(Editor's Note: This article was originally published on May 22, 2008. Your comments are welcome, but please be aware that authors of previously published articles may not be able to promptly respond to new questions or comments.)
When I say that the common wild violet is a lawn invader, this doesn't mean it is non-invasive everywhere else but the lawn. I have it in my asparagus bed, where it is hard to dig out; I have it in my strawberry bed, where it is hard to find. It has successfully invaded the territory of the rudbeckia, and is issuing a challenge to the lily of the valley. But it seems to cause most trouble when it invades the grass.
I do admit, when the spouse is out of earshot, to actually liking the violets. In this, I am not alone. It is the official state flower of my state, Illinois, as well as Wisconsin, Rhode Island and New Jersey. I have a bed of them that I actually encourage, a bed that I don't ask them to share with any other flowers. This bed does not border the lawn at any point, but the violets won't willingly stay put, and once they have conquered the lily of the valley, the lawn is the next line of resistance if I relax my vigilance. From that point on, it is world conquest all the way.
Identification of violets isn't easy. While there are a lot of fancy violet species and cultivars, many of them look pretty much alike, with heart-shaped leaves sprouting from a crown and flowers that look just about any other violet flower. The fragrant sweet violet, Viola odorata, is often accused of the invasive crimes of its wild relative, but it is a European import, although widely naturalized, while the common wild blue violet, now known as Viola sororia, is a North American native. To complicate identification even further, some of the species cross-breed freely. The common wild variety has, well, violet flowers, but they can also be white, or white with violet centers. Whatever the color, the flowers are small, held on upright stems, and the entire plant is no more than six inches high. Its usual use in the garden is as a groundcover, which it does well, spreading rapidly under favorable conditions. It prefers a moist and shady location. The flowers in the spring are attractive, and after flowering the foliage lasts until late fall, covering the ground as a good ground cover should.
But. It spreads. While some varieties of violet spread by underground stolons, the wild violet's real secret weapon of propagation is by seed. And herein lies a strange tale. Many violet species, including the common wild violet, have two kinds of flowers. The first, normal [chasmogamous] flowers are the pretty ones we see in the spring. These are usually pollinated and set seed in the normal manner; this is how the wild violets hybridize with others. But later in summer, the violet develops a second kind of flower hidden under its leaves. These are called cleistogamous flowers, from the Greek for "hidden." They are unusual in that they never open. Inside the closed pod, the flower fertilizes itself with its own pollen. The seedpods are three-cornered and held together with a sort of springy hinge. In fall, the pods pop open, flinging hundreds of seeds far and wide, often many feet away. The next spring, a mature violet plant will be surrounded by a horde of its seedlings, but it is likely that some of them are also germinating in the lawn.
Once the violets get into the lawn, they are hard to control. They are too low for the lawnmower to reach. The leaves have a waxy coating that resists herbicides. Some people add a sticker/spreader to make herbicides more effective, but I don't know if this also makes it more effective at killing the grass. My own weapon of choice is a paring knife. Violets are relatively easy to dig out of the ground. In spring, the flowers give the location of the plants away and make it easy to attack. It is vital to get the violets up and out of where they aren't wanted before the seedpods open. But most important is not to let them get started where you don't want them. If you don't want violets in the lawn, then dig them out as soon as you spot them there. A few violets are pretty and charming. A lawn full of them may lose its charm rapidly, and some mature specimens will develop fleshy above-ground rhizomes that choke out any other plants, especially grass.
So far, I have been fairly successful at keeping the violets out of the lawn we have now. In the spring, I enjoy the sweet, pretty anemones in the grass, instead.