(Editor's note: this article waas originally published 2-1-08. Your comments are welcome, but be aware that the author may not be able to promptly respond to new comments or questions)
The genus Primula is vast with many hundreds of species. They hail primarily from the northern hemisphere with the greatest diversity found in the Himalayan region, especially Yunnan, Sichuan and Burma. However, they extend from the subtropics all the way to north of the Arctic Circle. Some need boggy soils, others relatively dry, well-drained soil. Although popular garden plants easily grown by many gardeners, most of the species are actually quite demanding in cultivation. As a rule, we think of primroses as spring blooming but there is one group that blooms from late spring through to mid-summer and offer easy culture, as long as you follow a few simple rules. Here, I am referring to the Candelabra primroses, commonly called bog primroses.
Taxonomically, primroses are placed into a number of different sections, based on floral similarities. The candelabra primroses belong to the section Proliferae. As a group, they are easy to recognize. All produce rosettes of upright leaves from which rises stiffly upright flower stems 30-60 cm tall. The flowers are produced in whorls along the length of the upper flower stem. Each whorl opens sequentially with 6-18 flowers per whorl and up to 8 sequential whorls. This flowering habit means the plants are in bloom over a period of 4-6 weeks. This section is native to wet meadows, marshes and mountain streamsides of SW China (Yunnan, Burma, Sichuan, Bhutan) with one species found in Japan. They are hardy to at least USDA zone 5, but some are even hardier.
Culturally, these primroses require an evenly moist, deep (30 cm), organically-rich soil. They are ideal for planting in bog gardens or along streams and garden ponds. These primroses look best when planted in groups. They grow in both sun or part shade but require more water if grown in the former situation. Some are evergreen but most are deciduous and overwinter as a tight, acorn-like knob. They are easily grown from seed especially if the seed are fresh. They need 4-6 weeks stratification for maximum germination. Mature plants may be divided after they bloom. The main pest is root weevil larvae which burrow into their thick, thong-like roots, causing the plants to collapse mid-summer. Biological control is available via predatory nematodes.
In THE GENUS PRIMULA by Josef Halda, he lists 25 species as belonging to this section. However, many of these are essentially unknown in cultivation although potentially, they should not be difficult to grow. The inaccessibility of the plants is the main reason for their rarity in cultivation. Thankfully, there are about a dozen species that are readily available and they come in a rainbow of colours including white, pink, red, purple, yellow and orange.
Among the deciduous, crimson to reddish-purple species are P. beesiana (aka P. burmanica), P. japonica and P. pulverulenta. All grow 60-90 cm tall. Primula beesiana has carmine-red to nearly neon-pink flowers with a yellowish eye. Primula japonica is perhaps the hardiest and largest of the candelabra primroses. The flowers of the wild form are purplish-red with a darker eye. However, there are several named cultivars available in shades of crimson, pink or white. All have dark reddish eyes. Primula pulverulenta is also reddish-purple with a dark eye but the stems and lower leaf surfaces are covered in whitish powder (called farina).
Above: Primula japonica in crimson, pink and white, along with P. pulverulenta.
Among the yellow to orange-red species are P. aurantiaca, P. bulleyana, P. chungensis, P. cockburniana and P. helodoxa. Primula aurantiaca is the shortest of the candelabra group, reaching about 30 cm (the others reach 60-90 cm). Its flowers are deep reddish-orange with a yellow eye. Primula bulleyana is the most robust of this colour group. From reddish-orange buds open bright orange flowers without a noticeable eye. Primula chungensis is very similar to P. bulleyana but the flowers are yellow-orange and slightly smaller. Primula cockburniana is a taller, more slender version of P. aurantiaca with reddish-orange flowers, but the eye of this one is darker than the rest of the flower rather than yellow. It is also short-lived, perhaps best grown as a biennial, so save seed for future stocks! Primula helodoxa is easy to recognize by its bright yellow flowers.
Above: Primula bulleyana, P. cockburniana and P. X bullesiana hybrid
These last group of candelabra primroses are the evergreen types. These are not as hardy as the others, producing smaller rosettes, smaller flowers and shorter but more slender flower stems. These include P. anisodora, P. poissonii and P. wilsonii. Primula anisodora has such dark purple flowers that from a distance they appear black. This contrasts with a yellowish to white eye. Really striking! The flowers smell of anise. Primula poissonii has bright purplish-crimson to magenta flowers with a distinct yellow eye while P. wilsonii is rich purple-pink (darker than P. poissonii but lighter than P. anisodora) with a yellow eye.
Above: Primula anisodora
The deciduous species often hybridize, both in the wild and in gardens, creating a mesmerizing assortment of lovely coloured primroses that make identification of the true species difficult at times. In particular are the Bullesiana hybrids (bulleyana X beesiana) and Chunglenta hybrids (chungensis X pulverulenta). These comes in a mixture of orange, reddish-pinks and in particular, apricot shades.
So if you are a primrose fan and want to extend the blooming season of this lovely group of perennials, then why not develop a spot to grow the elegant candelabra primroses!
(I would like to thank bootandall for the use of the Primula cockburniana picture)