(Editor's Note: This article was originally published on April 10, 2008. Your comments are welcome, but please be aware that authors of previously published articles may not be able to respond to your questions.)
Ask a gardener to name a blue flower and gentians often spring to mind. Most gardeners have heard of them even if only a few actually grow them. They are symbols for alpine flora and as most hail from mountainous regions, they are ideal subjects for the rock garden. The ultimate example of their alpine nature is the fact that gentians grow on the slopes of Mt. Everest at an elevation of 18,000 feet! As a genus, they are indeed primarily alpine in nature, occurring in the mountains of western North America, Europe, Andes, Himalayas and Australia/New Zealand. However, with well over 200 species, there are actually some gentians which are better suited for the perennial border or woodland garden. They range from just a few centimeters to giants up to 2 m. The predominant colour is blue, but they do come in white, yellow, purple, red and even green. Strangely, while blue is the dominant colour, those which hail from Australia/ New Zealand are almost all white-flowered! Their trumpet-like flowers are primarily adapted to pollination by bees, which, as a rule, are attracted to blue flowers.
The genus got its name in honour of King Gentius of Illyria who created a remedy against plaque from the leaves and roots of a gentian. Extracts from gentians (in particular G. lutea) are still used in herbal medicines in the modern day. Gentian extract is used to increases the appetite, stimulates digestive juices, decreases intestinal inflammation, treats indigestion, heartburn, liver and spleen disorders, promotes menstruation, strengthens and builds the body, and is helpful for gout and arthritis.
Culturally, some gentians are easy while others are extremely difficult. Some need acidic soil, other alkaline, while others don't seem to care. They all require well-drained, yet evenly moist soil with a reasonable organic content. They detest hot, dry sites. In cool summer areas, full sun is best but in warmer climes, they should be shaded from hot afternoon sun. Oftentimes, the soil pH seems to be the most critical factor to success.
Despite some 200 species, only a handful are common in cultivation. As a rule, they are either spring bloomers or late summer-fall bloomers, but a few will bloom mid-summer to fill the gap. Let's start with the spring bloomers. The spring bloomers are divided into two groups; the ‘acaulis' group and the ‘verna' group. The spring trumpet gentians are from the acaulis group and are characterized by low mats of evergreen foliage and large, nearly stemless, solitary flowers in the deepest, richest shades of blue. This group is composed of 7 species; G. acaulis, G. alpina, G. angustifolia, G. clusii, G. dinarica, G. ligustica and G. occidentalis. All hail from the mountains of Europe. There are also several hybrids derived from these species. As a rule, all look quite similar but from a gardening perspective, the easiest to grow is G. acaulis (prefers slightly acidic soil), G. angustifolia (a lime-lover; see picture above), G. clusii (either acidic or alkaline) and G. dinarica (a lime-lover). They are propagated from seed or by division. The other spring-bloomer, G. verna, forms tufted plants with smaller, more exquisite blue flowers. While charming, it is challenging to keep in cultivation. They require acidic, peaty soil and will wither at the slightest hint of drought.
The mid-summer blooming gentian species all look quite similar, making identification difficult. Even nurseries selling these gentians often get the species names incorrect. There most common species is G. septemfida (G. lagodechiana offered by some nurseries is now classified as G. septemfida). This one has the showiest blooms of the summer bloomers. Plants are herbaceous in nature, forming tufted plants with somewhat sprawling stems to 30 cm terminated by a cluster of bright blue flowers. Some dwarf varieties have solitary blooms. It is perhaps the easiest gentian to grow as they are not particular about soil pH. The most floriferous plants are those grown in full sun and such specimens can be extremely showy. They can be used in the rock garden or the perennial border. Less showy, but equally easy in cultivation is the cross gentian, G. cruciata. The plant habit is similar to G. septemfida but the flowers are a bit smaller, the stems more upright and the plant easy to identify since each flower has 4 petals rather than the standard 5.
