Five have undivided leaves (some may have small lobes at the base):
Two have undivided leaves without stalks:
Bidens laevis, Smooth Tick-seed
Petals (ray flowers) over 1/2" long. Coastal plain only.
Bidens cernua, Nodding Tick-seed
Petals not more than 1/2" long.
Three have undivided leaves (sometimes lobed) with stalks 1/2" or more long:
Bidens comosa, Three-lobe Tick-seed
No petals (or very small.) Usually in dry areas.
Bidens connata, Purple-stem Tick-seed
Very similar to B. tripartita; status uncertain in Delaware, but thought to be only on the coastal plain.
To ID see http://www.woodlotalt.com/publications/BotNotesv1n9.pdf
Bidens bidentoides, Estuary Tick-seed Also see Bidens mitis below.
No petals; usually in shallow water. Rare.
Eight have leaves divided into three or more leaflets:
Four have divided leaves and no, or very small, petals (ray flowers):
Bidens bipinnata, Spanish Needles
Leaves divided more than once, into many leaflets.
Bidens vulgata, Tall Tick-seed
"Outer phyllaries 10-20, with stiff-hairy margins; awns 3 - 9.5mm long."
Bidens frondosa, Devil's Tick-seed
3-5 leaflets; "Outer phyllaries 5-10, margins ciliate"
Bidens discoidea, Swamp Tick-seed
No petals; "Outer phyllaries 2-5, margins without cilia". Coastal plain only.
Four have divided leaves and petals 1/2 - 1" long:
Bidens trichosperma, Northern Tick-seed Sunflower (Bidens coronata)
"Achenes not wing-margined ... Inner achenes 4.5-9mm long ..." Coastal plain tidal marshes.
Bidens mitis, Small-fruited Tick-seed
"Achenes not wing-margined ... Inner achenes 2.5-4.5mm long ..." Leaves stalked, sometimes undivided. Rare plant of coastal plain tidal marshes.
,
"Achenes wing-margined ... Outer phyllaries 8-12 ... tips blunt or simply acute."
Coastal plain only; adventive.
Bidens polylepis, Tick-seed Sunflower
"Achenes wing-margined ... Outer phyllaries 10-20(25) ... tips long tapering ..."
Petals slightly orange at the base.
Invasive adventive.
Sources: The Flora of Delaware, Herbaceous Plants of Maryland.