This month my knowhow article encourages the roof top veg plotter to aim high. It is a wonderful way to increase the crop space in a small garden, it provides shade in summer for salad crops, as well as for your own forehead as you sit in a deckchair on hot summer days.
The climbing vegetable I've had most success with is the runner bean. It likes its feet in moist rich soil and, as long as the young plant is protected from the ravages of spring gales it will climb through trellises and wigwams like a trooper.
Originally imported into this country as an ornamental, runner beans have delicate shell like flowers in a myriad of colours from pure white, through pinks and purples to deep red. I like red flowers, so I grow the red emperor, which is an allotment standard, reliable in any weathers, decorative and producing long succulent mid green pods.
Planted alone, I don't think runner beans do as well as if they are treated to the companionable proximity of sweet peas. This duet seems to work well. My guess is that the flowers attract insects and the combination of two different nitrogen fixers keep the fertility up.
Plant the seeds, two per 2” pot in early spring in the greenhouse. If both come up, pinch out the weaker seedling early. The remaining one should be planted in very rich open ground as soon as the danger of frosts is past. At the same time as I sew the seed, I dig out my bean beds and lay semi-composted kitchen waste in a trench before replacing the top soil. Even though all climbing legumes seem happy in 150mm deep beds, I heap up soil as high as possible to give them the very best start. This is as much about maintaining moisture in the soil as giving the roots a good run.
Baby runner beans don’t like the cold. Plant them out when all danger of frost is past, in my garden that is usually mid May. So protect them with fleece if spring is chilly or windy (a problem on the roof top). After a week or two they tend to look a bit weedy and the leaves might become yellowish. This is the sign to start feeding. One feed with a seeweed supplement or worm tea may well be enough. After that the roots will have developed and the nitrogen cycle will have started. After that they will need very regular and copious watering, but may need no further feeding.
The only other task is to help them climb by attaching the tendrils to strong canes, strings or wires. And do remember to pinch out the tops; otherwise you will be balancing precariously on ladders in order to get at your crop. The first few weeks of flowering is nail-biting. Will the pollinators come, will they set? Eventually, after a few false starts, tiny beans will be visible. Then your only problem will be what to do with all those runner beans. Invest in a good runner bean stringer, a little kitchen gadget that cuts the beans and discards the woody string at both edges. Pick regularly and when the beans are still young. http://www.edinburghfoody.com/2010/07/31/favourite-gadget-runner-bean-slicer/
Because these little beauties are heavy croppers, it is highly likely that you will get fed up with them by mid-September. Beans that you have missed start growing gargantuan and stringy. Then it is time to leave the beans on the storks to ripen seed for next year, or use the beans as a winter vegetable. The coloured varieties are tasty, but cook down to a rather unappetising grey colour. White cultivars, like Czar (The Organic Gardening Catalogue), will produce a white butter bean, that tastes delicious in a casserole.