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Genera - O
Oclemena
Odontites
Oenothera
Oligoneuron (see Solidago)
Omaltheca
Onoclea
Onopordum
Onosmodium (see Lithospermum)
Ophioglossum
Opuntia
Orchis (see Amerorchis & Galearis)
Origanum
Ornithogalum
Ornithopus
Orobanche
Orontium
Orthilia
Oryzopsis (see also Patis and Piptatheropsis)
Osmorhiza
Osmunda (see also Osmundastrum)
Osmundastrum
Ostrya
Oxalis
Oxybasis
Oxycoccus (see Vaccinium)
Oxydendron
Oxyria
Oxytropis
OCIMUM (Family: Lamiaceae)
OCIMUM BASILICUM Linnaeus - Sweet Basil. 2n = (30), 48, 52, 72.
Waste places. Possibly from Africa.
OCLEMENA (Family: Asteraceae)
Oclemena acuminata (Michaux) Greene - Mountain Aster. 2n = 18.
Woods, often acidic. [Aster acuminatus Michaux]
Oclemena nemoralis (Aiton) Greene - Bog Aster. 2n = 18.
Sphagnum bogs and shores. [Aster nemoralis Aiton]
Oclemena hybrid
Oclemena X blakei (Porter) G.L. Nesom - . 2n = 18.
[O. acuminata (Michaux) Greene X O. nemoralis (Aiton) Greene; Aster blakei (Porter) House]
ODONTITES (Family: Orobanchaceae)
ODONTITES VULGARIS Moench - Red Bartsia. 2n = 18, (20).
Dry fields, roadsides, waste places. From Eurasia. [O. SEROTINUS misapplied; O. VERNUS (Bellardi) Dumortier subsp. SEROTINUS misapplied]
OENOTHERA (Family: Onagraceae)
OENOTHERA AFFINIS Cambessèdes - . 2n = ?
Waste places. From South America.
Oenothera biennis Linnaeus - Common Evening-Primrose. 2n = 14.
Roadsides, fields, waste places, river thickets. [O. biennis var. pycnocarpa (G.F. Atkinson & Bartlett) Wiegand]
Oenothera clelandii W. Dietrich, P.H. Raven, & W.L. Wagner - . 2n = 14.
Fields, sandy soil.
OENOTHERA CURTIFLORA W.L. Wagner & Hoch - . 2n = 14.
Waste places. From farther west. [GAURA PARVIFLORA Douglas ex Lehmann]
OENOTHERA FILIFORMIS (Small) W.L. Wagner & Hoch - . 2n = 14.
Waste places, roadsides, fields. From farther south and west. [GAURA BIENNIS Linnaeus var. PITCHERI Pickering]
Oenothera fruticosa Linnaeus subsp. fruticosa - Southern Sundrops. 2n = 28, 42, 56.
Salt marsh margins, meadows, swamps, fields. [O. fruticosa var. eamesii (B.L. Robinson) S.F. Blake; O. fruticosa var. humifusa Allen; O. fruticosa var. linearis (Michaux) S. Watson; O. tetragona Roth var. longistipata (Pennell) Munz]
Oenothera fruticosa Linnaeus subsp. tetragona (Roth) W.L. Wagner - . 2n = 28.
Gravelly, sandy eskers, dry fields and banks, woodland margins, waste places. [O. fruticosa Linnaeus subsp. glauca (Michaux) Straley; O. tetragona Roth var. tetragona]
OENOTHERA GAURA W.L. Wagner & Hoch - . 2n = 14.
Roadsides, waste places, dry thickets. From farther south and west. [GAURA BIENNIS Linnaeus]
OENOTHERA GLAZIOVIANA Micheli - . 2n = 14.
Roadsides, waste places, fields. From farther south and west. [O. ERYTHROSEPALA Borbás]
OENOTHERA GRANDIFLORA L’Héritier - . 2n = 14.
Roadsides, waste places. From farther south. [O. BIENNIS Linnaeus var. GRANDIFLORA (L’Héritier ex Aiton) Torrey & A. Gray]
OENOTHERA GRANDIS (Britton) Smyth - . 2n = 14.
Fields. From farther west.
OENOTHERA LACINIATA Hill - . 2n = 14.
Waste places, roadsides, fields, usually in dry, sandy, open soil. From farther south and west.
Oenothera nutans G.F. Atkinson & Bartlett - . 2n = 14.
River thickets. [O. biennis Linnaeus var. nutans (G.F. Atkinson & Bartlett) Wiegand]
Oenothera oakesiana (A. Gray) J.W. Robbins ex S. Watson & J.M. Coulter - . 2n = 14.
Open, sandy areas of beaches, dunes, roadsides and fields. [O. cruciata Nuttall ex G. Don var. stenopetala (E.P. Bicknell) Fernald; O. parviflora Linnaeus var. angustissima (R.R. Gates) Wiegand; O. parviflora Linnaeus var. oakesiana (A. Gray) Fernald)]
Oenothera parviflora Linnaeus - . 2n = 14.
