QI have tried growing butterfly bush several times but each time it dies over winter. Last fall I put nearly a foot of straw over the crown of the plant, but the plant still died after a mild winter. What else can I do?
AButterfly bush (Buddleja davidii) is marginally hardy here, but it's worth trying to grow, especially if you want to attract butterflies to your garden. Butterfly bush has a rather unkempt growth habit, sending out many long, floppy shoots as it grows, but its numerous long, narrow panicles of small flowers are its main attraction. The flowers are purple, blue-purple, lavender, magenta or white, depending on the cultivar, and they are an incredible magnet for all sorts of butterflies.
Butterfly bush is a woody shrub in warmer climates, but in zone 4 (average lowest winter temperatures from 20 to 30 below zero) it acts, at best, like an herbaceous perennial. It requires special protection to get it through the winter. I've heard reports from some people who grow butterfly bush successfully with little winter protection, but their gardens are in relatively protected sites within the metro area. A foot of dry straw piled over the plant's crown around mid-November may do the trick. Because this wasn't effective for you, you may need to add even more protection. Try piling a cone of soil, perhaps 8 to 10 inches deep and several feet in diameter, over the crown of your butterfly bush. The best time to do this is late October or early November, before the soil freezes. Bring the soil from another site, such as your vegetable garden, so that you don't dig up or expose some roots while covering others.
Once the soil starts to freeze, pile a foot or more of dry straw over the soil cone. In the spring, gradually remove the straw and pull the soil back off the crown of the butterfly bush. Wait until new growth appears on the plant (probably just shoots from the base), then cut back the dead stems to that point.
QWhat should I do with the foliage on my asparagus?
AWhen the ferny foliage of asparagus has turned brown in the fall the stems can be cut back right to the ground. Cleaning up and disposing of (not composting) asparagus stems may help reduce the population of the asparagus beetle, a pernicious pest. If you have no problems with asparagus beetles, you could leave the stems standing over winter and cut them back in early spring before new asparagus spears emerge.
It's important to allow the foliage to turn brown in the fall because the stored energy that has accumulated in the green foliage all summer needs to move down to the asparagus crown. This helps ensure a vigorous crown and good production year after year.
QIn a Wisconsin park we picked up some nuts that reminded us of shagbark hickory nuts. However, these nuts were smaller and tasted more bitter than shagbark hickory. Do you know what kind of nuts we may have picked up?
AThe answer is in your description of the taste of the nuts. What you found was the fruit of bitternut hickory (Carya cordiformis), a tree native to woodlands in Wisconsin, Minnesota and much of the eastern half of the United States. Unlike shagbark hickory (Carya ovata) with its distinctive plates of bark that curl out and away from the trunk, bitternut hickory has fairly smooth bark with shallow indentations, similar to a green ash. The rounded nuts are smaller than shagbark hickory. The outer husk has four riblike protrusions at quarter intervals.
As the common name implies, the nuts are quite bitter and are not favored by humans or wildlife. Perhaps the best identification characteristic for bitternut hickory at this time of year is the bright sulfur-yellow buds on the tips of twigs. No other hickory species have this distinctive winter bud color.
QWhen is the best time to divide astilbes?
AAstilbe is a popular perennial, valued for its airy plumes of white, pink, peach, or red flowers in midsummer. Astilbes tolerate quite a bit of shade but also will grow in full sun. They need soil moisture as well as good soil drainage. Wet soil through the winter, however, can kill astilbes. Clumps of astilbe spread fairly slowly, so frequent division is not needed.
Most astilbes grow three to five years or more before they need to be divided. The best time to do that is early spring. Dig up the entire clump with a shovel or spading fork, then divide the clump into smaller sections and replant these divisions. Astilbes also can be divided in late summer or early fall, but don't delay into late fall or the replanted divisions may not have time to establish themselves before the ground freezes.
-- Nancy Rose is a research horticulturist at the Minnesota Landscape Arboretum. She spends her spare time gardening, inside or outside, depending on the weather. To ask her a gardening question, call 612-673-9073 and leave a message. She will answer questions in this column only.