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Mung beans need roughly 90 to 110 days from sowing to the end of the harvest, and the sequence is compact. They germinate in three to five days in warm soil, branch out over four to six weeks, flower for two to three, and then produce pods that ripen a few at a time over a further three to six weeks. That last stretch is the long one, and it is why the crop feels bigger than its 100 days: you are picking over the row every two or three days for more than a month.
Everything upstream serves that ripening period. It needs warm, dry, airy weather, and if it collides with a humid or rainy spell the pods mold on the plant. You cannot move the weather, so you move the sowing: work back roughly 100 days from the point when your settled dry late-summer weather ends, and that, or the first day afterwards that the soil is above 65F (18C), is when the seed goes in.
The calendar below is written by stage rather than by month so it reads the same in either hemisphere. In a short-summer climate the honest answer may be that the seed crop will not finish; in that case grow the plants as a green manure instead, cut at flowering around six to eight weeks, and take the soil benefit rather than the beans.
| Stage | Trigger | What to do | What can go wrong |
|---|---|---|---|
| Bed choice | Before sowing | Full sun, free-draining, unenriched, no legume for three years | Rich or wet ground gives leaf, root rot and few pods |
| Sowing | Soil above 65F (18C), two weeks or more after last frost | One inch (2.5 cm) deep, seed every two to three inches (5 to 8 cm), rows 18 to 24 inches (45 to 60 cm) apart | Cold soil rots the seed instead of germinating it |
| Emergence | Three to five days after sowing | Net against birds until the first true leaves open | Seedlings pulled out whole and not replaced |
| Thinning | First true leaves | Thin to four to six inches (10 to 15 cm), pinching at ground level | Crowded plants give a humid canopy and mildew later |
| Branching growth | Two to six weeks after emergence | Weed hard, mulch once soil is hot, add no nitrogen | Feeding produces a large plant with few pods |
| Flowering | Small yellow flowers, six to eight weeks in | One deep soak a week if dry; check tips for aphids twice weekly | Drought now drops flowers and leaves short pods |
| Pod fill | Slender green pods lengthening | Keep water steady; open the canopy; prop sprawling branches | Water stress gives half-filled pods that never recover |
| First pods darkening | Pods turning brown then black | Cut watering sharply and start the picking routine | Rain now molds pods and can sprout seed inside them |
| Picking over | Every two to three days for three to six weeks | Take only fully black brittle pods; dry each picking on a tray | A slow round loses beans to shattered pods |
| Clearing | Flowering stops, plants yellow | Take all remaining pods, cut at the base, leave the roots in | Debris left standing shelters next year's pests |
| Storing | Beans hard enough not to dent | Shell, winnow, jar airtight, freeze a week against weevil | Damp beans mold in the jar and will not sprout |
The three moments that decide the crop
Most of this calendar is undemanding, and a mung bean row will look after itself for weeks at a stretch. Three moments carry nearly all of the risk, and knowing which they are lets you relax about the rest.
The first is the sowing date, which is entirely a soil temperature decision. Sow into ground above 65F (18C) and the crop is up in under a week and away; sow into cool damp ground because the calendar said so and you get rot, gaps and a stand that never catches up. The second is water through flowering and pod fill, the three to five weeks when a dry spell costs you pods that no later watering will restore. The third is the discipline of picking over every two or three days once pods start turning black, because that is the difference between beans in a jar and beans on the soil.
Everything else, the weeding and the mulching and the occasional aphid wash, is ordinary garden work that can slip a few days without consequence. Put your attention on the three that cannot.
- Count back 100 days from the end of your settled dry weather That date, or the first day after it that the soil is above 65F (18C), is your sowing day. If the two dates cannot be reconciled, grow the crop as a green manure instead.
- Diarise the thinning at first true leaf Roughly ten to fourteen days after sowing. Thin to four to six inches (10 to 15 cm) then, not later, because a plant that has begun branching resents the disturbance.
- Set a weekly deep soak from first flower About an inch (2.5 cm) of water a week at the base of the plants in the morning, right through pod fill. This is the window where water turns directly into beans.
- Switch to every-other-day picking when the first pod turns black From that signal the crop is in its long harvest. In hot dry weather a ripe pod shatters within two days, so the interval is set by the weather, not by convenience.
- Do not jar beans that dent under a thumbnail The thumbnail test is the last gate in the season. Beans sealed at 14 percent moisture mold in the jar and will not sprout, which undoes the whole point of growing them.
Where mung beans fit in the rest of the garden
Because the crop wants heat and occupies its ground for a single warm season, it slots neatly after an early crop. Ground cleared of garlic, broad beans, overwintered onions or early potatoes is usually free at exactly the moment the soil is warm enough to sow, and a mung bean block will finish on it before autumn. The plants leave nitrogen-fixing nodules behind, so leave the roots in the soil when you clear the tops.
The same speed makes it one of the better summer green manures. Six to eight weeks from sowing to flowering means it can fill a gap that is too short for most cover crops, smothering weeds and improving the bed while it does it. On a small plot it is worth deciding at sowing which job a given block is doing, because a green-manure block is sown dense and cut early, while a seed block is sown at spacing and left to ripen.
Keep to the three-year legume rotation either way, and mark the bed on whatever plan you keep. Soil-borne root rots build up quickly where beans follow beans, and this is a crop that is easy to sow repeatedly in the same convenient hot corner without noticing.
The record that makes the second year better
Seven entries cover the whole season: the sowing date and the soil temperature you sowed at, the emergence date, the first flower, the first black pod, the last picking, and the cleaned bean weight against the row length. Written down as they happen, those turn a novelty crop into one you can plan, and they answer next winter's real question, which is how much row you need to keep your sprouting jar going until the following summer.
Add two notes alongside them. Record whether the ripening period collided with wet weather, because if it does two years running the fix is an earlier sowing and nothing else. And record which plants stayed clean and which showed mosaic, mildew or borer, so that the beans you set aside for next year's seed come only from the good ones.
Keep the bed position on the same note. It matters for the three-year rotation, and it matters because the ground you grew mung beans on comes out of the season better than it went in, and the crop that follows deserves the benefit.