From the Lady Bird Johnson Wildflower Center website: "Mistflower grows to 3 feet high, but often lower, with leaves opposite, somewhat triangular in shape, and bluntly toothed. At the top of the plant the branches, with their short-stemmed clusters of flowers, form an almost flat top. Disk flowers are bright blue or violet, about 1/4 inch long. There are no ray flowers. Blue Mistflower attracts bees and butterflies. However, this wildflower spreads quickly and can become a pest."
Cell phone photography at twilight - hard to photograph clearly.