Onagraceae, the Evening Primrose Family
Herbs with opposite or alternate simple leaves and regular flowers; sepals and petals each 4 (or 2 in one genus), stamens 8 (or 2 in one genus), attached to the summit or inside of a tubular receptacle; ovary 2-4-celled, inferior.
| 1a. Aquatic plant of shallow water or muddy ground, with prostrate stem (flowers minute, axillary; petals small or none, summer) | Water Purslane, Ludvigia palustris. |
| 1b. Land plants with erect or ascending stems — 2. | |
| 2a. Petals minute, greenish (3-8 dm. high; late summer) | False Loosestrife, Ludvigia polycarpa. |
| 2b. Petals yellow — 3. | |
| 2c. Petals white, pink, purple, or red — 9. | |
| 3a. Sepals borne at the summit of the ovary (5-10 dm. high; summer) | False Loosestrife, Ludvigia alternifolia. |
| 3b. Sepals borne at the summit of the slender tubular receptacle, which is prolonged beyond the ovary — 4. | |
| 4a. Stamens all equal in length (Evening Primrose) — 5. | |
| 4b. The alternate stamens longer (3-8 dm. high; summer) (Sundrops) — 8. | |
| 5a. Leaves deeply dentate or pinnatifid (2-5 dm. high; early summer) | Evening Primrose, Oenothera laciniata. |
| 5b. Leaves entire or undulate or finely toothed (5-12 dm. high; summer) — 6. | |
| 6a. Hairs on the stem with broad reddish bases | Evening Primrose, Oenothera muricata. |
| 6b. Hairs on the stem none, or without swollen bases — 7. | |
| 7a. Stem and foliage glabrous, or with sparse spreading hairs | Evening Primrose, Oenothera biennis. |
| 7b. Stem and foliage densely but closely appressed-pubescent | Evening Primrose, Oenothera rhombipetala. |
| 8a. Petals 5-10 mm. long | Sundrops, Oenothera pumila. |
| 8b. Petals 14 mm. long or more | Sundrops, Oenothera fruticosa. |
| 9a. Petals 2, reverse heart-shape, stamens 2 (flowers small, white, summer) (Enchanter’s Nightshade) — 10. | |
| 9b. Petals 4; stamens 4 or 8 — 12. | |
| 10a. Leaves rounded at the base, denticulate (3-8 dm. high; fruit prickly) | Enchanter’s Nightshade, Circaea lutetiana. |
| 10b. Leaves cordate at the base — 11. | |
| 11a. Fruit 2-celled, bristly (2-4 dm. high) | Enchanter’s Nightshade, Circaea intermedia. |
| 11b. Fruit 1-celled, with soft hairs (delicate plant 2 dm. high, or less) | Enchanter’s Nightshade, Circaea alpina. |
| 12a. Petals entire (summer) — 13. | |
| 12b. Petals notched at the end (flowers white or pinkish, less than 1 cm. broad, in summer) (Willow Herb) — 15. | |
| 13a. Flowers 2-3 cm. wide, purple (7-20 dm. high) | Fireweed, Epilobium angustifolium. |
| 13b. Flowers about 1 cm. wide — 14. | |
| 14a. Flowers red (2-5 dm. high) | Gaura, Gaura coccinea. |
| 14b. Flowers white, turning pink when old | Gaura, Gaura biennis. |
| 15a. Leaves entire, the margins usually somewhat revolute — 16. | |
| 15b. Leaves toothed, flat (4-9 dm. high) — 18. | |
| 16a. Plant densely pubescent with spreading hairs (3-8 dm. high) | Willow Herb, Epilobium molle. |
| 16b. Plant pubescent with appressed or incurved hairs — 17. | |
| 17a. Leaves linear, the margin revolute (3-5 dm. high) | Willow Herb, Epilobium densum. |
| 17b. Leaves narrowly lanceolate, not revolute (2-4 dm. high) | Willow Herb, Epilobium palustre. |
| 18a. Seeds tipped with a tuft of reddish-brown hairs | Willow Herb, Epilobium coloratum. |
| 18b. Seeds tipped with a tuft of white hairs | Willow Herb, Epilobium adenocaulon. |