According to The National Gardening Association, in order to care for an Astilbe plant, a gardener must apply compost regularly, water it well during the summer and thin the plants every few years. The Astilbe are thinned or divided by unburying them and separating the clumps.
In order to care for the Astilbe plant, a gardener has to compost and mulch the plants each year, preferably in the spring. A thin layer of compost is applied first, followed by a layer of mulch, which helps to keep the soil moist. The Astilbe needs at least an inch of water per week, meaning its soil should be moistened to that depth below the surface. Because this plant grows at such a rapid pace, it needs to be thinned every three or four years. Often, the crown of the plant rises above the soil as it grows and needs to be reburied.
An Astilbe needs to be planted in a shady spot where the soil is rich and moist. During planting, there should be anywhere from 1 to 3 feet separating it from other plants to allow for the plant to spread out. A gardener needs to dig a hole 15 inches deep and fill it with a layer of compost before adding the plant. The root ball should not be visible above the soil. After the planting, it needs to be watered thoroughly.