Streambanks, thickets, roadsides, railroads, fields, dry sand or gravel. [O. biennis Linnaeus var. cruciata (Nuttall ex G. Don) Torrey & A. Gray; O. cruciata Nuttall ex G. Don var. cruciata]
Oenothera perennis Linnaeus - Little Sundrops. 2n = 14.
Dry fields, clearings, roadsides, waste places, meadows.
OENOTHERA PILOSELLA Rafinesque - . 2n = 56.
Dry woodlands, fields, roadsides, rocky shores, waste places. From farther west and south.
OENOTHERA SERRULATA Nuttall - . 2n = 14.
Railroads. From farther west. [CALYLOPHUS SERRULATUS (Nuttall) P.H. Raven]
OENOTHERA SPECIOSA Nuttall - White Evening-Primrose. 2n = 14, 28, 42.
Waste places, roadsides. From farther west and south.
OENOTHERA VILLOSA Thunberg subsp. VILLOSA - . 2n = 14.
Fields, roadsides, waste places, beaches. From farther west. [O. DEPRESSA Greene]
OMALTHECA (Family: Asteraceae)
Omalotheca supina (Linnaeus) de Candolle - Alpine Cudweed. 2n = 28.
Granitic alpine areas. [Gnaphalium supinum Linnaeus]
Omalotheca sylvatica (Linnaeus) Schultz-Bipontinus & F.W. Schultz - Heath Cudweed. 2n = 56.
Clearings, rocky slopes, woods borders. [Gnaphalium sylvaticum Linnaeus]
ONOCLEA (Family: Onocleaceae)
Onoclea sensibilis Linnaeus - Sensitive Fern. 2n = 74.
Swamps, low woods, alluvial thickets, meadows, in neutral to slightly acid soils. [O. sensibilis var. obtusilobata (Schkuhr) Torrey] CONN herbarium image
ONOPORDUM (Family: Asteraceae)
ONOPORDUM ACANTHIUM Linnaeus subsp. ACANTHIUM - Cotton Thistle. 2n = 34.
Waste places, roadsides, gravelly, sandy tidal shores. From Eurasia.
OPHIOGLOSSUM (Family: Ophioglossaceae)
Ophioglossum pusillum Rafinesque - Northern Adder's Tongue. 2n = 960.
Peaty or grassy swales, meadows, damp thickets, damp sands. [O. vulgatum Linnaeus var. pseudopodum (S.F. Blake) Farwell] NEBC herbarium image
Ophioglossum vulgatum Linnaeus - Southern Adder's Tongue. 2n = 240, 480, ca. 486, ca. 540.
Rich wooded slopes, floodplain woods. PH herbarium image
OPUNTIA (Family: Cactaceae)
Opuntia humifusa (Rafinesque) Rafinesque var. humifusa - Eastern Prickly-pear. 2n = 22, 44.
Dry, exposed ledges and sandy fields.
ORIGANUM (Family: Lamiaceae)
ORIGANUM VULGARE Linnaeus subsp. VULGARE - Wild Marjoram. 2n = 30, 32.
Waste places, roadsides, railroads, fields, woodlands, brooksides, meadows. From Eurasia, northwestern Africa.
ORNITHOGALUM (Family: Asparagaceae)
ORNITHOGALUM NUTANS Linnaeus - . 2n = 14, 18, 19, 35, 45.
Low meadows, fields. From western Asia. CONN herbarium image
ORNITHOGALUM UMBELLATUM Linnaeus - Star-of-Bethlehem. 2n = 16, 18, 27, 36, 45, 52, 54, 72, 90.
Old house sites, roadsides, thickets, fields, meadows. From Europe. CONN herbarium image
ORNITHOPUS (Family: Fabaceae)
ORNITHOPUS SATIVUS Brotero subsp. SATIVUS - Serradella. 2n = 14.
Sandy, open, disturbed soil. From southwestern Europe, northern Africa.
OROBANCHE (Family: Orobanchaceae)
Orobanche uniflora Linnaeus subsp. uniflora - One-flowered Cancer-root. 2n = 18, 36, 48, 72.
Damp woods and thickets, streambanks, parasitic on roots of various plants.
ORONTIUM (Family: Araceae)
Orontium aquaticum Linnaeus - Golden Club. 2n = 26.
Shallow water of ponds, sandy, muddy or sphagnous shores. NEBC herbarium image
ORTHILIA (Family: Ericaceae)
Orthilia secunda (Linnaeus) House - One-sided Pyrola. 2n = 38.
Rich woods. [Pyrola secunda Linnaeus; P. secunda var. obtusata Turczaninow]
ORYZOPSIS (Family: Poaceae)
Oryzopsis asperifolia Michaux - . 2n = 46, 48.
Dry woods.
