The Q&A Archives: Purple Fountain Grass And Ageratina Altissima

Question: 1. I bought three large purple fountain grass plants which have flourished in my zone 6 garden, thinking that they are winter hardy here. Now I am given to understand that they are not. If that is so, could I dig them up with any success for next year? They are magnificent and I hate to lose them.
2. I have many ageratina altissima (white snakeroot) growing here. I read that they are not border friendly and should be allowed to grow in a woodland setting. They seem to flourish for me in both sun and partial shade, even very shady areas. Is there any way I could move some of them around to expand the areas where they grow and beautify some naked spots, especially in shady areas? This is really a beautiful fall flower along with aster frikartii.
3. Should I divide perennials now or wait until Spring? The cimicifuga, lysimachia clethorides and anemone tomentosa I planted late last fall all grew beautifully this year.

Many thanks for your help in the past and for your advice here.

Answer: The purple fountain grass is not hardy in northern areas, and it can be quite difficult to overwinter unless you have a cool greenhouse in which to hold it. If you want to try, pot up divisions and keep the soil barely damp. It can be quite difficult to prevent it from rotting through the months when it is not in active growth.

The Ageratina altissima (old name was Eupatorium rugosum) can be moved or divided in the early spring. "Border friendly" can mean different things to different people. If you like the look, then grow it there.

Most fall division work would be done in early to mid September to allow for ample reestablishment time. Mid October is a bit late in the season since frost is imminent in many areas; the general rule is to finish dividing about six to eight weeks before the average frost date. In any case, most of the very late bloomers are typically divided in the spring.

Cimicifuga is usually planted and left in place rather than divided or transplanted at will. Once it reaches its mature clump size of usually about two to three feet, it will stop expanding. Young plants should not be disturbed, and yet it can in fact be very difficult to divide once it has matured and put on some size. If you try it, do it in very early spring and be willing to risk loss of the plant.

Lysimachia clethroides is also rather invasive and will quickly spread through an area, especially if the soil is rich and/or moist. It can be divided at almost any time during the growing season and you will need to keep a careful eye on it to prevent it from over-running more desirable plants. In my experience, this plant is not "border friendly" and you might have second thoughts about growing it at all.

Anemone tomentosa (earlier called A. vitifolia) is usually divided in the early spring. It will grow in partial shade but may need staking in either a shady spot or a windy spot. This is a lovely plant for borders or woodland gardens.

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