How to Sow Black Soybeans: Why Late Sowing Beats Early Sowing

This is a long-season crop that punishes early sowing

A black soybean is a bigger, slower plant than the ordinary pale-seeded soybean, and it wants a different sowing date rather than the same one. Where a standard crop runs 100 to 140 days to dry beans, a black-seeded one runs 150 to 180, and it grows a frame to match: waist to chest high, heavily branched, and carrying pods that are noticeably fatter with two or three large beans in each rather than three small ones.

The trap is assuming that a longer season means sowing earlier. It does not. These are strongly late types that hold off flowering until the days start shortening after midsummer, so an early sowing does not flower any sooner. It simply spends those extra weeks building stem and leaf, and by the time flowering starts you have a sprawling plant that shades its own flowers, falls over in the first storm and sets fewer pods than a plant half its size.

Sow into early summer warmth rather than late spring warmth. Wait until the soil at sowing depth has held above 60F (15C) for several days, the last frost is clearly behind you, and night temperatures are staying above 55F (13C). That is often two to four weeks later than you would sow a green-picked bush bean in the same garden, and the later sowing consistently yields more.

The limit at the other end is your first autumn frost. Count back 170 days from it. If that date falls before your soil is warm enough, you have the season for a green picking but not for dry beans, and you should plan the crop accordingly from the start rather than discovering it in autumn.

Read the soil with a thermometer at sowing depth mid-morning on three consecutive days. One warm afternoon is not a soil temperature, and with a crop that gains nothing from being early there is no reason at all to gamble.

Depth, seed handling and getting an even stand

Black soybean seed is large, often half again the weight of ordinary soybean seed, and the seed coat is thin and splits easily. Handle it dry and handle it gently. Do not soak it before sowing: a big seed with a dark, brittle coat takes up water fast, ruptures along the coat and emerges damaged or not at all. It draws all the moisture it needs from damp soil within a few hours.

Sow one to one and a half inches (2.5 to 4 cm) deep, going to the deeper end only in light sand that dries out at the surface. The whole seed is lifted above ground on the stem as it germinates, and because these seeds are heavy that lift takes real effort. Anything sown past two inches (5 cm), or capped under a crust after heavy rain, produces seedlings that arrive bent, weak and a week late.

Sow direct into the bed. A black soybean makes a deep taproot within days and resents being moved even more than the standard crop does, because it has more top growth to support when it checks. If your season genuinely forces a start under cover, use deep modules, sow two seeds each and plant out at the first true leaf, no later.

Put two seeds at every station and thin to the stronger seedling once both have a true leaf. Large dark seed loses germination faster in storage than most legumes, and a single seed per station leaves gaps that a late-sown, long-season crop gives you no chance to fill.

1. Sow dry seed, never soaked The large seed with its thin dark coat splits when pre-soaked. Sow it dry into moist soil and let it take up water at its own rate over the first few hours.
2. Water the drill, then sow into it Run water along the open drill, let it soak away, sow, then backfill with dry soil. That gets moisture under the seed without capping the surface into a crust.
3. Two seeds a station, an inch (2.5 cm) down Firm the soil over each station with the flat of your hand so the heavy seed sits against damp soil rather than in an air pocket.
4. Thin at the first true leaf Snip the weaker seedling off at ground level. Pulling it tears the taproot of the one you are keeping, and this crop has months of growing left to do.

Spacing: give it far more room than an ordinary soybean

This is where the two crops diverge most sharply. A dry soybean block is planted tight, four to six inches (10 to 15 cm) apart, so the plants close over and hold each other up. Do that with a black soybean and you get a dense thicket that never dries out inside, drops its lower pods to disease and lodges as one mass in the first wind.

Space plants ten to fifteen inches (25 to 38 cm) apart in rows two to two and a half feet (60 to 75 cm) apart. The plant branches from low down and fills that space entirely by flowering. What you are buying with the extra room is light on the lower branches, where a large proportion of the pods sit, and air moving through the canopy during the humid late summer weeks when the pods are filling.

In a raised bed, a staggered grid at twelve inches (30 cm) each way works well and is easier to walk around than rows. Whatever the layout, leave yourself a path. You will be opening pods weekly through pod fill, and a block you cannot reach into is a block you stop inspecting.

If you intend to pick the crop green rather than take it to dry beans, keep the same wide spacing. Green picking strips whole branches at once, and you need to be able to get at the plant from both sides without treading on its neighbor.

The first three weeks, and what to write down

Large seed leaves carrying half a big black bean each are conspicuous, and pigeons, jays and mice all recognize them. This is the commonest single cause of a failed sowing and it is fully preventable: peg fine mesh or row cover over the row on the day you sow, leave it slack so the seedlings can lift it, and take it off once the plants have three or four true leaves and the seed leaves have yellowed.

Slugs do different damage and the cover does not stop them. Leaves stripped with a bare stem left standing is birds. A stem cut clean through at soil level with the top lying beside it is slugs, and the answer is three consecutive nights out after dark with a torch, plus clearing the shelter along the bed edges.

Sow the block once. Unlike a fresh green crop, there is nothing to gain from staggering a long-season bean over a fortnight, because the flowering date is set by day length rather than by the sowing date and a later sowing simply arrives at the same point as a smaller plant. If you must re-sow a gap, do it within ten days.

Write down the sowing date, the soil temperature you sowed at, and where in the garden the block sits. Then, later in the season, add the date the first flowers opened. The gap between those two is the number that matters: on a day-length-driven crop it tells you how much of your early sowing was wasted growing leaf, and whether next year's sowing should move later still.