An invasive weed which is poisonous to livestock. It is often found growing in large clumps. Each stem can bear many (25–100) bright yellow, 5-petaled flowers, which often develop a row of tiny black dots along the petal margins. The leaves provide an important diagnostic feature; they appear to be perforated, the result of the presence of many small, clear oil glands. At first sight the leaves bear a resemblance to French Broom (Genista monspessulana), but the plants are unrelated.