Pollinating and keeping young durian fruit: operations at the right time

Durian is mainly pollinated at night, the natural fruit setting rate is only 20-40%. Additional manual pollination increases 60-80%. Instructions on how to practice correctly + how to keep young fruit after beans.

Summary: Durian has bisexual flowers but weak self-pollination. Pollen dries quickly in the sun and rain, flowers bloom mainly at night (15-22 hours), depending on flower bats and some nocturnal insects. In many orchards in the West, natural enemies have decreased, causing the natural fruit setting rate to be only 20-40 percent. Additional hand pollination increases this rate to 60-80 percent, especially with Monthong and Musang King varieties.

Applies to: Durian orchards that are 5 years old in business in the Southwest and Southeast. Especially orchards growing hybrid varieties need a high passing rate such as Monthong, Musang King.
Duration: 7-15 days at the peak of ovulation of a plant. The whole orchard can last 20-30 days.
Difficulty level: Advanced. Need to understand flower biology and work at night.
Estimated additional costs: 1-3 million VND per hectare for additional pollination.

Understand the biology of durian flowers

Before pollination, you need to know how durian flowers "operate":

Bisexual flowers, having both pistils (female) and stamens (male) on the same flower.
The pistil ripens 2-3 hours before the stamen — this is a natural mechanism to avoid self-pollination.
Flowers start blooming from 2-3 pm, stamens release pollen from 7-10 pm.
Pollen dries quickly in the sun. The next morning, the flower drops its petals and is no longer able to pollinate.
The flowers of this tree can pollinate the flowers of other trees if many varieties are grown.

The mechanism of "pistils ripen before stamens" makes durian dependent on cross-pollination. The orchard only has one variety = low pass rate. The orchard has 2-3 varieties mixed together = much higher success rate.

When is additional pollination needed

Not every orchard needs it. Consider:

Recommended: orchard of a single variety, orchard far from the forest (few flower bats), orchard in a heavily sprayed area (few insects). Especially Monthong and Musang King varieties — the rate of natural beans is very low.
Can be omitted: mixed multi-variety orchard (Ri6 + Monthong + Thai durian), orchard near natural forest with flower bats, pure Ri6 orchard (this variety self-pollinates better).

Decision based on previous crop yield. If fruit set is consistently below 40 percent — additional pollination is needed. Over 60 percent — not needed yet.

Additional hand pollination process

Prepare before pollination night

Survey the orchard during the day — determine if the plants are starting to shed their pistils. Flowers that are about to release their pistils have slightly separated petals and the pistil extends beyond the petals.
Prepare the tools:
Headlamp (head lamp) — keep both hands free.
Soft brush or small paintbrush.
Small box containing collected pollen.
Notebook.
Wear clothes that cover your arms and legs — there are insects and snakes at night.
Start work from 7:30 p.m. to 10:00 p.m. — the time when pollen is at its freshest.

Collect pollen for

Pollen from other plants:

Look for male or bisexual flowers that have fully bloomed and the stamens have released pollen (usually at 7-9 p.m.).
Use the brush to gently touch the stamens, the pollen will stick to the pen.
Or cut the stamens lightly and collect them in a small box.
Store pollen in a cool, out-of-sunlight place. Pollen keeps for 6-12 hours in good conditions.

Pollination on the female pistil

Look for a flower with a ripe pistil — the pistil extends beyond the petal, there are drops of mucus on the tip of the pistil (a sign that the pistil is ready).
Use a brush to touch the pollen to the stigma. Tap lightly a few times to make the powder adhere evenly.
Each plant can pollinate 30-50 flowers in one night.

Technical Note

Do not take pollen from the same plant for pollination. Pollen from other plants has a much higher success rate.
Do not pollinate if it is going to rain in the next 2-3 hours. Rain washes away the chalk.
Do not pollinate if the wind is strong — the pollen will fly away.
Mark pollinated flowers with a small colored string — for tracking later.

Keep the young fruit after it sets

After pollination, the flowers lose their petals within 2-3 days, and young fruits begin to develop. This is a sensitive period — young fruit loss is the biggest problem.

Common causes of young fruit loss

Water shock: watering too much or too little. Especially after fruiting, the tree is sensitive to moisture fluctuations.
High nitrogen fertilization: plants push out young shoots instead of growing fruit. Young fruit falls because there is no nutrition.
Heavy rain followed by dry: biological stress. Young fruits drop en masse after 2-3 days.
Spraying while fruiting: chemicals cause shock, causing fruit to fall.
Competition between fruits: fruits are too thick on the branches, some fruits are not nutritious enough and have to fall.
Stalk borers: dark stems, falling fruit.

Measures to keep young fruit

Keep soil moisture stable — neither suddenly dry nor waterlogged. Drip irrigation is ideal.
Reduce nitrogen for 30 days after fruit set — only apply potassium, phosphorus, and micronutrients.
Foliar supplementary spray: amino acid + calcium-boron + potassium nitrate (KNO3) 1 percent. Spray in the cool afternoon, 2 times 7-10 days apart.
After heavy rain do not water immediately. Wait until the soil is dry before watering.
Monitor for stem borers — spray if symptoms appear.

Preliminary pruning of fruit after 25-30 days

The young fruit is about the size of a duck egg:

Discard deformed fruits (distorted, misaligned).
Remove fruit that is crowded in a stuck position.
Let the fruit grow towards the sky (easy to fall due to gravity + sunlight).
Keep the fruit's stem firm, dark red, balanced in shape, growing downward or horizontally.

This is a "preliminary" pruning — a second pruning when the fruit is the size of a chicken egg (45-60 days after beans) will make the final decision.

Monitor and evaluate

[ ] Fruit setting ratio = number of fruits set / number of flowers pollinated. Count 50 flowers on 5-10 random plants. Target above 60 percent.
[ ] Rate of young fruit dropping after 30 days. Target less than 15 percent.
[ ] Left stem — firm, dark red is good. Soft, dark spots are at risk of falling out.
[ ] Sign of a tree "budging" — young buds appear at the tip of fruit-bearing branches. If seen, need to intervene immediately.

Common mistakes

Get pollen from the same plant that needs pollination: the success rate is much lower than pollen from other plants.

Pollination when the pistil is not yet ripe: the pistil does not have mucus yet, pollen does not stick.

Daytime pollination: pollen has dried, stigma has dried. Not effective.

Using nitrogen right after fruit set to "nourish the fruit": counterproductive. The tree pushes its buds and drops its young fruit.

Mass spraying during fruit setting: causes shock and fruit drop. Only spray when there are clear pest symptoms.

Take notes

Night of pollination — day, number of plants, number of flowers pollinated.
Pollen-donating plants and pollen-receiving plants.
Fruit setting rate compared to the crop without additional pollination.
Rate of young fruit dropping after 30 days.
Yield at the end of the season.

References

*Floral biology and durian pollination* — Southern Fruit Institute, 2022.
*Instructions for flower pruning, fruit pruning and additional pollination for durian* — Department of Horticulture, 2023.