When to sow runner beans

Two different species climb poles in our gardens. Runner beans (Phaseolus coccineus) have scarlet, white or bicoloured flowers, rough, broad pods and big, often mottled seeds. They are the classic in British gardens. Climbing French beans (P. vulgaris), the Stangenbohnen that dominate German gardens, have small white or lilac flowers and smoother, rounder pods.

They grow the same way, with a few differences worth knowing. Runner beans germinate in slightly cooler soil and cope better with a cool, exposed site, but they need bees and bumblebees to pollinate their flowers, and they set pods poorly in hot, dry weather. Climbing French beans pollinate themselves and want more warmth.

Beans are killed by frost and rot in cold, wet soil, so there is no point rushing. Runner beans germinate from about 50–54°F; climbing French beans want nearer 59°F.

In the UK, sow runner beans outdoors from mid-May in the south and early June further north. In Germany, Austria and Switzerland, sow from mid-May, after the Ice Saints cold spell around 11–15 May, until mid-June.

For a head start, sow one or two seeds per pot indoors from mid-April to May and plant out in late May or early June, once frost is over. Pot-raised plants also dodge bean seed fly and slugs at their most vulnerable stage.

Choose a sunny spot sheltered from strong wind: a full row of beans on 98 in canes catches a lot of it. Dig in plenty of compost or well-rotted manure, ideally the autumn before, so the soil holds moisture through summer.

Put the supports up before you sow. The British way is one cane per plant: a double row of canes 6–12 in apart, the rows 18–24 in apart and tied together at the top, or a wigwam of five or more canes. Sow two beans 2 in deep at the foot of each cane and keep the stronger seedling.

The traditional German way is one tall pole per clump: four to six beans in a ring 1.2–1.6 in deep around each pole, with poles about 16–24 in apart. Either way, supports should stand 87–98 in tall.

In a 120 × 80 cm raised bed, a double row of 8–10 canes along the north side gives you a solid crop without shading the rest of the bed.

Guide the young shoots to their canes; after that they climb on their own. Beans need little water until they flower, but from flowering on, water regularly and generously in dry spells, and mulch to keep the soil moist.

Flowers but no beans? With runner beans, that is usually hot, dry weather or too few pollinators. Water well at the roots in dry spells, and don't hope for miracles in a heatwave: set improves once it cools down.

When the plants reach the top of their supports, cut off the extra growth.

Runner beans usually start cropping in midsummer, from July; climbing French beans about 70–75 days after sowing. Pick runner beans at about 8–10 in long, before the seeds bulge, and climbing French beans younger, when they snap cleanly.

Pick every two or three days. Plants left with old pods stop flowering, but picked regularly they keep going for eight weeks or more.

Never eat the beans raw. Both species contain lectins that cause vomiting and diarrhoea; cooking destroys them.

Slugs and snails can strip young plants overnight; planting out sturdy pot-grown plants helps. Black bean aphids gather on shoot tips, and red spider mite can build up in hot, dry summers.

Bean seed fly maggots eat germinating seed in cold soil. Sow into warm soil or raise plants in pots.

Rust brings brown to black pustules under the leaves, usually late in summer. Halo blight and anthracnose spot leaves and pods and are carried on seed, so don't save seed from diseased plants.

Runner and climbing beans are legumes. Leave three to four years before any legume returns to the bed.

Summer savory is the traditional partner for beans and is said to deter black bean aphids; we found no trials behind it.

Practical advice: tall beans cast a lot of shade, so plant them on the north side of a bed, and keep other legumes out of the same bed.