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Within three to four weeks of emergence a mung bean has formed nitrogen-fixing nodules on its roots and is supplying itself. Anything you add on top of that is worse than wasted. A fed plant grows large, dark and leafy, flowers later than it should, sets fewer pods and holds so much foliage that the ripening pods sit in damp shade. You end up with a magnificent plant and a poor crop.
If the leaves look slightly pale for the first three weeks, that is normal and it corrects itself as nodulation gets going. Resist the urge to intervene. The one exception is genuinely exhausted ground, where a low-nitrogen dressing supplying phosphorus and potassium before sowing does help pod set; even then, keep nitrogen out of it.
Mung beans prefer a soil pH around 6.2 to 7.2. On strongly acid ground the nodule bacteria work poorly, which produces exactly the pale, stunted growth people try to cure with fertilizer. If your beans are consistently yellow and small on a bed where other crops do fine, test the pH before you reach for a feed.
This is a crop that repays being left alone. If you feel you should be doing something for the plants, weed around them or open up the row rather than feeding them.
Water steadily, then stop on purpose
Mung beans are more drought tolerant than most beans, with a deep taproot that finds moisture well below the surface, but they are not indifferent to water. The period that matters runs from the first flowers opening through to the pods reaching full size, roughly three to five weeks. Drought in that window drops flowers and leaves you with short, half-filled pods, and no amount of watering afterwards fills them.
Through that stretch, aim to give the bed the equivalent of about an inch (2.5 cm) of water a week if rain does not supply it, delivered as one thorough soak rather than daily sprinkling. Water at the base of the plants in the morning. Wetting the foliage in the evening in warm humid weather is how powdery mildew and leaf spot get started on a crop that is otherwise fairly clean.
Then stop. Once the first pods start losing their green and turning toward brown and black, the crop changes from a growing operation into a drying one. Continued watering at that stage keeps the canopy damp, slows ripening, encourages mold on the pods and can cause seed to sprout inside a pod that got rained on. If you are growing in containers, this is the hardest part, because a pot dries in a day and the temptation to keep it wet is strong. Reduce watering rather than cutting it dead in a pot, and move it under cover in a wet spell if you can.
| Stage | Water | Feed | What you are watching for |
|---|---|---|---|
| Emergence to first true leaves | Keep the surface just moist | None | Birds pulling seedlings, slugs, cold nights |
| Branching growth | Only in a dry spell | None | Weeds, and a pale color that is normal at first |
| Flowering | About an inch (2.5 cm) a week, one deep soak | None | Small yellow flowers, and any dropping without setting |
| Pod fill | About an inch (2.5 cm) a week if no rain | None | Short or flat pods, which mean water stress |
| First pods darkening | Reduce sharply | None | The start of the picking-over period |
| Main ripening | Stop completely | None | Mold in the canopy, shattered pods on the soil |
Keeping the plant open and off the ground
A mung bean is a bush, not a climber, so it needs no trellis. What it does need is air. By flowering the plants have branched out and met across the row, and a dense humid canopy in late summer is where mildew, leaf spot and pod rot begin. Anything you do to keep air moving through the row pays for itself twice over: healthier leaves, and pods that dry cleanly on the plant instead of going gray.
The main tool is spacing, decided at thinning. After that, strip the lowest leaves and any weak inward-facing side shoots once the plants meet in the row. Pull weeds out from under the plants rather than leaving them to add to the thicket. If a plant has sprawled outward under the weight of pods so that its lower branches lie on the soil, prop them with a short stick or a loop of string on a stake. Pods touching damp soil discolor and rot, and they are also the ones slugs find first.
In a very wet or humid climate, a low cloche or a sheet of clear cover over the row during the ripening weeks is genuinely useful, provided both ends stay open. It keeps rain off drying pods without trapping moisture. It is not needed at all in a dry-summer climate, where the plants will do the job unaided.
- Weed hard for the first six weeks A young mung bean is easily buried by summer weeds. Hoe shallowly between rows until the plants meet, after which they shade the ground themselves and weeding mostly stops.
- Mulch once the soil is thoroughly warm An inch (2.5 cm) of straw or dry grass clippings after the plants are six inches (15 cm) tall keeps moisture even through flowering. Do not mulch early: it insulates cold soil and slows a heat-loving crop.
- Open the row when the plants meet Remove the lowest leaves and any spindly inward shoots. You are aiming to see soil through the row when you look down at it, not a solid mat of foliage.
- Give one deep soak a week through flowering Water at the base in the morning, enough to wet the soil six inches (15 cm) down. Frequent light sprinkling keeps roots shallow and wets the leaves, which invites mildew.
- Cut the water as the first pods turn The moment pods start going from green to brown, the crop wants dry air. Prop any branches lying on the soil at the same time, before pods start rotting underneath.
Growing them as a green manure instead
Mung beans make an unusually good short-season green manure, and it is worth knowing that even if you are growing them for seed. Sown thickly on an empty summer bed, they cover the ground in three weeks, smother weeds, fix nitrogen and can be cut and dug in at the start of flowering, around six to eight weeks after sowing. That is short enough to slot between two main crops in a way that most cover crops cannot manage.
For that use, sow at roughly twice the density you would for a seed crop, broadcast rather than in rows, and rake in. Do not thin. Cut the whole stand down at ground level when the first flowers show, which is when the nitrogen in the plant is at its highest and before the stems get woody, then either dig the tops in or leave them on the surface as a mulch. Leave the roots in the ground: the nodules are the point.
Allow two to three weeks between digging in and sowing the next crop, so the green material has begun to break down. If you are short of time, laying the cut tops on the surface and planting through them works nearly as well and disturbs the soil less. A bed treated this way in high summer noticeably improves the crop that follows it, and it costs one packet of cheap seed.
What to record during the growing season
Write down the date the first flowers opened and the date you saw the first pod turn dark. Set against your sowing date, those two say whether the crop is on schedule to ripen before your weather turns, and if it is consistently late, they say to sow a fortnight earlier next year or to accept a green-manure crop instead of a seed crop.
Note how much water the bed got through flowering and whether pods came out short. Short, half-filled pods almost always trace back to a dry fortnight at flowering, and a season's record turns that from a mystery into a fixable mistake. Note the row position as well, for the three-year legume rotation and because the following crop in that ground will benefit from what the nodules left behind.