Ask Mr. Smarty Plants

A quick look at the Butterfly Center website will lead you to some regional garden guides with a good selection of plants that provide food for butterfly larvae or adults.

Two of the flowers mentioned are Butterfly mistflower (Chromolaena odorata) or Mistflower (Eupatorium (Conoclinium) coelestinum). Perhaps this is what you spotted in their garden.

Chromolaena odorata is a native shrub  that is also called Jack in the bush, Fragrant boneset, Fragrant mistflower, Crucita, and Blue mistflower.

Blue mistflower or crucita is a deciduous sub-shrub in North America. (It is a shrub in the tropics.) Branched stems curve upward and are 2-6 ft. in height. Somewhat triangular-shaped leaves are virtually evergreen in extreme S. TX. Lilac to bright purplish-blue flowers cluster together into showy, ageratum-like flower heads. Dies back to the roots in hard winters. It is native from Tropical America to the Gulf States, SE Texas & the Rio Grande Plain. You can see the Texas counties where this plant is native at the USDA website.

Conoclinium coelestinum just to be confusing is also called Blue mistflower as well as Wild ageragum, Blue boneset and Mistflower.

Blue 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. This plant is native to a large section of the middle of the United States from Texas to Michigan. And the USDA webpage for this plant does indicate that it is native to many counties in Central Texas including Williamson. This plant should be right at home in your garden.