Why is harvesting stinging nettle leaves in early spring generally preferred over harvesting them later when the plant flowers?

Leaves become much tougher, and the plant may accumulate more oxalates once it begins setting flowers.

The quality and palatability of many edible wild plants change significantly throughout their life cycle, a pattern strongly linked to seasonal shifts and resource allocation. For stinging nettles, while the tender young leaves in early spring are prized for cooking as greens, harvesting later is discouraged. Once the plant initiates flowering, its biological priority shifts toward reproduction. This shift results in the leaves generally becoming significantly tougher or more coarse in texture. Furthermore, the concentration of certain compounds, specifically oxalates, can increase in the leaves after the initial vegetative growth phase, negatively impacting culinary desirability and safety compared to the early, tender harvest.