There may be several reasons why the dragon fruits are falling off. The first one is when the fruits have been left on the trees for too long. They naturally drop off then because they are over-ripe.
When the mother branches are not strong enough to nourish the fruits, they abort them. This is to direct energy on making sure of the plant survival. If the fruits don't fall off, the plants may get exhausted and die. Let's see some more:
Lack of water
Although dragon fruit is a type of cactus, they like good moisture around the base but not standing water. This interesting thing has been observed by a dragon fruit grower in Australia. Her friends say, 'Oh dragon fruit is a cactus, why do we need to water it a lot'. From her insights, watering moderately actually helps with fruiting & fruit sweetness, something that her friends haven't had for a while.
If the plants don't get enough water to nourish themselves, then they basically can't support their juicy dragon fruits. Thus, they let them go & drop the fruits. Maybe later our dragon babies.
You can see when buds, flowers & fruits are forming they suck the water out of the branches like they've been left hungry for weeks. The mother branches may get thinner, less plumb & dried up. It takes a lot of energy.
So approaching the fruiting stage, water lightly but frequently. Allow them time to absorb the moisture. Check if the soil is well-draining. Also, see if the weather in your local area is too sunny or a bit cooler. Then, adjust the water amount accordingly. Water, with CO2 & sunlight, helps with photosynthesis to create the sugar for the fruits & plants.
Too much water
On the other extreme, over-watering might cause fruit drop. This happens in farms where they've been severely flooded for quite some time. It might be worse if the water has a high salt content.
Fruits that are nearly ripe or still young will begin to fall off gradually one by one. This may be a signal that the mother plant roots are in some trouble & starting to rot. This can spread to the stem of the plants.
As this signals to the plants, 'Baby fruits, we may not be able to support you & your future growth in a situation like this', they drop the fruits to conserve some energy for ensuring survival.
So don't water the plants too much. If there's too much water 10-15 days before the fruits reach their peak, they may crack. Make sure your soil drains well & doesn't have hardened clayey stuff underneath.
Not enough nutrients
Maybe you've been feeding your plants enough food, but check if there are any little suckers in the soil. These are the fungi or bacteria that may be enjoying the food with your dragon fruits.
Check the soil pH as well. If it's around 6-7, then it's good. If it's quite low, around 4-5, then it may be too acidic. Acidic soil is a great home for the bad guys. It inhibits nutrients intake by the roots.
This may also be caused by the design of design fruit posts. If the in-ground land to plant the dragon fruit posts is clayey & hardened, then nutrient intake may be low.
You may be able to tell this by looking at the mother branches while carrying fruits. Are they crinkled up & much thinner than usual.
As the plants approach flowering & fruiting stage, feed them some more phosphorous & potassium. Chicken manure is a great source of potassium. Growers usually top dress their soil with this stuff. It also helps decompose the hay.
Pests & Fungi
Fungi like the dragon fruit canker Neoscytalidium dimidiatum can get onto the dragon fruit. When they penetrate deep inside, there's almost no way to save the fruits then. The fungi suck the good juice out of the fruits. The fruits then may turn dry & fall off. This disease spreads really fast.
We think this might relate to other environmental factors. Not sure how to treat this for now. But we think preventative measures are better. We'll share with you more as we find out more about this.
Some folks use copper-based spray. They make sure the water amount is not too high, especially in rainy season when there's high humidity. Don't over-feed the plants too much. This weakens their natural defense system & thus they grow dependent on extra nutes. The excess amount which the plants don't take in any more is enjoyed by the fungi. Some folks say that over-feeding might also be a reason for fruit dropping.
Maybe it's all natural
Just like the phenomenon of bud aborting, mother nature may change her mind & decides to drop the fruits as conditions are not supportive or sufficient. If you've already checked everything from the soil, the water, the roots, the pests and your plants are doing okay, then it may be too many fruits and the mother plants need to shed some off. Don't worry too much in this case. It will be okay.
Weak pollen from the start
Sometimes, if the pollen is weak from the beginning, then fruits may form but they grow quite weak. They don't look like they're going it make it. Other than genetics, rain may affect the quality of the pollen. Fresh ones work better than some older or not-well-stored ones. You can see an example here:
To prevent dragon fruits from dropping off, try:
- Water lightly but more frequently as fruits are forming
- Off gas the chlorine for some days if you use tap water
- Not watering too much as it may cause rot & cracking
- Rainbow sprinkle the base so it's just moist
- Feed P & K during flowering & fruiting stage
- Some chicken manure to boost K & decompose hay
- Make sure the soil is not too acidic & is well-drained
- Don't over-feed (love but not too much love)
- Use fresh strong viable pollen
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