Runner Bean Care: Watering, Feeding and Why Flowers Fail to Set Pods

Flowers that set no pods

A runner bean in full flower that drops every one of them and sets nothing is the single most common problem with this crop, and it is worth being clear about the cause: it is almost never a disease and there is nothing to spray. Three things cause it, usually acting together. The roots are dry, the nights are too warm, or there are not enough bumblebees working the flowers.

Dry roots are the commonest and the one you control. A runner bean carrying its full canopy moves an extraordinary amount of water, and when the supply falls short the plant abandons its flowers first, because a flower is the cheapest thing to abort. Notably the leaves can look perfectly fine while this is happening, so wilting is not the warning sign. The soil eight inches down being dry is the warning sign.

Night temperature is the second cause. Runner bean pollen loses viability when nights stay above about 68F (20C), and if the days are also above 86F (30C) the flowers simply drop. Nothing you do rescues that particular week, but the plants recover completely when the weather breaks, so do not pull them up. Keep them watered and picked and they flower again.

Pollination is the third. Runner beans are not properly self-fertile in the way that green beans are; the flower needs a heavy insect to force the keel open, and in practice that means bumblebees. A garden with few bumblebees sets few pods, and short-tongued bees make it worse by biting a hole through the base of the flower to steal the nectar without ever touching the pollen. You will see those neat slits at the back of the flowers if it is happening.

Misting the flowers with water is often recommended and does very little. What actually helps is water at the roots and a deep mulch, because the plant's water status is what decides whether it holds a flower.

Watering: deep and infrequent, never little and often

Once flowering starts, a row of runner beans needs the equivalent of about four gallons per square yard (22 liters per square meter) every four or five days in warm weather. That is a large amount and it has to go in slowly enough to soak down rather than running off. A light sprinkle every evening is worse than useless: it keeps the top inch damp, which encourages surface rooting, and leaves the deep roots dry.

Water at the base of the plants, in the morning if you can, and let the foliage stay dry. Wet leaves overnight in a dense canopy is the invitation halo blight and gray mold are waiting for. If the row is on a slope, scoop a shallow basin around the base of each plant so the water sits and sinks instead of running down the path.

Mulch matters as much as the watering. Three to four inches (8 to 10 cm) of compost, grass clippings or straw laid over damp soil once the plants are a foot tall halves how fast the bed dries out, and holds the root zone cooler through the hot spells that cause flower drop. Lay it on wet soil, never on dry.

1. Check by finger, not by eye Push a finger or a trowel in to a depth of four inches beside the row. If it comes out dry at that depth, water heavily, regardless of how the surface looks.
2. Water in one long soak Give the whole allowance in one go every few days rather than a little each evening. Deep soaking pulls the roots down to where the moisture stays.
3. Mulch as soon as the soil is warm and wet Wait until the plants are about a foot tall and the soil is thoroughly damp, then mulch thickly right up to but not touching the stems.
4. Increase during flowering, not before Before the first flowers open, moderate watering is fine and encourages deeper rooting. From first flower to the last pick, water heavily and consistently.

Feeding, and the nitrogen trap

Runner beans fix nitrogen through nodules on their roots, so they need far less nitrogen fertilizer than their size suggests. Feeding a general high-nitrogen fertilizer gives you exactly what you do not want: an enormous mass of dark leaf with flowers thinly scattered through it, and a canopy so dense it stays wet and shades out its own lower flowers.

Once the first flowers open, switch to a high potash liquid feed, the sort sold for tomatoes, every ten to fourteen days. Potash supports flowering and pod fill and it does not push leaf. If the trench under the row was prepared with compost, that plus a potash feed is the whole feeding program.

Yellowing between the veins of the older leaves in midsummer is usually magnesium, common on light soils that have had a lot of potash. A foliar spray of Epsom salts at an ounce per gallon (7 g per liter) corrects it within a fortnight. Uniformly pale plants with poor growth from the start are more likely a nitrogen shortage from nodules failing in cold wet ground, and those recover as the soil warms.

Records that pay for themselves

Write down the date the first flower opened and the date you picked the first pod. The gap between them is normally twelve to eighteen days, and if in some year it stretches out or no pods come at all, the flowering date tells you exactly which week of weather to blame. That turns a vague memory of a bad year into something you can act on.

Also log how often you watered and how much, plus a note of any run of hot nights. Over two or three seasons the pattern becomes obvious: the years with good set are the years the row was never allowed to dry out during flowering, and seeing that written down is far more persuasive than being told it.