A rather large genus of mostly aromatic plants, some of which provide well-known spices or flavorings, such as tarragon, absinth, and the bitter wormwood. Several species have been used as medicine, vermifuges, and insecticides. Others are cultivated for their ornamental foliage. Most species have ± deeply pinnatifid or at least lobed leaves. Sometimes there is a pair of lobes at the very base of the leaf, superficially suggesting stipules (and if these are the only lateral lobes, as is sometimes the case, the leaf may be misinterpreted as simple).
1. Principal leaves unlobed or pinnately lobed (the axis and/or lobes mostly 4 mm or more broad); leaves strongly tomentose (at least beneath).
2. Apex of leaf (and of lobes) rounded; heads with involucres 5.5–7 mm long.
2. Apex of leaf (and of lobes if any) acute; heads with involucres (2.5–) 3–4.2 (–4.5) mm long.
3. Stem (at least upper part) and upper surface of leaves at least somewhat tomentose; leaves unlobed or at most once-pinnate with entire lobes.
3. Stem above and upper surface of leaves glabrous or nearly so; leaves either ± bipinnately lobed or evenly serrate.
4. Leaves ± bipinnately lobed; plants clumped.
4. Leaves unlobed, evenly serrate; plants colonial from rhizomes.
1. Principal leaves finely dissected or bipinnatifid (the axis and lobes less than 4 mm broad); leaves tomentose or otherwise pubescent or glabrous.
5. Leaves on both surfaces and stems glabrous to puberulent or loosely hairy, not densely tomentose or silky.
6. Leaves bipinnatifid or pinnatifid with toothed segments, the segments all flat; annual or biennial, from a taproot.
7. Involucres ca. 1–1.8 mm long; heads all on short pedicels in an open, much-branched inflorescence; leaves sweetly aromatic, much reduced in the inflorescence.
7. Involucres (at least the largest) ca. (1.8–) 2–2.5 mm long; heads mostly sessile or subsessile, in tight spike-like clusters; leaves only weakly if at all aromatic (like faint A. absinthium), well developed in the inflorescence, usually exceeding the spike-like clusters of heads.
6. Leaves 1–3 times pinnatifid, often with ± revolute linear-filiform entire segments mostly less than 1.2 mm broad; biennial or (usually) perennial.
8. Stem and leaves (at least when young) ± closely puberulent; stem woody below, usually branched; plant a rare aromatic escape from cultivation.
8. Stem and leaves glabrous or with at least scattered long hairs; stem herbaceous, unbranched (except in the inflorescence); plant a common nearly scentless native on dunes and other sandy ground.
A. campestris (in part)
5. Leaves densely silky or tomentose at least beneath.
9. Heads with long white hairs on the receptacle, among the florets; leaf segments (or unlobed leaves in the inflorescence, except for A. frigida) clearly flat, mostly 1.3–3 mm broad.
10. Leaf segments flat, over 1 mm broad; plant coarse and spreading, usually much taller, strongly aromatic.
10. Leaf segments linear-filiform, less than 1 mm broad; plant relatively small and slender (but woody at base), ca. 3.5 dm or less tall, mildly aromatic.
9. Heads with naked receptacle; leaf segments narrowly linear-filiform, mostly less than 1.2 mm broad.
11. Involucre usually glabrous; leaves 1–3 times pinnatifid, the ultimate segments many times as long as broad; a common native of sandy places, biennial or perennial from a taproot or woody base.
A. campestris (in part)
11. Involucre ± densely tomentose; leaves once-pinnatifid and/or the ultimate segments appearing as prolonged teeth ca. 2–4 times as long as broad; a rare introduction, perennial from slender rhizomes.
12. Leaves once-pinnatifid (or entire or with only a basal pair of lobes).
12. Leaves bipinnate, the ultimate segments ca. 2–4 times as long as broad.