Ask Mr. Smarty Plants

Mr. Smarty Plants isn’t going to be of much help on this one. You see, the mission of the Lady Bird Johnson Wildflower Center is to increase the sustainable use and conservation of native wildflowers, plants and landscapes. Verbena bonariensis   is an introduced plant from South America, that has made it into The Invasive Plant Atlas of the United States. So no flowers = no seeds which makes the plant less invasive.

However, there are plants whose flowering is inhibited by an imbalance in the the nutrients that are available in the soil, particularly the ratio of nitrogen to phosphorus (N/P). Too much nitrogen may result in poor flowering. If the plant has been getting regular lawn fertilizer which often has a high N/P ratio, this could account for the lack of flowering. Lets look at fertilizer a bit more closely.

The most common elements that are found in fertilizers are nitrogen (N), phosphorus (P), and potassium (K), and fertilizers containing these three elements are termed complete fertilizers. Fertilizers differ in the relative amount of these elements that they contain, and this information is found on the label of the fertilizer container (bag, bottle, etc.) It is represented by three numbers eg 1-1-1, or 5-10-5, or 3-1-2, which are the percentages of the elements; N, P, and K in that order.
This link to Central Texas Gardening has some good tips about proper use of fertilizers .