Astilbes originated in Asia. False goatsbeard (Astilbe biternata) is the only North American native. According to Young American Growers, a grower in Pennsylvania, astilbes in their native habitat grow beside streams in semi-shaded areas. They like acidic soils and plenty of water. Cornell University rates astilbes as "moderately difficult to grow." Proper cultivation throughout the growing season may be the most important requirement for winter survival.
Astilbes require plenty of water. If you satisfy this need throughout the growing season, plants won't suffer a moisture deficit as winter approaches. Even as the cold withers astilbe foliage, the water need may still be there if rainfall has been scarce. Monitor autumn rainfall amounts and supplement as needed, up until the ground freezes. According to the University of Illinois Extension, astilbes set their flower buds for the next year in October. Water-stressed plants in fall may mean poor flowering the next season.
Soil and Soil Amendments
A soil that is well-drained is crucial for astilbes, especially as fall becomes winter. Too much winter moisture, coupled with a heavy soil such as clay, can be fatal. Avoid planting astilbes in dry, compacted sites. The soil should also contain a great deal of organic matter. Don't add lime, or plant astilbes too close to a lawn that you regularly lime because of the risk of runoff.
Astilbe thrive on fertilizer more than many other perennials. Cornell recommends feeding astilbes with a high nitrogen fertilizer in the fall. The University of Illinois Extension recommends an application in spring as well. The fall application will give the plants a boost as they set flower buds in October. Read labels and apply according to directions--too much of a good thing may harm your plants.
Astilbes need to be divided every three to four years. There's some debate as to when this should be done. In a northern climate, the Young American Growers advise to divide in spring or just after flowering rather than in the fall. If you have no choice but to divide in the fall, lift clump gently and divide so each section has plenty of roots and replant. Again, water and good soil are crucial for success.
Weeding, Mulch and Hardiness Zones
In the fall, cleaning the area of any weeds and diseased foliage is important. Astilbes don't get many diseases, but they do get powdery mildew (a white film on the leaves during humid weather) and a bacterial leaf spot, according to Cornell. Otherwise, diseases and weeds may appear in spring. Pruning healthy but dead foliage isn't necessary. Astilbes are hardy in USDA Hardiness Zones 4 to 9. If you garden in a colder climate, such as a borderline Zone 4, mulching is a good idea. Young American Growers recommends using pine boughs, but snow is the best protection.