Tall tumblemustard is a plant with many branches growing from a single stem and a stout taproot. It is initially a low-growing rosette of basal leaves, but later becomes tall and lanky in appearance, growing up to five feet in height. The round stems are light green and mostly smooth, except for widely scattered white hairs.

In the Sage Hills, whitetop often forms a sea of sweet-smelling white flowers. Whitetop grows upright from a single stem eight to twenty inches tall. The flower head is typically flat-topped and dense with white flowers. Flowers have four widely-spaced petals.