Lobelia: choosing and growing the best lobelia

By Daniel Griffiths

Published: Wednesday, 24 November 2021 at 12:00 am

Plant Profile: Lobelia

What
Lobelia diverse genus of more than 400 species of annuals, perennials and shrubs. The more cold-tolerant species have been greatly important to the development of ornamental cultivars. These x speciosa hybrids exhibit a vibrant range of colours from white, blue and deep purple through to vibrant red, and tend to send their flowering stems up 40cm to 1.2m above fleshy, evergreen rosettes.

Origins
Most are concentrated in tropical or warm temperate regions but a few extend their range into cool temperate zones.

Season
The most popular hybrids flower from mid-July until well into September.

Size
There is a wide range in size from the tall and shrubby to diminutive, sprawling species with wiry stems.

Conditions
The most popular cultivars tolerate sun or partial shade but require consistently moist soil in the summer. They are best kept slightly drier through the colder months, in a sheltered position or in a dry mulch to protect them from frost.

Hardiness
Generally hardy, RHS H3-H5. The North American Lobelia siphilitica is one of the hardiest, coping with temperatures down to –10°C (RHS H4, USDA 8a-9b).

"Lobelia
Lobelia inflata. Annual species from the east coast of the USA with small, light-blue flowers. May behave like a biennial but tends not to overwinter well. It’s used as an emetic plant and has the rather graphic common name of pukeweed. 15cm-1m.
"Lobelia
Lobelia spicata. A highly variable perennial from North America with elegant spires of icy, lavender-blue flowers that can appear as early as late May and persist until the beginning of August. It tolerates dry soils better than most. 60cm. RHS H4.
"Lobelia
Lobelia siphilitica. f. albiflora ‘Alba’. White form of L. siphilitica with less vigour but still producing tall flower spikes above bright-green foliage. Can be planted as a marginal pond plant with its roots fully submerged. 1.2m. RHS H4. 
"Lobelia
Lobelia x speciosa ‘White Witch’. An exciting new cultivar, but with limited availability. The large, white flowers have a dash of pink in the throat. Barry admires its showy flowers, but admits it is a weak grower. 60-80cm. RHS H4.
"Lobelia
Lobelia cardinalis ‘Elmfeuer’. A dramatic, dark-leaved and red-flowered cultivar, often noted by nurseries as synonymous with L. cardinalis ‘Queen Victoria’, although breeder Barry Clarke suggests it is a definite hybrid. 60-80cm. RHS H3.
"Lobelia
Lobelia x speciosa ‘Ruby Slippers’. An early backcross from 1989 by Thurman Maness of Wildwood Nursery in North Carolina, USA. It may not have the largest of blooms, but it is one of the best red-flowered lobelias. Very long lived. 80cm. 
"Lobelia
Lobelia laxiflora subsp. angustifolia. This shrubby lobelia thrives in sunny/drier situations adapted to southern US states and Mexico where it is found at fairly high altitude. A narrow-leaf form, more cold-tolerant than L. laxiflora. 1m. RHS H3.
"Lobelia
Lobelia x speciosa ‘Pink Elephant’. A fantastic grower from Ireland, with strong, evergreen rosettes, that retains much L. siphilitica vigour. Clear-pink flowers are held on tall stems. Established clumps spread up to 60cm. An improvement on the popular L. ‘Compton Pink’. 1.2m. AGM*. RHS H5†.
"Lobelia
Lobelia urens. Rare, British native, mainly found in Devon among grass on poor, acid heathland. Sometimes called ‘flower of the Axe’, it thrives on intermittently disturbed ground. Pretty, small, blue-purple flowers. 40cm. RHS H4.
"Lobelia
Lobelia x speciosa ‘Hadspen Purple’. Named by Nori Pope after the Hadspen estate, where he and his wife Sandra based their famous nursery, this hybrid has gained deserved popularity since it was released in 2002. Velvety-purple flowers held atop strong stems. An easy grower. 60cm-1m. RHS H5.
"Lobelia
Lobelia x speciosa ‘Red Demon’. A new, red cultivar with very large flowers that is not yet widely available. Barry Clarke admits it is not the most vigorous but deems it an improvement on the ever popular L. cardinalis ‘Queen Victoria’. 1m.
"Lobelia
Lobelia angulata. This diminutive, creeping species grows in woodland and alongside streams in its native New Zealand. Small, white flowers from June to September. Good for a container on the north side of the house. 10cm. RHS H4, USDA 6a-9b.
"Lobelia
Lobelia tupa. With its claw-like, orange/red flowers held on tall, upright stems that have narrow, hairy, grey-green leaves this is one of the most iconic lobelias. Known as devil’s tobacco, this Chilean native appreciates a sunnier, drier site than most. 2m. RHS H4.

