Sowing Mung Beans: Warm Soil, Spacing and How Fast They Come Up

Wait for warmth, then move quickly

Mung beans are a hot-weather crop and they behave like one. Seed sown into cold, damp ground sits, swells and rots rather than germinating, so there is nothing to gain by sowing early. Wait until the top four inches (10 cm) of soil are reliably above 65F (18C) and night temperatures have stopped dropping below about 55F (13C). In practice that is a fortnight or more after the last frost, at the point when you would be sowing sweetcorn or okra rather than peas.

The reward for waiting is speed. In soil at 70 to 80F (21 to 27C) mung beans emerge in three to five days, and a week later they are already branching. The whole crop runs roughly 90 to 110 days from sowing to the first dry pods, which is short enough that a late sowing still finishes in most warm-summer climates. It is also short enough to follow an early crop: ground cleared of garlic, broad beans or early potatoes will take a mung bean sowing and still ripen it.

Sow direct. Mung beans have a taproot that resents disturbance, and transplanted seedlings sulk for a fortnight, which cancels out any head start you thought you were getting. Modules are only worth the trouble if your soil warms very late and you can hold the plants to no more than two true leaves before planting out.

In a climate with a long hot season you can make two sowings three to four weeks apart. The second one extends the picking window without doubling the daily work, because a mung bean crop is picked over repeatedly rather than harvested in one go, and two overlapping blocks quickly become more picking than a home garden wants.

Do not soak mung bean seed before sowing. It germinates fast enough without help, and pre-soaked seed going into cool or wet soil splits and rots far more readily than dry seed does.

Depth, spacing and the shape of the plant

Sow one inch (2.5 cm) deep, or an inch and a half (4 cm) on light sandy ground that dries fast. Space the seed two to three inches (5 to 8 cm) apart along the row and thin to four to six inches (10 to 15 cm) once the seedlings have their first true leaves. Rows go 18 to 24 inches (45 to 60 cm) apart. That is wider than it looks like it needs to be at sowing, and the reason becomes obvious in six weeks.

A mung bean is a bushy, branching plant 24 to 30 inches (60 to 75 cm) tall, wider at the base than a bush snap bean and inclined to sprawl outward once it is carrying pods. Crowded plants shade their own lower pods, hold moisture in the canopy and give you mildew and pod rot at exactly the point in the season when you are trying to dry pods on the plant. Air moving through the row is worth more than a few extra plants.

Give it full sun and free-draining ground. Do not enrich the bed with manure or a nitrogen feed: mung beans fix their own nitrogen within a few weeks of emergence, and added nitrogen produces a large soft leafy plant that flowers late and sets poorly. An ordinary bed that grew a hungry crop last season is ideal, with no further preparation than raking it level.

1. Check the soil temperature, not the calendar Push a soil thermometer four inches (10 cm) down in the morning. Above 65F (18C) two mornings running means sow. Below that, wait a week and check again; nothing is lost by sowing into properly warm ground.
2. Rake level and draw shallow drills Drills one inch (2.5 cm) deep, 18 to 24 inches (45 to 60 cm) apart. Do not dig in manure or compost first. Rich ground gives leaf at the expense of pods on this crop.
3. Sow every two to three inches (5 to 8 cm) Sow at roughly double your final density to allow for gaps. Cover, firm the soil down with the back of the rake and water once, thoroughly, unless rain is due.
4. Cover against birds until the seed leaves open Emerging beans push their seed leaves up whole and birds pull them out of the ground. Row cover, netting or even a sheet of chicken wire laid flat for the first ten days is enough.
5. Thin to four to six inches (10 to 15 cm) at first true leaf Pinch surplus seedlings off at ground level rather than pulling them, so the taproots of the keepers are not disturbed. Do it early; thinning a plant that has started branching sets it back.

Sowing figures at a glance

The numbers below cover the whole sowing decision. Nothing here is a fine judgement call except the soil temperature, which is the one worth being strict about, because a cold sowing either fails outright or produces a slow, uneven stand that never fully catches up.

Seed to sow, and what to write down

Mung beans are almost entirely self-pollinating, so seed saved from your own plants comes true and germinates well for three or four years if it was dried hard and stored cool. Once you have grown the crop for one season you should never need to buy seed again, and that is worth planning for at sowing: keep a note of how much seed a given row length took, so you know how much to hold back from the harvest.

Record the sowing date and the soil temperature you sowed at, and then the date the first seedlings broke ground. Those three figures together tell you next year whether you sowed too early. A gap of more than a week between sowing and emergence almost always means the ground was still too cool, and the answer is to sow ten days later next season rather than to blame the seed.

Note the bed position too. Like all legumes, mung beans should not come back to the same ground for three years, and unlike most of them they leave the soil visibly better than they found it, so it is worth knowing which corner has had the benefit.