Answer: The sun exposure sounds right, as does plentiful moisture. If your astilbes appear to be dwarfed, I'd look into the type of soil they're planted in. Heavy clay soil will inhibit root growth, which will keep the tops on the small side. If you suspect your soil is poor, you can amend the soil in late fall after the foliage of your astilbes dies down, or in the early spring just as new foliage is beginning to develop. I'd dig a good amount of compost or aged manure into the planting area. If you have a grouping of astilbes, dig them all out and then spread a 4-5" layer of organic matter over the surface of the bed, digging it in to a depth of 8-10". Then level the soil and dig the holes for your plants. After replanting, water well to help settle the soil. It's a lot of work but your astilbes will reward you with lush growth and tall plumes of color. As the plants begin to grow in the spring you can fertilize with a 10-10-10 or 20-20-20 fertilizer (in amounts as suggested on the label). Best wishes with your garden!
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