All about lobelia

Adorning patios and pub frontages across Britain, the cascading blue of Lobelia erinus may be a much-loved symbol of summer but there is so much more to this diverse genus than merely colourful filler for hanging baskets. Found predominantly in warmer climes, the 400-plus species have a wide distribution, with one species, L. urens, found in parts of southern Britain. There are shrubby, annual and perennial species, with monocarpic giants such as the spectacular L. deckenii from Tanzania contrasting with the tiny, mat-forming L. angulata endemic to New Zealand. Their abundant flowering, colour range and tendency to hybridise has also resulted in many garden-worthy hybrids.

Origins

The name Lobelia may have been coined by Swedish scientist Carl Linneaus in 1753 for the Flemish botanist Matthias de l’Obel, but the plant has a longer history as a purgative in the sacred rituals and medicinal repertoire of indingenous people. Certainly, all parts of the plant are toxic and, due to poisonous compounds such as Lobeline, induce vomiting if ingested. This may be behind the name devil’s tobacco, given to the Chilean L. tupa, or the association in Victorian England that lobelias, included in a floral arrangement, implied ill will. Thankfully this has done little to discourage breeders from creating new, highly ornamental cultivars.
Morphologically, lobelias tend to have simple leaves held alternately with flowering stems topped with two-lipped, tubular flowers divided into five lobes. Usually the upper lip is split in two while three lobes fan out to form the lower lip. Inevitably with such a broad genus the taxonomy has fluctuated with various other genera, including Pratia and Isotoma, now included under the umbrella of Lobelia.

"Lobelia
Lobelia siphilitica. f. albiflora ‘Alba’

Most common types of lobelia

By far the most familiar group of garden lobelias are the x speciosa hybrids. These are large-flowered crosses between L. cardinalis and L. siphilitica, both hardy North American species, and have justifiably become firm garden favourites. Barry Clarke, botanist at the Sir Harold Hillier Gardens in Hampshire, grows an impressive number of cultivated forms and explains that, because of Lobelia’s readiness to hybridise, these cultivars should only be vegetatively propagated.

As a result, there is often confusion over some, particularly older, cultivars – L. x speciosa ‘Vedrariensis’ being a good example. Barry also holds the National Collection of species lobelias. His passion for the genus came from a wanderlust that took him around the world where he was able to see a wide range of species growing in the wild. With a few notable exceptions, many of these require specialist care and are described by Barry affectionately “as quite challenging”.

Where to plant lobelia

While their large, colourful flowers easily justify their place in the border, modern hybrids tend to be short-lived. Most wild species such as L. urens and L. spicata may not compete in flower power but have an elegance that blends easily into a naturalistic planting scheme.

Once established, their glossy rosettes are highly decorative but can be prone to molluscs and are worth a bit of spring protection. Your efforts will be repaid, however, when they begin flowering in mid-July – providing a useful bridge from mid- to late summer – usually requiring no staking, and injecting welcome colour often well into September.

How to cultivate lobelia

Most hardy lobelias are best grown in rich, moist, slightly acid soils in partial shade. They will tolerate full sun in cooler
climates as long as the soil does not become too dry. Almost any ground being prepared to receive lobelias will benefit from the incorporation of organic matter, leaf mould or regular garden compost. This will help improve the structure of soil, preventing waterlogging in the winter but increasing water retention in the summer.

When to plant lobelia

Many species are easy enough from seed sown in late winter and should make flowering plants in their first year but the seedlings will be variable. The rosettes of named hybrids should be divided in early spring. If winter temperatures regularly fall below freezing during winter, a dry covering of leaves will help reduce the risk of rotting. After the worst of the frosts have passed, these leaves should be cleared away. After some decent rain in late spring it is beneficial to mulch plants to help retain moisture into summer. It is suggested that watering the rosettes as the season transitions into autumn improves their hardiness.

Planting combinations

Lobelia siphilitica, L. cardinalis and their hybrids (L. x speciosa) are all well adapted to the wooded edges of streams. While there are
a number of cultivars that show remarkable longevity, many of the newer introductions, impressive in the size and colour of their flowers, seem to fade away after a few years. These larger flowers make a bold statement in a border, while the more demure and variable flowers of species such as L. spicata and L. siphilitica appear more comfortable in a naturalistic planting scheme.

The same can be said of the older, smaller-flowered cultivars. One of the most useful lobelias is L. cardinalis, which grows wild along woodland streams of North America and provides an intensity of red rarely found in the shade garden.

Some of the smaller, creeping forms are useful woven through the moist leaf litter of a woodland garden. Plants such as L. angulata will work themselves among the leaves of larger plants, sending up masses of white, star-like flowers. Barry has had success plugging these creeping forms into meadow grass. Other species also grow well in grass, such as the rare, native L. urens.

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The best lobelia for your garden

1

Lobelia x speciosa ‘Pink Elephant’

"Lobelia
Lobelia x speciosa ‘Pink Elephant’

A fantastic grower from Ireland, with strong, evergreen rosettes, that retains much L. siphilitica vigour. Clear-pink flowers are held on tall stems. Established clumps spread up to 60cm. An improvement on the popular L. ‘Compton Pink’.

1.2m. AGM*. RHS H5†.