(Editor's note: This article was originally published on April 4, 2008. Your comments are welcome, but please be aware that authors of previousl published articles may not be able to respond to your questions.)
There really is much reason to recommend Convallaria majalis as a groundcover for shady, woodsy areas. The flowers not only have a delightful scent, they are attractive little white or pink bells on short upright scapes. Of course, when it comes to scents, what is delightful for some is overpowering or cloying to others. If you are considering whether to plant lily of the valley, you should first make sure which group you belong to. But for those of us who enjoy it, nothing is more pleasant than a spring day with its fragrance on the breeze, or cutting a half-dozen stems to fill a tiny vase and perfume the house. It is also suitable for forcing in winter, like the equally-fragrant paperwhite narcissus.
Unfortunately, the flowers are not long-lasting and the bright orange fruits are too small to be significant in the garden. But the foliage remains standing as a groundcover throughout the year, each plant a small, upright cluster of dark green, lanceolate leaves, about six inches in height. Some cultivars, such as "Albostriata," are available with variegated leaves, and double-flowered versions, e.g., "Prolificans," have now been produced by breeders. An isolated lily of the valley plant is an insignificant object, but a single plant usually does not remain single for long. If it does not languish and die, it spreads. And spreads, and spreads. This, of course, is what makes Convallaria useful as a groundcover. But the experienced gardener knows that "makes a good groundcover" in the plant catalog is really a euphemism for "invasive as Genghis Khan." Once established under favorable conditions, the spread of lily of the valley is close to unstoppable.
Like many other invasive groundcovers, Convallaria propagates itself vegetatively from underground rhizomes. While it does produce seeds, the cross between daughter plants descended from a single parent seems to be sterile, and it spreads so vigorously that an entire bed might be daughters of the same original parent. The rhizomes are thin and twisty, root-like rather than fleshy, and they spread out horizontally in all directions, pausing every few inches to throw up a new plant. These pips, as they are called, are easily separated to transplant in new locations. They are best planted as early as possible in the spring.
Lily of the valley prefers a cool, moist, shaded area with a soil containing organic matter. Its ability to thrive in the shade is one reason for its popularity as a groundcover, although it may not flower in extremely dense shade. It does not like a situation that is too sunny or too hot or dry, although it is more able to tolerate these conditions later in the summer than in the spring. These factors mean that it is less likely to thrive in the warmer, drier zones, where the leaves may turn brown and die back in midsummer. In cooler zones, lily of the valley usually doesn't go dormant until the frost season, only to emerge again in early spring, usually several feet beyond where it used to reach.
Lily of the valley is not generally bothered by insect pests, perhaps because all parts of the plant are toxic. Neither is it very susceptible to most diseases. With regard to control, it is susceptible to herbicides, but this carries the usual risks and drawbacks of herbicide use. In addition to which, the nature of its invasiveness, by sneaking underground in all directions, often causes it to emerge unwanted among other, more desirable perennial plants that would likely be harmed by any herbicide application.
Thus the best, most practical method of eradication in such situations is to dig invader out by the roots. In loose soil, you may be able to rip out two or three emergent pips with a single pull. If they are not removed entirely, however, every root of them, it is likely that they will return. And in many cases, it is not possible to extricate the invasive rhizomes from the other plants without damage. Once the lily of the valley is growing up between the roots of the hostas, it may be to late to entirely eliminate it.
It is still possible to slow the spread, however, by pulling out every visible plant above the ground - making sure the plants have opened far enough that you can tell the lily of the valley from the unfurled hosta! If it happens to invade the lawn, mowing will also have the same effect.
Even better is keeping the problem from arising in the first place. If you plant Convallaria as a groundcover, make sure it is not in any position where it can invade nearby [or not so nearby] beds. Be prepared to have yards and yards of it covering the ground. Also be aware that attempts to contain it may not be successful. I have tried in the past to restrain its spread by installing deep edging as a barrier, but the rhizomes tunneled underneath to escape with vexing ease. Another method is to use a sharp-edged shovel around the boundary of the planting to sever the rhizomes, but all too often, they have already made their break for the freedom beyond.
Above all, be very wary of the thought that a nice, tidy little clump of lily of the valley would be a good addition to your perennial woodland garden. In that tidy little clump is the beginning of a ruthless invasion.