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The main reason that rhododendron buds are falling off is usually drought or lack of water when grown in pots. Usually, they develop buds around June and July, so ensure they have enough water during this time as they form. More on this below. Drought is not the only cause of bud drop, so in this article, I also identify other causes and what you can do about all of this.
Not getting enough water
Rhododendrons usually develop flower buds from mid to the end of the summer. However, if there’s been a dry spell up until then, the plant may not have enough water to form the buds correctly. They could only partially form and then drop off.
What to do
The only solution here is to prevent this environmental situation by ensuring that you water your rhododendrons well all throughout the growing season. Remember that these plants like moist but well-drained soil, so check that they haven’t become water-logged after a watering session.
You can adjust the ground to prevent rapid evaporation of moisture by placing a layer of acidic mulch around the base of the plant. Bark chipping is good for this. Keep the mulch about 20cm to 30cm away from the trunk of the plant so that a warm, moist environment doesn’t develop there, as this can cause offer issues. If grown in pots, make sure you keep them well watered. You might also want to consider using a soaker hose in your beds and borders if you live where drought to more common.
Vine weevils are beetles which eat the leaves of the rhododendron. They’re generally most active in the spring and again at the end of the summer. A lack of photosynthesis from the affected leaves can cause a lack of energy for the plant to keep the flower buds going, and in the worst cases, they drop off.
What to do
Regularly check your plant for these little brown bugs and pluck them off the leaves. You can trap the adults by putting sticky traps around the plants and even in the rest of your garden.
To determine if you have other pests that are harming your flower buds, read my guide on Rhododendron pests and diseases.
Bud blast spread by leaf hoppers can cause bud drop
Bud blast is a disease thought to be spread by the rhododendron leafhopper insect. The insect itself doesn’t do any real harm to your plant, but it carries this disease. Bud blast is a fungal disease that kills the flower buds.
The leafhoppers are cream coloured wingless insects that cling to the underneath of the rhododendron leaves. You can find them there if you look closely. But you probably more often come across the cream skins that they shed. The problem is that the female leafhopper makes cuts in next year’s buds in the autumn and lays her eggs there. The bud blast disease infects these incisions. The infected flower buds turn brown, die and drop off.
It’s worth noting that some gardening experts have some doubts abut the connection between leafhoppers and bud blast disease.
What to do
You can’t do anything about the leafhoppers themselves. However, pick off the infected flower buds and get rid of them. This reduces the amount of fungal spores around the plant and so there are fewer spores to spread and cause damage.
Encourage birds and ladybirds/ladybugs into the vicinity as these like to feed on the leafhoppers. You can also use an organic spray such as natural pyrethrum.
Root rot is a problem with many plants. This is a soil fungus that eventually causes the leaves to wilt, turn yellow and die. If you notice this happening in your leaves as well as your flower buds falling off, check for root rot. Root rot occurs if the soil is too wet. To check, uncover some of the roots and check if they, and the surrounding soil, are very wet.
What to do
If you catch the root rot early enough, you can save the plant by removing the affected roots carefully. Adjust the soil by adding sand or grit so that it drains properly before covering up the roots again. This is usually an issue with newly planted rhododendrons. Keep a close eye on the wetness of the soil when you water the plant.