What characteristic defines annual weeds?

The defining characteristic of annual weeds is that they complete their life cycle in less than one calendar year. This means that these plants germinate from seeds, grow, flower, produce seeds, and die all within a single growing season, typically lasting from spring to early fall. This rapid lifecycle allows annual weeds to proliferate quickly and can make them particularly challenging to manage, as they often produce a large number of seeds that can remain dormant in the soil until conditions are favorable for germination.

In contrast, the other options describe characteristics of different categories of plants. For example, plants that complete their life cycle in longer than one year would be classified as perennial weeds, while those that live for two years would generally fall under the category of biennials. Germination in the late summer does not specifically apply to the definition of annual weeds, as these plants can germinate at different times depending on the species and environmental conditions. Therefore, the essence of annual weeds lies in their ability to grow and reproduce quickly within one season, which is why this specific choice accurately captures their defining characteristic.