What blessing is said for a pleasant scent from an herb (annual plant)?

For pleasant scents from herbs and annual plants: like mint, basil, or lavender, the blessing is 'Borei isvei besamim' ('Creator of fragrant herbs'). The distinction from 'atzei besamim' is botanical: herbs (isvei) are soft-stemmed plants that typically complete their life cycle in one season, while trees (atzei) have woody stems that persist. If you know the plant is an herb, say 'isvei besamim'; if it's a tree, say 'atzei besamim.'