If you plant lily of the valley, you had better love lily of the valley. And your neighbour had better love it, too. If not, your neighbour had darned well better love you.
Lily of the valley (Convallaria) is a plant that doesn’t do things by halves, and it’s no respecter of fences. Given the right conditions (it doesn’t ask for much; light soil and a little elbow room will do), it will march right through your garden… and into the next one.
It falls into the category of invasives. With these, you should be careful when you plant it, and where. Goutweed (Aegopodium podagraria) is the classic example of an invasive plant. If someone offers you a snippet of this pretty, variegated groundcover, back off and keep running. Unless it’s kept firmly contained, such as in a concrete planter, it’s everywhere, including your neighbour’s yard, within a few years.
There are some very useful groundcovers that can still be poor neighbours, if your neighbour is a wild space. In a city garden, far away from such things, the periwinkle or Vinca is one of the few perennials that will reliably give you flowers (including white, purples and the eponymous periwinkle blue) as well as foliage (including some with variegation) even in dry shade. Or, at least, will give you these “once established.” They require attention to watering until they do.
However, if you’re living at the edge of a ravine, periwinkle is likely to sneak off and join nature’s circus, hogging the limelight (and nutrients and space) from more deserving native plants. So, periwinkle + ravine = bad neighbours.
Likewise violets (Viola). They’re no shrinking violets when it comes to self-seeding with great energy and enthusiasm. In my dry shade garden, they’re very happy and very welcome. In my friend S.’s ravine-side plot, not so much.
Although I advised her when she bought the place to embrace her inner violet (knowing she’d have to live with them a while), S. is doing absolutely the right thing by yanking up the violets whenever they appear. Same equation applies.
By the way, never, ever plant goutweed near a ravine or other native habitat.
Please, think about where your plants might spread before you introduce them to your garden. And do your research before you install those free giveaways from someone else. The good-neighbourliness you save might be your own.