Two years ago, we put in some salvia plants in my friends' garden. Sorry, I don't know which kind, but it's one of the blue, spikey flowers, that I've seen a lot of over the years. Last summer they came right back, in a slightly larger cluster.

This year, not one salvia. But the neighbors had a salvia patch suddenly appear in their yard! About the same area my friends' covered last summer, but more sparse. And this weekend, the neighbors had even more, growing along their drive!

I've searched a good bit to find out how salvia spread, but have found nothing. We're all perplexed...did my friends' salvia "travel" to the neighbors' yard? If so, how?

Thanks for your help.