Lisianthus has large gentian-like convex flowers with flaring pale purple petal-like lobes. Eustoma, commonly known as lisianthus or prairie gentian. The flowers can grow up to two inches across and can be found in a variety of colors. flowers are either single-flowered or double-flowered. Lisianthus flowers will begin to bloom in early summer and some will continue to bloom throughout the later months of the summer.