OSMORHIZA (Family: Apiaceae)
Osmorhiza berteroi de Candolle - Mountain Sweet Cicely. 2n = 22.
Rich woodlands. [O. chilensis Hooker & Arnott]
Osmorhiza claytonii (Michaux) C.B. Clarke - Hairy Sweet Cicely. 2n = 22.
Rich woodlands.
Osmorhiza depauperata Philippi - . 2n = 22.
Moist, deciduous woodlands. [O. obtusa (J.M. Coulter & Rose) Fernald]
Osmorhiza longistylis (Torrey) de Candolle - Anise-root. 2n = 22.
Rich, moist woodlands. [O. longistylis var. brachycoma S.F. Blake; O. longistylis var. villicaulis Fernald]
OSMUNDA (Family: Osmundaceae)
Osmunda claytoniana Linnaeus - Interrupted Fern. 2n = 44.
Moist woods, margins of swamps, moist thickets, occasionally in the open, in acid or neutral soils. CONN herbarium image
Osmunda regalis Linnaeus var. spectabilis (Willdenow) A. Gray - Royal Fern. 2n = 44.
Swamps, peaty thickets, shores, meadows, bogs, mostly in acid soils. CONN herbarium image
Osmunda hybrid
Osmunda X ruggii R.M. Tryon.
[O. claytoniana Linnaeus X O. regalis Linnaeus var. spectabilis (Willdenow) A. Gray]
OSMUNDASTRUM (Family: Osmundaceae)
Osmundastrum cinnamomeum (Linnaeus) C. Presl - Cinnamon Fern. 2n = 44.
Swamps, damp thickets, stream banks, and other moist, shaded places in acid soils. [Osmunda cinnamomea Linnaeus; O. cinnamomea var. glandulosa Waters] CONN herbarium image
OSTRYA (Family: Betulaceae)
Ostrya virginiana (Miller) K. Koch - American Hop-Hornbeam. 2n = 16.
Rich, dry woods. [O. virginiana var. lasia Fernald]
OXALIS (Family: Oxalidaceae)
OXALIS CORNICULATA Linnaeus - Creeping Wood-Sorrel. 2n = (12), (16), 24, (28), (32), 44, 48.
Waste places, roadsides, fields. From unknown origin.
Oxalis dillenii Jacquin - . 2n = 18, 20, 22, 24.
Dry fields, sandy thickets, woodland margins, roadsides, waste areas. [O. stricta misapplied]
Oxalis florida Salisbury - . 2n = 16.
Fields, woodland margins, sandy thickets, roadsides, waste areas. [O. filipes Small]
Oxalis montana Rafinesque - Mountain Wood-Sorrel. 2n = 22.
Rich, damp woodlands. [O. acetosella misapplied]
Oxalis stricta Linnaeus - . 2n = 18, 24.
Waste places, roadsides, fields. [O. europaea Jordan var. europaea; O. europaea var. bushii (Small) Wiegand]
Oxalis violacea Linnaeus - Violet Wood-Sorrel. 2n = 28.
Rich, moist woodlands, rocky slopes. [O. violacea var. trichophora Fassett]
Oxalis hybrid
Oxalis dillenii Jacquin X O. stricta Linnaeus.
OXYBASIS (Family: Amaranthaceae)
Oxybasis glauca (Linnaeus) S. Fuentes, Uotila & Borsch var. glauca - Oak-leaved Goosefoot. 2n = 18, (36).
Waste places, roadsides, railroads. [Chenopodium glaucum Linnaeus var. glaucum]
Oxybasis rubra (Linnaeus) S. Fuentes, Uotila & Borsch var. rubra - Coast Blite. 2n = (18), 36.
Salt marshes. [Chenopodium rubrum Linnaeus var. rubrum]
Oxybasis rubra (Linnaeus) S. Fuentes, Uotila & Borsch var. humilis (Hooker) Mosyakin - . 2n = 36.
Saline or brackish soil. [Chenopodium humile Hooker; C. rubrum Linnaeus var. humile (Hooker) S. Watson]
OXYBASIS URBICA (Linnaeus) S. Fuentes, Uotila & Borsch - Upright Goosefoot. 2n = 18, (36).
Waste places, wool waste. From Eurasia. [CHENOPODIUM URBICUM Linnaeus]
OXYDENDRON (Family: Ericaceae)
OXYDENDRUM ARBOREUM (Linnaeus) de Candolle - Sourwood. 2n = 24.
Woods. From farther south and west.
OXYRIA (Family: Polygonaceae)
Oxyria digyna (Linnaeus) Hill - Alpine Mountain-sorrel. 2n = 14, 42.
Alpine areas.
OXYTROPIS (Family: Fabaceae)
Oxytropis campestris (Linnaeus) de Candolle var. johannensis Fernald - 2n = 32, 48.
Rocky or gravelly river shores. [O. johannensis (Fernald) Fernald]
Designed by Ray Angelo. Revised March 16, 2023