What is the primary characteristic of herbaceous plants?

Herbaceous plants are characterized primarily by their non-woody stems, which indeed die back to the ground during the winter months in temperate climates. This dieback is a significant adjustment to seasonal changes, allowing the plant to conserve energy during periods of unfavorable conditions. When spring arrives, these plants typically regrow from their roots or rhizomes, creating new shoots.

The other characteristics listed do not accurately define herbaceous plants. For instance, woody stems are characteristic of trees and shrubs, not herbaceous plants. While some herbaceous plants may remain green throughout the year, many do not, particularly perennials that die back in winter. Herbaceous plants also do not necessarily require constant moisture; their water needs can vary greatly depending on the species and their natural habitat. Thus, the option describing their tendency to die back each winter accurately captures the essence of herbaceous plants.