The nigella plants that were such an enchanting cloud of pastel colors last month have now dropped their petals. The seed heads have swelled into what looks like an alien rosebud with tentacles. They sway in the breeze, still surrounded by a mist of feathery leaves that are drying and changing color. Flower arrangers love these seed heads. But they will remain in this narrow space along a silvery fence, sustenance for birds and other small creatures.