Q. I am looking for a suitable ground cover to plant between concrete steppingstones in an area that gets full sunlight. I want the plants to keep low without mowing.
A. One of the best ground covers for this type of situation is creeping mazus. The white-flowered form is more vigorous than the lavender version, but both are lovely. Mazus, however, will die back in hot, dry conditions and requires rich soil that retains moisture.
If your site is hot and you can provide excellent drainage, woolly thyme is a good option. If you are patient, dwarf mondo grass is another great choice; it will give you deep green color all year and choke out most weeds once it is established.
The thyme and mazus are sprinters. Both have stems that grow along the ground, rooting where they contact the soil surface. You can dig up rooted sprigs of either of them and move them to bare areas to help cover the bare ground more quickly.
Dwarf mondo grass spreads by short rhizomes just under the soil surface. These may grow only an inch or so in a year.
If you choose the mondo grass, therefore, you will want to start with more plants and place them closer together.
Q. I have planted a perennial garden, but I remain confused about when perennials should be cut back: In the fall? Early spring? Right after flowering?
A. There are no hard and fast rules for cutting back all perennials. Each species is a little different. In nature, perennials never get cut back. Rain, snow, wind and animals may beat the dead portions of the plant back to the ground, where they gradually decay. In the wild, a perennial in fresh spring growth may have some remnants of its previous year's stems.
We cut perennials back for aesthetic reasons and to prevent insect and disease problems. Many perennials can be cut back as soon as the foliage dies in late fall or early winter.
In mild climates, the foliage of many perennials is somewhat evergreen and some of it may be left in place. As it develops a shabby appearance, it can be removed just as new growth is emerging in spring. Other perennials grow vigorously in spring and take a break when summer's heat arrives. Iris, bleeding hearts and Oriental poppies fit this description. Some of the outer leaves of iris may die off, and bleeding hearts and poppies may lose their foliage entirely. It's best to remove this as it dies off.
Some perennials, such as chrysanthemums and asters, don't like to have their foliage removed in winter. The foliage helps to trap air around the crown of the plant and moderates harmful temperature fluctuations. Others, such as penstemon, dianthus and hellebores, are somewhat evergreen and need their foliage all winter to grow well in spring.
Part of knowing whether to cut plants back is watching them and observing the signs of aging: yellowing leaves, primarily. Books on perennials don't treat the subject of cutting back very uniformly. I have found that Allan Armitage's book "Herbaceous Perennial Plants" has good tips for many species.
Scott Aker is a horticulturist at the U.S. National Arboretum.