(Editor's Note: This article was originally published on January 6, 2008. Your comments are welcome but please be aware that authors of previously published articles may not be able to respond toyour questions.)
In part 1 of the article entitled: Primroses: Diversity is Their Key to Popularity, I described the main cultural requirements of primroses and how they may be used in the garden. In this article I will describe in more detail the main groups of primroses that are easily grown in the garden. There are over 500 species of Primula and plant taxonomists have broken these down into 37 groups based on area of origin, flower form and leaf form. Some of these groups are near impossible to grow in a garden setting as their cultural requirements are so specific. In fact, only a few groups are easily grown in the average garden. However, even this limited number of groups can provide you with a wonderful display of flowers over an extended season from mid-winter to mid-summer, depending on where you live.
The most popular group of primroses are those referred to as polyanthus primroses. Botanically these are called the Vernales Group. This group will provide colour from mid-winter through May. In the Pacific Northwest, they are popular for providing winter blooms in container plantings. They are also popular indoor pot-plants during the winter months and may be later planted in the garden to provide colour in future years. Polyanthus primroses are usually hybrids derived from Primula vulgaris, P. veris, P. elatior and P. acaulis. These primrose species are all native to Europe. They are available in a rainbow of colours, including green and brown! They include single, semi-double or double-flowered forms.
Polyanthus primroses do best in a highly fertile, humus-rich soil and in lightly shaded areas. Because they do not like acidic soil, lime should be applied to the planting area. These plants also demand constant moisture. Allowing them to become wilted will severely set back the plants. To help maintain soil moisture, mulch the plants with 3 - 5 cm of old compost/leaf mould or well decomposed manure. This mulch will also keep the organic content high in the growing area. For best results, divide and replant polyanthus primroses every 3 - 4 years.
A variety of Polyanthus primroses
Another popular and easily obtained group of primrose are the Auriculastrum Group, commonly called auriculas. Like the polyanthus group, they too are natives of Europe. The most popular auriculas are hybrids available in a wide range of colours in single, semi-double or fully double forms. Unlike the tufted leaves of the polyanthus primroses, auriculas produce a rosette of leaves from a stout rhizome.
Auriculas demand more sun than the polyanthus types and are ideal subjects for a rock garden. While the hybrid auricula are relatively large, species such as P. marginata, P. hirsuta and P. pedemontana, are fairly small in size. Most of the species in this group hail from limestone regions; thus the addition of lime in the growing area is beneficial.
Even though the auriculas are among the most drought-tolerant of all primroses, they still prefer a moist, well-drained soil. Old leaves often remain at the base of the rosettes and should be carefully removed to keep diseases to a minimum. They flower about the same time as the polyanthus primroses.
Another diverse and increasingly popular group of primroses are the candelabra or Proliferae primroses. As the name implies, plants produce whorls of bloom on tall slender stems. A good plant may produce 4 - 6 whorls, each whorl lasting about a week. As such, they are among the longest blooming primrose. This group has the added bonus of late spring-early summer blooms, generally May to July. Flowers are available in red, purple, pink, orange, yellow and white.
Plants may be grown in full sun or light shade, but flower colours will be richer if plants are lightly shaded. The candelabra primroses include the species P. bulleyana, P. beesiana, P. japonica and P. pulverulenta as well as hybrids of these. In the wilds of eastern Asia, these primroses often grow in boggy situations, hence constant moisture is a must. They are ideal subjects for bog gardens and waterside plantings. These primroses overwinter as a tight acorn-sized bud.
A selection of candelabra primroses
The Sikkimensis group of primroses are valued for their attractive, heavily farinose (powdery), fragrant, pendent flowers that are available in shades of violet, cream, yellow, pink, copper and red. Easily grown in damp, fertile soil in sun or light shade, these primroses bloom about the same time as the candelabra and as such, are great companions for each other. Species in this group P. alpicola, P. florindae, P. secundiflora and P. sikkimensis are also native to Asia. Although they are rarely available locally, they may be available through mail order catalogues. As a note of interest, P. florindae is among the tallest of any primrose, often reaching over 1 meter in height. These primroses disappear completely in winter.
Two colour forms of P. sikkimensis
These primroses are among the smallest and most dainty primroses. Many only obtain a height of a few inches. Botanically they are referred to as the Farinosae Group as most plants have leaves heavily covered in whitish powder called farina. These primroses have the widest distributional range of any groups, being found throughout North America, Europe and Asia as well as the Falkland Islands and southern tip of South America. Of the groups mentioned, these are a little more challenging to grow. However, they are popular among the more avid primrose growers. The colour range is more restricted; mauve, purple, pink, pale yellow or white. They prefer moist to boggy conditions and are apt to be short-lived so collecting seeds is recommended. With their small size, alpine troughs might be the recommended way to grow them. Seed exchanges or specialty nurseries would be the best sources for these primroses. Among the recommended species are P. halleri, P. frondosa, P. luteola, P. farinosa and P. algida. They are spring-bloomers.
Some examples of Farinosae primroses include P. farinosa, P. frondosa, P. halleri and P. luteola.
There are several other popular primroses that belong to a mixture of primrose groups. However, the one feature they all have in common is that they hail from Asia. Technically, the candelabra and sikkimensis groups can also be included in the Asiatics, but the following are additional species.
The drumstick primrose, P. denticulata, is native to the Himalayas. As the name implies, flowers are carried in dense, spherical heads. They are among the earlier primroses to flower in spring. They are also among the most robust primroses, producing quite large leaves by mid-summer. As a result, they require quite a lot of space in the garden. The colour range of drumstick primroses are more restricted than the polyanthus or auricula, being available in shades of mauve, pink, magenta and white. Like the candelabra types, this primrose also overwinters as an acorn-sized bud.
Some varieties within P. denticulata.
The Cortusoides Group contains some lovely choice primroses that are late spring-early summer bloomers. These are woodland species with softly-hairy, rounded leaves that are vaguely reminiscent of Geraniums or Pelargoniums. The colour range is rather limited to pink shades or white, but they have relatively large flowers in loose, rounded clusters. Among the most easily obtained are P. cortusoides, P. jesoana, P. polyneura and P. sieboldii.
Primula chionantha, P. melanops, P. capitata and P. glomerata.
The last primrose to be discussed is the exotic (if not bizarre) P. vialii. This species hardly looks like a primrose at all. The leaves are narrow and held quite upright. In August or even September, plants produce a tall stem topped with a Kniphofia-like spike of flowers. The flowers have a red calyx which protects the developing blossoms, and the petals themselves are lavender-blue. Together, the calyx and petals create a pleasing effect. Primula vialii is among the latest primrose to sprout in spring. They are not the easiest of primroses for the home gardener to grow and even if you are able to keep them alive through their first winter (they are notorious for rotting in winter), plants are apt to be short-lived. However, they are still a welcome addition to the variety of primroses which can add beauty to any garden.