Gadenin日本語Get the appWait for 54F (12C) at sowing depth
Runner bean seed is large and starchy, and it drinks in water within a few hours of going into the ground. In soil colder than about 50F (10C) it swells, sits, and rots before the root ever breaks the coat. Push a soil thermometer down two inches in the morning, not the afternoon, and sow only when the morning reading has held above 54F (12C) for several days running. At that point emergence takes eight to twelve days; sow a fortnight earlier into cold ground and you will be resowing instead.
There is very little to gain by rushing. Runner beans climb and flower on warmth, not on how long they have been in the ground, and a stand sown into warm soil catches up with an early sowing well before the first flowers open. The plants are also killed outright by frost at any size, so the real earliest date is a week after the last frost, not the last frost itself.
If you want a head start, sow into deep pots or root trainers under cover three to four weeks before that date and plant out after hardening off. Runner beans transplant far better than most beans as long as the root ball is not broken up, but the root run has to be deep. A shallow cell tray produces a plant that stalls the moment it is planted.
| Soil temperature at 2 in (5 cm) | Days to emerge | What happens |
|---|---|---|
| Below 50F (10C) | Usually never | Seed swells and rots. Do not sow |
| 54F (12C) | 10 to 14 days | Workable but slow and patchy in wet ground |
| 61F (16C) | 8 to 10 days | The reliable target |
| 70F (21C) and up | 6 to 8 days | Fast and even. Keep the surface damp |
Never presoak runner bean seed. A soaked seed splits its coat under the pressure of its own swelling and lets soil bacteria straight in. Sow it dry and let the ground do the wetting at its own rate.
Depth, and why two inches is right
Sow runner beans two inches (5 cm) deep, deeper than most garden seed. The seed is big enough to push that far, and the depth does two useful jobs: it puts the seed in soil that stays evenly damp instead of drying out between waterings, and it anchors the base of a plant that will be carrying eight feet of wind-catching growth by midsummer.
Too shallow and the seed coat dries and stays clamped over the seed leaves, so the plant emerges bent and stunted. Too deep, past three inches, and the seed uses most of its reserves getting to the light. In heavy clay stay at the shallower end of two inches; in light sand go to two and a half.
Water the bottom of the drill or hole before you sow, drop the seed onto damp soil, then backfill with dry soil and firm it with the flat of your hand. Watering after sowing instead caps the surface into a crust that emerging beans have to fight through.
- Put the supports up first Drive the canes or poles in before you sow, not after. Pushing a cane in beside an established seedling tears through the root plate on one side and the plant never quite recovers its vigor.
- One seed per cane, plus spares Sow one seed at the foot of each cane, two inches from the base on the inside of the row. Sow four or five extra seeds at the end of the row as transplant spares to fill any gaps.
- Cover against birds and mice Mice take sown bean seed out of the ground overnight and pigeons pull emerging seedlings for the seed still attached. Netting or a cloche until the first true leaves open stops both.
- Thin to one plant per cane If both a seed and a spare come up at the same station, cut the weaker one off at ground level with scissors rather than pulling it, so the root of the keeper stays undisturbed.
Spacing, and the soil underneath
Runner beans are hungry and, above all, thirsty. The single most useful thing you can do before sowing is dig a trench a spade deep along the line of the row in late winter and fill it with compost, rotted manure or even kitchen waste, then let it settle and cover it back over. That reservoir of moisture-holding organic matter under the row is what keeps flowers setting in a hot dry spell later on, and it is far more effective than anything you can do from above once the plants are growing.
Space plants six to ten inches (15 to 25 cm) apart along the row. Closer than six inches and the canopy stays wet after rain, which is where halo blight and gray mold start. Wider than ten inches and you waste the support. A double row of canes needs eighteen to twenty-four inches (45 to 60 cm) between the two rows so you can walk in and pick from both sides.
Do not add high-nitrogen fertilizer to the trench. Runner beans fix a good deal of their own nitrogen, and an excess produces a wall of leaf carrying very few flowers. Phosphate and potash, or simply well rotted compost, are what you want.
| Layout | Plant spacing | Row or structure spacing |
|---|---|---|
| Double row of crossed canes | 6 to 8 in (15 to 20 cm) | 18 to 24 in (45 to 60 cm) between rows |
| Wigwam of six to eight canes | One plant per cane | Canes 10 in (25 cm) apart at the base |
| Single row on netting | 8 in (20 cm) | One plant per 8 in of net |
| Large container | 3 plants per 15 in (38 cm) pot | One wigwam of three canes per pot |
What to write down at sowing
Record the sowing date, the soil temperature you actually measured, and how many days passed before the first seedling broke the surface. Those three numbers together tell you next year whether your usual sowing date is early, late or right for your own ground, which no general advice can tell you.
Also note where each row sat in the plot. Runner beans should not follow beans or peas on the same ground for at least three years, because root rots and foot rots build up in the soil, and by the following spring nobody remembers exactly where last year's row ran. A quick sketch with a measurement from a fixed edge is enough.