Above are Gentiana cruciata and two pics of G. septemfida
Several look-alike species that are often confused are G. bigelovii, G. dahurica, G. decumbens, G. gracilipes and G. waltonii. These also have tufted growth 20-30 cm with upright to slightly arching stems with both terminal and axillary clusters of smallish, more star-shaped blue flowers. Again, these are generally easy in cultivation and while not as showy as some gentians, do provide a welcome spot of mid-summer blue. All of these mid-summer bloomers are best grown from seed.
Shown above are Gentiana dahurica, G. waltonii and G. decumbens
The fall-blooming gentians fall into two main groups; the willow gentians and the Chinese gentians. One of the most spectacular and larger gentians is the willow gentian, G. asclepiadea. It blooms in late summer with arching stems up to 1 m in length. The upward-facing flowers are produced in pairs along the upper leaf axils. The flowers all open simultaneously, lending a spectacular blue fountain appearance to the plant. They also come in white or rarely, pink. Moist, peaty, acidic soil in part shade will result in the most lush plants. These have deep roots and resent transplanting so start with young plants and leave them, since like a great wine, they improve with age.
Above are the white and standard blue forms of G. asclepiadea
The Chinese gentians are very late blooming, sometimes blooming trough November! Plants are very frost tolerant. They generally produce solitary, quite large, brilliant blue flowers at the ends of trailing 20-30 cm stems. The outside of the flowers are exquisitely striped in various shades of blue, along with white. There are several species and hybrids included in this group; G. sino-ornata, G. ornata, G. farreri, G. hexaphylla, G. ternifolia and G. veitchiorum are the species but the hybrids are often more spectacular than the species. Among the more common hybrids are ‘Kingfisher', ‘Drake's Strain' , 'Juwel', and ‘Delft'. These prefer full sun and demand acidic, moist soil. Unlike most gentians, these are easy to divide.
Above are Gentiana 'Kingfisher', 'Juwel', 'Drake's Strain and G. sino-ornata
Another fall bloomer which should be mentioned is G. paradoxa. This species was only recently discovered in a small region of the Caucasus Mountains. Plants are somewhat like G. septemfida but the relatively large flowers are solitary and the leaves are very narrow and crowded along the stems. They readily hybridize with G. septemfida and often, G. paradoxa offered in the trade are actually these hybrids.
There are a few miscellaneous gentians you may encountered in seed exchanges, specialty nurseries and even local nurseries. The North American native bottle gentian, G. andrewsii and G. clausa, produce upright stems to 50 cm with a terminal cluster of mid-blue, ‘bottle'-shaped flowers. The blooms never open fully. It is certainly not the showiest species, but suitable for a woodland or wildflower garden. Gentiana tibetica produces large rosettes of leathery lance-shaped leaves and stout stems topped by a cluster of greenish-white to greyish-white flowers. This one is grown mostly as a curiosity. Perhaps the most imposing species is yellow gentian, G. lutea. This is the giant of the gentians, with stout stems reaching up to 2m! The leaves are large, leathery and ribbed (somewhat like Veratrum spp.). Plants have a thick taproot and cannot be transplanted once mature. They must be grown from seed. It may take many years for a plant to bloom, but when it does, look out! The yellow flowers are produced in dense axillary clusters and while not as showy as their blue relatives, still make an impressive display just for their size. This species is not fussy about soil as long as it is well-drained and they are situated in full sun. They bloom mid-summer.
Above are Gentiana triflora, G. andewsii and G. lutea
This is just a taste of the gentians that exist. You may come across other species in seed exchanges and specialty nurseries. If you have not tried them, give them a shot. You will certainly enjoy their bright blue flowers and if growing a variety, you can have a splash of blue in the garden from spring through fall.
I would like to thank Bootandall for the use of Gentiana 'Drake's Strain', dpacifici for the Gentiana lutea pictures and kmenzel for the G. andrewsii picture. Your pictures helped complete the article!