Primroses are one of the most popular groups of garden flowers. With over 500 species and countless hybrids, they exhibit great diversity and lend themselves to a variety of garden settings. This article will describe the culture of this lovely group of perennials and their use in the garden. A future article will describe in more detail, the main easily-grown groups of primroses.
What is a Primula?
Primula or primroses are a group of perennial, herbaceous (not woody) plants which may be evergreen or deciduous. Most are low-growing with leaves produced in tufted rosettes. The showy flowers are generally held in open clusters although some produce a single flower per stem, a dense rounded head, a candelabra arrangement or a dense spike of flowers. These blooms are usually upright or outward-facing and open funnel-shaped or less commonly, bell-shaped and pendant. Typically there are five petals, but semi-double and double forms do exist. Several have delightful fragrances.
Care and Maintenance
In nature, primroses inhabit a wide diversity of habitats including rocky alpine crags, scree slopes, stream-sides, dry to moist forests, damp open meadows, swamps and even sub-tropical rainforests. Despite the 500 or so species only certain groups of primroses are easy to cultivate in the garden. All primroses prefer fertile soil, rich in organic matter. The soil pH should be slightly acidic to slightly alkaline. A good soil mix would consist of 2 parts organic material, 2 part topsoil and 1 part coarse sand. Additional grit could be added to the planting area of the alpine primroses as they demand excellent drainage.
Primroses prefer a cool location and are intolerant of high temperatures or dryness during the growing season. Because they are susceptible to summer drought, plants should be well-watered if rainfall is scarce. Do not lightly sprinkle with water every day. To ensure deep root growth, give a thorough watering once a week.
A group of Japanese or candelabra primroses
In warmer areas, primroses are best grown in part shade; an eastern exposure with early morning sun or dappled shade is ideal. However, in northern areas primroses perform better if grown in full sun. This is especially true of the alpine types.
Primroses prefer mild winters or barring that, a steady snow cover in winter. Unfortunately, depending on where you live, this is not always possible. In fact, the cycle of alternate freezing and thawing in winter is a significant problem and is probably the #1 killer of primroses. In addition, those with evergreen leaves are susceptible to winter desiccation. Thus a sheltered planting area is recommended. In more exposed areas covering plants with evergreen boughs, leaves like oak or beech or dry straw will help prevent these problems. It is important to wait until the ground is frozen before adding winter protection.
Pests and Diseases
Primroses are not prone to many diseases. Crown-rot can occur if the soil is not well-drained, especially in winter. Protection against alternate freezing and thawing in winter and the addition of coarse sand or grit to the soil of the planting area will keep rot to a minimum.
The leaves of primroses are quick to produce patches of dead tissue (a symptom of drought) if allowed to get too dry. Proper watering will prevent this physiological problem from occurring.
Slugs and snails are among the most serious pests and they will undoubtedly eat the flowers first! Slug bait is available from garden centers, but ensure you following the manufacturers recommended application procedures because these baits are poisonous to pets and birds. Alternatively, you can physically remove them.
In some areas, the larvae of vine weevils can also do serious harm to primroses. The small, grub-like larvae burrow into the roots and cause the plants to collapse. The adults also feed on the plants, leaving scalloped edges on the leaves. There is no simple cure for this pest, however, predatory nematodes are available as a biological control.
Cowslip primroses are popular in woodland settings
Primroses may be propagated by seed or by division. The latter method is usually preferred, since this is a relatively simple process. In addition, division may even help maintain the health of the plant. Primroses are best divided after flowering. Carefully dig up the plant and cut the clump into two or more smaller divisions using a sharp knife. Some roots may be lost, but when placed into fresh soil, primroses are generally quick to send out new ones. The addition of bonemeal to the planting hole is often beneficial. Remember to keep the plants well watered after dividing, especially in mid-summer.
Most groups, such as the auricula, sikkimensis and candelabra usually look better if left as a large colony while the polyanthus types will bloom better if divided every 3 - 4 years. Never divide the polyanthus types to just a single growth; it is better that each division have 3 - 4 growths. The other primrose groups are more forgiving and if need be, can be divided into single growths without any serious set-backs.
Growing primroses from seed can be challenging. The polyanthus types are readily available as seeds and germinate without too much fuss. Other primroses may need a chilling period (4 - 6 weeks) after sowing before the seeds will break dormancy. The key to success is to sow fresh seeds. When ready to sow, sprinkle the seeds on the surface of a pot filled with a high organic, sterilized mixture. Place a very light layer of soil over the seed. Maintain even moisture and if the seeds are fresh, they should germinate in 2 - 3 weeks. Seeds which have been stored for several months or longer will sprout irregularly and may need to overwinter outdoors before they will germinate.
Primroses in the Garden
Many people regard primroses as a typical "spring" flower. However, with the great diversity within this genus and depending on where you live, primroses can be an attractive feature of any garden from mid-winter through to mid-summer.
The 500 or so species of Primula are classified into 37 groups based on area of origin, flower form and leaf form. There are several main groups that are popular as garden ornamentals, including the polyanthus, auricula, drumstick and candelabra primroses (part 2 will give a more detailed account of the most popular primrose groups).
A group of drumstick primroses
The polyanthus, auricula and drumstick primroses are useful additions to the perennial border. Due to their small stature, they are best placed in the front of the border. They combine particularly well with spring bulbs and other early flowering perennials such as lungwort, rockcress and leopard's-bane. It is useful to interplant these primroses with annuals to provide summer colour. The best primroses for a rock garden are the auricula group. They benefit from the additional drainage afforded by most rock gardens and the plants themselves do originate from alpine environments. The beauty of the sikkimensis and candelabra primroses is stifled when grown in a perennial border. In a lightly shaded woodland bed, these primroses combine attractively with shooting star, hosta, astilbe, columbine and ferns. If your garden includes a water feature, such an area will bring out the natural beauty of the candelabra and sikkimensis primroses.
Some gardeners devote entire portions of their gardens to primroses. Others only grow a few scattered here and there. Either way, they will add to the beauty of any garden.
About Todd Boland
I reside in St. John's, Newfoundland, Canada. I work as a research horticulturist at the Memorial University of Newfoundland Botanical Garden. I am one of the founding members of the Newfoundland Wildflower Society and the current chair of the Newfoundland Rock Garden Society. My garden is quite small but I pack it tight! Outdoors I grow mostly alpines, bulbs and ericaceous shrubs. Indoors, my passion is orchids. When not in the garden, I'm out bird watching, a hobby that has gotten me to some lovely parts of the world.