Question
hello James,
  I have about twenty astibes that I divided two years ago. I've had them for five years. Although I water them well,I can tell that they need something. I would like to know how can I amend the soil for their best growth?

Answer
Hi Sharon, Astilbes like moist but well drained soil. By mid July, mine look pretty bad from the heat, so I shear them off at the ground and fertilize them with Milorganite or a similar slow release fertilizer. Given enough water and in cooler climates, shearing might not be needed if the foliage looks good.  Just fertilize them and look forward to bigger, better plants next spring.  Jim


Sorry, I didn't cover the amendment part of your question. I've answered how to amend soils so many times on this web site, I assume readers read other answers.
I use finely ground pine bark mostly.  Almost all nursery stock and perennials are grown in a bark based mix, so ideally your transition zone between your soil and the bark is a combination of both.  With shade loving perennials, I use about 40% bark and 60% existing soil.  I'll throw in some leaves, if they are available. When dividing (usually in 3-5 years), I'll add an additional 20% bark.
In areas with heavy root competition from large trees, I'll add a little water holding polymer (Soil Moist) when planting or dividing.
Jim