The third blue-flowering September plant is sometimes called a perennial ageratum (Eupatorium coelestinum), but it seems to operate more like a self-seeding annual. I had coveted this one ever since I first heard about it, so when a friend told me last summer that she would dig up a couple plants for me, I was tickled, er, pink. But as she handed her two little pots over the fence, she warned me, “Beware. They can take over.”

This was last summer and I now see what she meant. Still, they’re easy to pull out, and I have had to do so in order to let the plants growing behind them continue getting some sun.

I recently returned from the annual symposium of the Garden Writers Association, held this year in Indianapolis. One of our garden visits was to the city’s stunning art museum, which adjoins the former estate of the Eli Lilly family with its very well maintained historic gardens. The formal garden in particular caught my eye because it featured a mix of blue and yellow-flowering plants, among them the blue ageratum. It stands 12 to 15 inches high and will keep blooming until frost.

—Carolyn Ulrich