any flowering plant that has not been genetically manipulated. Generally the term applies to plants growing without intentional human aid, particularly those flowering in spring and summer in woodlands, prairies, and mountains. Wildflowers are the source of all cultivated garden varieties of flowers. Although most wildflowers are native to the region in which they occur, some are the descendants of flowering plants introduced from other lands. The bright flowers characteristic of the Hawaiian Islands, for example, are nearly all native to other parts of the tropics and subtropics. Most were taken purposely to the islands for cultivation but spread rapidly into the fertile lowlands, displacing the less-colourful native species and leaving only the steep mountainsides to the original flora. In the lowlands of the United States and Europe most wildflower species are native; others are migrants.

Disturbance of the native flora by humans began in prehistoric times. For example, fires that escaped from the control of their human makers are thought to have burned off native vegetation and made way for aggressive species from the same or other areas. For instance, one of the best-known buttercups of northern Europe, Ranunculus acris, probably became more abundant and widespread as the forests were burned away. In the lowlands of northern Europe, this species probably became modified during the Stone Age into some new forms better adapted to habitats created by human actions. Two forms occurring in the northern United States and Canada, which had been introduced into eastern North America by the early 19th century, gradually spread across the continent, one becoming common in the state of Washington only within the 20th century.

The distinction of weeds (weed) from wildflowers depends upon the purpose of the classification. A weed is a plant that, from a human perspective, is out of place; that is, one growing where it is unwanted. Sunflowers are looked upon as weeds when growing in cultivated fields or on grazing land of the Great Plains of North America but as wildflowers in uncultivated valleys. The sunflower also is a crop plant cultivated for its seeds; in some places it is a garden flower.