Astilbe x arendsii is a perennial plant that produces plumes of white, pink and red flowers. Also known as "false spirea," it thrives in moist soil and partial shade that protects it from the afternoon sun. Plant it in a shady border where the summer blooms will add a pop of color. Astilbe x arendsii is hardy in United States Department of Agriculture Hardiness Zones 4 through 8. Astilbe x arendsii should be planted in the spring, after all danger of frost has passed.
Amend the soil with 3 inches of compost. If the soil is slow to drain after a hard rain, add 2 inches of coarse sand or peat moss. Use a garden fork to mix the amendments into the top 10 inches of soil and rake the bed smooth.
Soak the astilbe rhizome in a bucket of water for three hours before planting.
Plant the astilbe x arendsii rhizome 1/2 inch deep and cover it with soil. When planting more than one, space them 12 to 24 inches apart.
Spread 3 inches of mulch over the soil around the astilbe when it sprouts. Place the mulch 3 inches from the base of the plant and spread it out within a 2-foot radius. Mulch helps protect the astilbe’s shallow roots from summer’s heat.
Keep the soil moist at all times. Astilbe x arendsii is not a drought-tolerant plant.