Most of us assume that when we plant shrubs, they will survive our winters and grow larger and more robust every year. In fact, one of the most common errors first-time gardeners make is not allowing enough room for those young woody plants to expand. (That's why you often see a modest one-story rambler dwarfed by a pair of gigantic arborvitae.)
However, not all woody plants grow that vigorously in Minnesota. Some are just hardy enough for their roots to survive winter while their tops die. Called "die-back shrubs," this category of plants falls somewhere between woody shrubs and flowering perennials. Most winters they behave like any other flowering perennial: Their stems die, often right down to the ground, only to sprout again in spring.
While many homeowners avoid plants that have an element of risk, more gardeners are looking to push the envelope and try new or unusual plants -- as long as there's a reasonable chance the plants come back each year. Many die-back shrubs fill the bill.
Endless Summer, the new big leaf hydrangea that's garnered so much press this year, falls into the category of die-back shrubs. In fact, some people who bought them last year became impatient this spring and ripped them out early in the season when the plants appeared dead. Many of those who waited longer were rewarded with new green shoots coming from the base of the dry, brown stems.
Because Endless Summer produces flowers on new stems, dying back to the ground doesn't handicap this popular plant. However, it does mean that it will never grow as large here as it would farther south.
Butterfly bush (Buddleia davidii) is another die-back shrub that's showing up more frequently in Minnesota gardens. Like Endless Summer hydrangeas, buddleias flower profusely on new stems. Unfortunately, this plant is very marginal here and frequently dies altogether. But because it grows so quickly, many gardeners find it worthwhile to replant.
Butterfly bush owes its popularity to the slender, long clusters of pink, purple, rose or magenta flowers it produces beginning midsummer. These flowers are absolute butterfly magnets! When the sun is shining, it's common to see several butterflies -- monarchs, red admirals, painted ladies, tiger swallowtails -- hovering around a buddleia's blossoms.
Russian sage (Perovskia atriplicifolia) also can be considered a die-back shrub. After a particularly mild winter, its 12- to 18-inch-tall stems may push out new growth from top to bottom. But after a typical Minnesota winter, only the lowest parts of the stems send out shoots. These shoots grow rapidly, though, and by July, they can easily reach a height of 3 feet or more. And from midsummer, they bloom until frost shuts them down.
A much taller plant, Eurasian smokebush or smoketree (Cotinus coggygria) never fails to attract attention once it's through blooming. Its big panicles of delicate flowers develop into large, puffy seed structures. From a distance, they look like clouds of hazy smoke. Up close, they look so silky you can't help but touch them.
Purple-leaved cultivars 'Velvet Cloak' and 'Royal Purple' are particularly showy, but the green-leaved cultivars usually are a little more hardy. Smokebush may die back only sporadically, which means it can grow quite tall -- about 10 feet -- before a severe winter kills the top growth again. But then, new stems will sprout up from the base and even though the plant won't produce flowers that year, its foliage will be lovely.
Die-back shrubs should be planted in fertile soil that drains well. Wet soil interferes with the winter hardiness of most shrubs and it's particularly damaging to those that die back most winters. In the fall, die-back shrubs need a thick application of mulch -- 6 inches of straw, wood chips or shredded bark, or a foot or more of dry leaves. Remove the mulch in the spring and, with any luck, these shrubs should come out of dormancy and begin the growth cycle once again.
Deb Brown is a horticulturist with the University of Minnesota Extension Service Yard and Garden Line. For help with garden, plant and insect questions, call the Yard and Garden Line at 612-624-4771.