Epilobium are hardy and half hardy perennials that usually flower in the middle of summer.
Due to being a very large genus of plants, plant sizes are very variable, though gardeners tend to like the smaller (10 to 20 cm) and larger (120 to 150 cm) varieties.
The smaller species usually grow in clumps and have funnel shaped flowers of red; the larger varieties of Epilobium have small flowers of white or pink.
Some of the common names for Epilobium members include French Willow, Rosebay, Fireweed and Willow herb.
Epilobium angustifolium - Fireweed / Rosebay Willowherb by wallygrom.
Common Names: Willoherb, Spike-primrose, Fireweed, French Willow, Rosebay, Willow Herb.
Life Cycle: Hardy perennial. Half hardy perennial. Annual.
Height: 4 to 96 inches (10 to 240 cm). Check this
Native: Temperate, subarctic and subantarctic regions.
Growing Region: Zones 2 to 9.
Flowers: Middle of summer.
Flower Details: Pink, red, orange, yellow, white, purple. Cone-shaped. Many species have tiny flowers.
Foliage: Ovate. Lanceolate. Opposite. Alternate.
Sow Outside: 1/8 inch (3 mm). Autumn using fresh seed. Germination time: two to four weeks. Seeds should first be sown into flats. Sink the flat into the ground in an area that offers shade, preferably close to a wall that faces north. Provide a glass/plastic covering. Keep an eye on the flats to ensure that the soil remains moist and to check if seedlings have emerged. Transplant Epilobium in the spring with a spacing of 12 to 24 inches (30 to 60 cm).
Sow Inside: No.
Requirements and care: Full sunlight or partial shade. Good drainage. Stony soil; dry soil for best results. Soil pH 6.0 to 7.0. Spreads rapidly so deadhead before they have chance to set seed. Propagate: Self-seeds readily. Softwood cuttings and division can be performed in the spring.
Miscellaneous: Many members of the Epilobium by many species of moth caterpillars.
It is best to grow French willow and other Epilobium plants outdoors in flats. The seeds should be planted at a depth of about 3 mm in the autumn using fresh seeds; the flats should then be covered with glass and be placed against a north facing shed or wall in the garden. Once seedlings emerge in the spring they should be allowed to grow for 6 or 7 weeks and then transplanted at a spacing of 30 cm (small) to 60 cm (large) into a sunny or lightly shaded area of the garden. Members of Epilobium such as the French willow prefer to grow in a soil that is of a stony and dry nature with a slightly acidic soil of pH 6 to 7.
Once established French Willow and other Epilobium will pretty much look after themselves, perhaps a little too well, so dead head Epilobium after flowering to prevent them from self-seeding and taking over the garden.