I still remember the first time I planted a rhododendron in my garden — I was captivated by its lush green leaves and stunning spring blooms. But after a few seasons, I noticed the flowers weren’t as vibrant, and the shrub looked a bit tired. That’s when I realized that simply planting it wasn’t enough — rhododendrons need specific care, especially when it comes to feeding. Over the years, I’ve learned through trial and error how to properly fertilize these sensitive plants, and now, each spring, I’m rewarded with full, healthy blossoms. In this guide, I’ll share exactly what I’ve learned about when, how, and what to feed your rhododendrons to keep them thriving.
Rhododendrons are highly sensitive to lime in the soil and therefore do not tolerate just any type of fertilizer. In this guide, you’ll learn when, how, and with what to fertilize these beautiful flowering shrubs to ensure lush growth and blooms.
In many gardens, rhododendrons impress with their abundant blossoms in spring. As evergreen shrubs from the heath family, they are anything but frugal — on the contrary, to produce plenty of flower buds, they need regular feeding.
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Quick Tips for Fertilizing Rhododendrons
- Best time to fertilize: Spring (March or April)
- For new plants: Use a mix of special rhododendron fertilizer and horn shavings (horn meal)
- Organic option: Coffee grounds work well
- For mature plants: Use long-lasting fertilizers and horn shavings
- Second application: Possible up until late June
Fertilizing Newly Planted Rhododendrons
Rhododendron breeder Holger Hachmann recommends fertilizing newly planted rhododendrons in March or April. If planting in autumn — which is not advised in colder regions due to potential winter damage — wait until spring to fertilize.
For plants around 30 to 60 cm (12–24 inches) tall, apply:
- 40 to 60 grams per square meter of a long-term fertilizer (e.g., Floranid Permanent or Osmocote Rhododendron Fertilizer)
- Additionally, mix in around 30 grams of horn shavings per square meter
Fertilizing Rhododendrons with Coffee Grounds
Coffee grounds are a great organic fertilizer for rhododendrons. They contain small amounts of all essential nutrients, help slightly acidify the soil, and enrich it with humus — all ideal for the lime-sensitive, humus-loving rhododendron.
- You can use coffee grounds alongside other fertilizers without needing to reduce amounts.
- A particularly effective blend: coffee grounds + horn meal
- Always work organic fertilizers lightly into the soil after spreading so they decompose faster.
Fertilizer Amounts for Mature Rhododendrons
For established rhododendrons around 70 to 120 cm (28–47 inches) tall:
- Apply in March or April during dry weather
- Use 90 grams of long-term fertilizer and 50 to 70 grams of horn shavings per square meter
For older, larger rhododendrons:
- Up to 120 grams of long-term fertilizer
- 50 to 70 grams of horn shavings
Note: These recommendations are for large-leaved varieties. Small-leaved rhododendrons, dwarf forms, and Japanese azaleas need only half the recommended amounts.
Healthy rhododendrons will have deep green, dense foliage and plenty of flower buds.
Follow-Up Fertilizing in Early Summer
A second fertilization is possible up until the end of June, using:
- A mineral fertilizer like Blaukorn Entec, or
- An organic product such as Oscorna
Don’t exceed 30 grams per square meter.
If you’re unsure whether a fertilizer is suitable for rhododendrons, check the label — if it contains lime, skip it. Rhododendrons are very sensitive to lime.
For peace of mind, just pick a special rhododendron fertilizer at your local garden center.
Tip: Fertilizing Around Mulch
If your rhododendron is mulched, carefully remove the mulch from the outer root zone before applying fertilizer directly onto the soil. If you apply fertilizer on top of mulch, it can cause faster decomposition and bind up nutrients, making them less available to the plant.
Caring for my rhododendrons has become one of my favorite spring rituals. By following these simple fertilizing tips, I’ve seen my plants flourish year after year. It’s amazing how a little attention and the right nutrients can make such a difference. If you take the time to give your rhododendrons the care they need, you’ll be rewarded with a vibrant, healthy display of blooms that will brighten up your garden for seasons to come. Trust me, it’s worth the effort! Happy gardening!
