Now here's an underused but very dependable perennial that is gaining in popularity as gardeners learn more about the diversity of heights, textures, bloom, and leaf and stem colors. Gail and I have been using them for years and both remember our parents using them successfully too.
Astilbes range in height from ten inch tall groundcover types such as the late blooming Pumila to the 3-4 foot tall Purpurkerze (aka Purple Candles), or to the 5 foot tall background astilbe 'Superba', with its purple scapes and fast moving feet. They all prefer compost-rich, moist soil and partial shade but will tolerate full sun here in Vermont given adequate rain or supplemental watering.
Several years ago we did a couple experimental plantings, one in an area known to flood for well over a month straight in the spring and the other adjacent to a vernal pond in an area that was quite damp all summer. In both instances what some might call "neglectful planting" resulted in waves of colorful bloom that only gets better each year.
Astilbes are most impressive in mass plantings where they do not have to compete with tree and shrub roots. We work leaf compost, aged manures and peat moss into the soil at planting time and then top-mulch with wood chips or more shredded leaves to maintain balanced moisture.
A typical 3" root division will become a mature plant within three years and if planted in well amended soil to begin with, it will continue to reach out for years to come. An astilbe root is a thick, fibrous root mass with a number of smaller, finer side roots. The strong root masses are often difficult to divide. This can be accomplished with a large knife, hand saw or small axe. I am forever purchasing the cheap grocery store brand serrated bread knives because they work well on this task. Recently I found a display of sheet rock a.k.a drywall knives in a box store and these are the best. Clearly the visual reminder of taking an axe to a plant may seem odd but over time astilbe roots do get tough!
Many gardening journals describe astilbes for the strength they lend to the garden. They are physically able to stand well even under unfavorable wind conditions. As the plants mature, they form a tight clump of stems and leaves which work well with hostas, daylilies, actaeas and eupatoriums. If you visit us you will see that we continue to work them into a variety of situations, from full sun to fairly difficult shade.
As you consider using astilbes, evaluate their mature size, texture and color as well as bloom time. In this part of New England the bloom time ranges from June through August and on into mid September with a couple later varieties. As with any plant there are a number of variables involved in actual bloom time and this is something you will have to work with at your location. No matter which you choose, we are certain you will enjoy this trouble free perennial.
Dividing every fifth year or when the center begins to degrade and bloom scapes diminish in number, will keep your astilbes looking good for a long time. Spring feeding around the plant perimeter with a fertilizer of your persuasion, organic or commercial, and fall clean up after several heavy frosts will keep them looking special. We wish we could suggest a good, picture-heavy book to serve as reference but as of now have not seen one. We try to have our display gardens well marked but don't always have saleable quantities of everything that's on display. Come. See. Enjoy!