot in an ordinary year is therefore partly fathered by my neighbours' beans, and its children can differ from their mother in flower colour, pod length and the pattern on the seed.
The distances nobody on an allotment has
The advice from seed libraries and serious growers is to separate runner bean varieties by hundreds of metres, and some sources ask for a kilometre or more before they would call the seed pure. I once walked our site with a measuring wheel. The furthest point from my plot is under two hundred metres away, and there are runner beans in every direction. Isolation by distance is simply not possible for anyone here, and anyone who says their allotment runner bean seed is pure should be asked how.
Time does not help much either. In theory you could sow very early or very late so that your beans flower when nobody else's do. In practice the frosts decide. The last frost in this part of London usually comes in April, but I have seen one in the second week of May, and a single night below freezing kills runner beans outright. Everyone plants out at the end of May, and everyone's beans flower from July until the first autumn frost.
What I do instead
There are three honest choices. The first is to agree with the neighbours. Two plot-holders beside me grow the same old bean I do, from the same original handful, so our three plots make a small block of one variety, and the bees that visit us mostly carry the right pollen. It is not isolation, but it tilts the odds. The second is to cover a few flower trusses before they open with small bags of fine mesh, and trip the flowers myself by pressing down the keel with a fingertip, the way a bee would. Some pods set. Many flowers simply drop.
The third choice is to accept crossing and say so. Most of the seed on my plot is what I call a plot population: runner beans grown and saved here for several generations, mostly one variety but with a little of everyone else's in them. On the swap table those envelopes are labelled plainly as crossed plot seed, and people take them happily. The bagged pods are kept apart and labelled with the variety name and the words bagged and hand-tripped.
- Bag a truss before the first flower on it opens, not after.
- Use fine mesh rather than paper; the flowers need air and light.
- Tie coloured thread round each bagged stem so its pods are not picked for dinner.
- Take the bag off once pods have clearly set, about a week later, so they grow straight.
Many plants, not the best plant
The other half of keeping a variety true is keeping it varied. If I saved seed only from the single best plant every year, I would narrow the variety to one family line, and after a few generations it would lose vigour, which is what happens to any small population of a plant that normally mixes its pollen. The usual advice for runner beans is to save from a dozen plants or more. On half a plot that is a real commitment: a double row of eighteen plants on canes, with the lower pods on every plant left to ripen for seed.
Which pods I choose matters too. I leave the first well-formed pods low on each plant, because they have the longest season to ripen, and mark them with a twist of garden twine. Anything odd is pulled out before it flowers: a plant with the wrong flower colour, a weak one, one whose pods come out curled and stringy when the variety should not be. Gardeners call this rogueing. It feels cruel in July, and it is the main reason the seed is worth sharing in February.
Records that someone else can read
None of this means much unless it is written down. I keep a notebook in a zip bag in the shed, with a page for each lot: the variety, where the original seed came from, the year, the number of plants, what flowered within about fifty metres, whether any trusses were bagged, and what was rogued out. At harvest I add the date the pods came in and how they were dried. It takes ten minutes a week in summer, and it is the part of seed saving most people skip.
The same information goes onto the envelope in shorter form, because a packet of beans with no history is only beans. A neighbour once gave me seed of a handsome bean with a black-and-purple seed coat and no notes at all. I grew it for three years before an older plot-holder mentioned that it had crossed with a white-seeded variety the summer before it reached me, which explained why a fifth of my plants kept coming up with white flowers. A single line in a notebook would have saved me two seasons of puzzling.
Written from the notebook in the shed, on half a plot in Walthamstow.