Guppy Grass, the common name for Najas guadalupensis, is a thin, airy aquatic plant composed of flexible stems lined with narrow, pointed leaves. Also called water nymph or simply Najas, it grows throughout the Americas in ponds, alkaline lakes, canals, and slow rivers. Najas guadalupensis often floats or anchors lightly, forming loose networks that provide crucial shelter for small fish such as guppies—hence its common name.

It reproduces both by seed and by fragile vegetative pieces that readily re-establish in new locations. This adaptability makes Najas guadalupensis well suited to dynamic or seasonal aquatic environments. Although delicate in appearance, it can become abundant and dominate portions of quiet waterways. Its free-form, branching habit fills mid-water spaces and contributes to habitat diversity.