Although soursop trees are relatively easy to take care of and usually don't require a lot of maintenance, they are quite difficult to get pollinated if left done naturally. This is why many growers hand pollinate the flowers. This is not only to increase the fruit set rate but also to improve the shape of the fruits and their quality.
But as a little side note, so I heard that bees don't seem to like to pollinate the guayabana (soursop) flowers. It's the beetle or something when they get pollen sticking around their body in one flower and then go pollinate other soursop flowers.
Anyway, let us check out how to pollinate the soursop trees now.
Choosing A Soursop Flower for Pollen
To select a flower for pollen, you should look for those that are near the top of a smaller branch or the flower itself is of a small size. This is because those flowers are relatively more difficult to get fertilized or even if so the fruit dropping rate is high or at times fruits don't get developed at all. Which means, although they might not be good candidates for fruit bearing, they could good pollen donors.
Have a look at those flowers that have their three petals slightly open outwards. Gently lift open one petal to take a look inside. If you see that the anthers are lightly darker and they are beginning separate, we can cut the flowers to collect the pollen.
The best time to pick the flowers is in the cool afternoon. After picking the whole flowers, we can lay them on a piece of paper in an airtight box or simply a paper bag to protect them from moisture loss or the wind blowing the pollen away. We can do the pollination tomorrow morning.
The next morning, you can break off the flower petals of the collected flowers. Shake the flowers a little bit so the dark gray anthers fall onto the piece of paper.
You can then use a dry cotton swab or Q-tip to gently rub the pollen out of the pollen sac (anthers). Usually, the pollen from one flower will be enough to pollinate 6-8 other flowers.
Now we can get on to:
Choosing A Flower to Pollinate
To choose a flower for pollination, you could look for those on the main soursop trunk. As a little fun fact, those fruits that grow directly on the soursop trunk are often the very best and sweetest.
We can also look for flowers on big branches, with big flower stems or with no pests or diseases for a good pollination.
Some growers choose flowers near the base of a branch or near the base of a fork branch (like near the meeting point on the letter Y) and not too far out from the main trunk. Because a soursop can weigh up to 4-7 lbs (1-3 kg) per fruit, this is so the branch doesn't have to droop or bend down way too much when bearing the fruits.
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When you see that the three flower petals are slightly open, the stigma may be of age and mature. Gently open the petals to see the stigma inside. If it seems wet and slimy, we can begin the pollination.
Let's Get to Pollinating
The best time to pollinate is around 8-9 AM in the morning. Just as well, the stigma of the female-stage flower is most receptive from 8-10 AM. In full bloom time, we can hand pollinate the flowers every 3-4 days.
When you do the pollination, you can use the index and middle finger to kind of hold the flower stem securely in place (like when someone is holding a sort of cigarette, although this might not be a good analogy for those of you who are non-smokers). But anyway then, use your thumb to slightly open one flower petal – any one is fine.
Then on the other hand, with a cotton swab in hand, dab a little pollen on. It is okay to dip the cotton swab head a bit in water. This can increase the pollen sticking rate.
Then, gently spread or rub the pollen onto the stigma. It'd be better if the pollen is spread evenly all around. Repeat this three times. The fruit will grow even and not too deformed or weird looking.
If there is a flower growing between two little fruits that have already set on the branch and you want to pollinate it, just be a little extra careful though. With a skillful pair of hands, it's okay.
However, if it's one of your first few times doing the job of a bee, then you may want to leave that sandwiched in-between flower alone. Some poking and drilling here and there could disturb the neighboring new fruits, causing them to drop. It'd be a real waste.
After Pollination
After pollination, just leave the flowers as-is and don't touch them. About 4-7 days after hand pollinating the flowers, you can have a check at the flower stem (or pistil).
If it looks still green and seems to have grown a little bigger, then congratulations this means the pollen takes and our pollination process is complete. We can then wait for the soursop fruit to form.
In another case, if the pollination isn't successful, the flower stem will turn more dark, dry up and eventually fall off. Better luck next time. We won't have any soursop to enjoy this time.
Also, if a female-stage flower has been pollinated, it will still grow and become a male-stage flower and develop anthers (with pollen). Some growers, a little bit greedily speaking, still go out and collect the pollen on these flowers. It unfortunately causes the flowers to drop. So no fruits nor pollen.
So when you have pollinated a flower just leave it as-is and don't go for more pollen. You can tie some sort of string or break off a small piece of the flower petal for marking which flower has receieved pollen. It'd be a safer bet.
After the male stage, some pollinated Annona flower petals will dry up and turn more dark. Some growers thought that these darkened flowers were dead and therefore plucked them off.
But before you do so, have a little sneak peak inside. If there seems to be a little fruit growing inside, then the pollination takes. The flower petals turning dark and dry after the two stages are normal. This is partly why you'd have a better chance of determining by looking at the flower stem or pistil (it would grow bigger and more woody). So just make sure to have a look in the right place so you don't waste any young new-forming fruits.
The Fruit
If the soursop tree is hand pollinated, the fruit set rate will generally be much higher. With a skillful pair of hands, the successful fertilization rate could be around 80-90%.
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