Flag by the lake

I think this flower would be called a flag, rather than an iris.  Do you think so?  Oh maybe it's a special kind of Iris.  The white stone wall keeps the bed from washing into the water below.  And that's not a streak across the water, but the string across the edge of the flower bed.

The sign tells about the Evening Primrose plants which will be enjoyed by strolling folks at dusk.