Excerpted from Perennials for Every Purpose, by Larry Hodgson
It's no wonder primroses are inseparable from spring in our minds. Not only are they among the first perennials to bloom—some even flower in late winter—but their very name implies earliness: Primula derives from the Latin word for "early." Most primroses produce a ground-hugging rosette of greenery, and their rounded flowers have five petals each. Their leaves range from thick, smooth, and waxy to narrow, hairy, and toothed. Some species bear one flower per stem while others have dome-shaped or rounded clusters of flowers on each stem.
Primroses have common cultural needs, namely moist soil and cool growing conditions. They thrive in full sun in cool-summer areas, but usually need partial shade elsewhere. And while they generally need moist soil, most also require good drainage.
The main exceptions are Japanese primrose (Primula japonica) and, to a lesser degree, drumstick primrose (P. denticulata): Both will do well in almost waterlogged soil. Although cold-hardiness varies from plant to plant, a winter mulch is wise almost everywhere to help primroses survive the winter. Gardeners in AHS Heat Zones 12 to 9 have trouble growing many primroses, not because of heat but because the plants need winter chill.
Division is the ideal way to propagate primroses and the only way to maintain specific cultivars. You can also grow primroses from seed, but their need for temperatures between 40 and 50 degrees F (4 to 10 degrees C) during the long period between sowing and the first blooms makes starting plants indoors impractical for most of us.
Suitable primrose companions for a moist, partly shaded spot include astilbes, ferns, hostas, Japanese iris (Iris ensata), forget-me-nots (Myosotis spp.), and pulmonarias. Drumstick primrose (P. denticulata) and Japanese primrose (Primula japonica) are suitable for wetland plantings—try them with marsh marigold (Caltha palustris), yellow flag (Iris pseudacorus), and cardinal flower (Lobelia cardinalis).
Problems and Solutions
Spider mites can be a real plague, especially if you grow primroses in full sun in a hot climate. Infested plants have yellow stippling on the leaves, and in serious cases, leaves turn brown. To keep spider mites in check, spray plants regularly with water.
Polyantha primrose (Primula X polyantha). Once upon a time, there were individual wild species of European primroses, such as the fragrant, deep yellow cowslip (P. veris) and the lemon-yellow English primrose (P. vulgaris). Today, though, these and other primrose species sold commercially are probably all hybrids, and I prefer to lump them together under the name of (P. X polyantha). This varied group shares wrinked, tongue-shaped leaves and stemless or stemmed flowers, borne singly or in clumps. They bloom abundantly in early to late spring and sometimes again, much more lightly, in the fall. Most gardeners grow unnamed plants from popular seed strains such as Pacific Giant Hybrids, with large flowers in the full range of colors, and Cowichan Hybrids, in solid colors (no yellow eye) and with reddish leaves.
Polyantha primrose: Primula X polyantha
Bloom color: Yellow, pink, purple, blue or white, depending on cultivar
Bloom time: Early to late spring
Length of bloom: 3-4 weeks or more
Height: 6-12 inches
Spread: 8-9 inches
Hardiness: USDA zones 3-9
Heat tolerance: AHS heat zones 8-6
Light preference: Partial shade (full sun in cool climates)
Soil preferences: Humus-rich, moist but well-drained soil
Propagation: Divide in early summer, after flowering
Garden uses: Container planting, edging, mass planting, rock garden, wall planting, woodland garden; along paths, on slopes, in wet areas.