Why are my durian flowers and young fruit dropping?

Durian trees often flower heavily and then shed most of what they set. Growers find flowers and small fruitlets covering the ground within days of flowering and worry that the tree is sick.

Some of this is normal. A study of durian reproduction in Malaysia noted that fruit set is usually very low, because many ovaries drop after the flowers open — either they were not fertilised, or they were damaged by animals visiting the flowers. Within a few days of pollination most young ovaries fall, leaving only one or two on each flower cluster.^[1]

So the real question is not whether flowers drop, but whether enough fruit holds to give you a crop. If almost nothing holds, season after season, something is limiting fruit set.

In Peninsular Malaysia durian usually has two main flowering seasons, around March–April and September–October, with smaller flowerings in between. The flowers open in the late afternoon and the stigma becomes receptive in the evening, so pollination happens at night.^[1]

• Poor pollination. Fruit bats, especially the cave nectar bat (Eonycteris spelaea), are the main pollinators of durian. Giant honey bees visit the flowers most often, but bats do most of the effective pollination. Flowers that are not pollinated drop within days.^{[1, 2]}
• Trees that cannot pollinate themselves. Research in southern Thailand found that most durian trees set very little fruit with their own pollen, although some clones are self-compatible. A block planted with a single clone can therefore flower well and still set poorly.^[2]
• Few bats nearby. Where bats are netted or their colonies destroyed, researchers have suggested that low natural fruit set may follow.^[2]
• Rain at flowering. Areas with more rain during flowering struggle with fungal and bacterial infections and with flowers dropping without setting fruit.^[3]
• Drought after flowering. Durian needs a short dry spell to trigger flowering. A study in Johor found flowering followed a spell of very little rain. But if the dry period drags on, trees drop developing fruit and can be damaged.^{[3, 4]}
• More fruit than the tree can carry. A stressed tree naturally aborts fruit, and very heavy fruit set can itself call for thinning.^[3]

Work through the causes above in order. Most of the fixes are orchard practice, not inputs.

• Water through fruit set. Once flowering has been triggered, irrigate if dry weather continues, so trees are not forced to shed developing fruit.^[3]
• Plant more than one clone. When planting or replanting, interplant clones so that trees can cross-pollinate. Ask your nursery or local agriculture office which clones flower at the same time in your area.^[2]
• Protect the pollinators. Do not kill or disturb fruit bats near the orchard; they are the durian's main pollinator.^[2]
• Consider hand pollination on high-value trees. Hand pollination with pollen from another tree is used to increase fruit set, and gives more even fruit with fewer empty segments. Durian pollen stays viable for about a day and can be collected and kept refrigerated.^[3]
• Thin very heavy clusters. Where set is very heavy, thin young fruit to one or two per cluster on each branch.^[3]
• Keep roots healthy. Good drainage, a layer of mulch under the canopy (kept away from the trunk) and weed control keep trees strong going into flowering. See our tip on durian roots and waterlogging.^[3]

If fallen flowers or fruit show rot, spots or insect damage, get them identified before you spray anything.

NEXGROW is a bio-organic, phosphate-solubilising fertiliser. For durian, the product booklet describes it as helping the tree form a strong root system, supporting the initiation and development of flowers, and possibly reducing the need for synthetic phosphorus fertiliser.

It is a soil and root input applied by tree age — after harvest for mature trees. It does not replace pollination or water management, which are the main levers for fruit set. The rates below are taken directly from the booklet.

Test the soil before changing your fertiliser programme, and whenever trees flower well but hold little fruit year after year even with good pollination and watering. The result tells you whether pH or a nutrient is actually short, so you correct that instead of adding more of everything.

Sample the root zone around the edge of the canopy, mixing several cores from across the block. Our team can arrange the test and go through the results with you.