Traditionally, several Native American groups planted sunflowers on the north edges of their gardens as a "fourth sister" to the better known three sisters combination of corn, beans, and squash.[9] Annual species are often planted for their allelopathic properties. However, for commercial farmers growing commodity crops, the sunflower, like any other unwanted plant, is often considered a weed. Especially in the midwestern USA, wild (perennial) species are often found in corn and soybean fields and can have a negative impact on yields.