The Q&A Archives: Fertilizing Astilbes

Question: Two years ago I moved about 12 astilbes of various
kinds to a new garden which is under a very large
red oak tree. My soil is very sandy, but I amended this
spot with compost and black dirt and then added wood chips
to the entire area. I know its very important to keep this
area well watered, but what kind of fertilizer would be
best for these astilbes? A recent soil test showed that
all nutrients are well balanced but these plants have not
had good blooms for a couple of years.
Thank you for any info you can provide

Answer: In my experience, astilbes do best with a rich, organic soil that is evenly moist. Since your soil is sandy, it would need a generous amount of organic matter such as rotted leaves, aged stable manure and bedding, or compost worked into it followed by annual top dressings of compost and a year round organic mulch to aid in a continuous feeding for the soil. The organic matter would also help the soil hold moisture better over a longer period of time. Keep in mind too that a sandy soil will leach nutrients relatively quickly, along with the moisture.

Since soil tests show that the nutrients seem to be sufficient, I would suspect that water has become the limiting factor in the blooming performance. If you still feel additional nutrients might be beneficial, you could add additional compost or well rotted well aged manure in early spring and/or late fall.

It is also possible that the tree is causing too dense a shade, since interestingly enough these plants will tolerate quite a bit of sun and bloom very well provided the soil is moist.

Finally, if the plants are very old and have grown into very large clumps, they might benefit from being divided to stimulate vigorous new growth. This would best be done in very early spring.

I hope this helps you trouble shoot.

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