What is the key feature of chickweed that helps to define its growth habit?

The defining feature of chickweed that characterizes its growth habit is the arrangement of its leaves. Chickweed displays opposite, oval-shaped leaves, which is a key identification trait that helps distinguish it from other similar plants. The growth pattern of having pairs of leaves directly across from each other is significant in understanding its overall morphology and how it competes within its environment. This leaf arrangement contributes to the plant's ability to effectively gather sunlight and nutrients.

While the other features mentioned may contribute to the plant's characteristics, they are not as definitive in identifying chickweed specifically. For instance, distinctive seed dispersal is a general trait many plants exhibit, and a fibrous root system is common among numerous species. Although upright square stems may apply to various plants, chickweed’s defining characteristic remains its unique leaf structure.