Horticulture Magazine

Lobelia Plant Care and Growing Tips

blue lobelia flowers

Lobelia Overview

Official Plant NameLobelia
Common Name(s)Cardinal Flower, Blue lobelia
Plant TypePerennial (Some Annuals)
Native AreaNorth America / South Africa
Hardiness RatingMostly H3
ToxicityMostly Toxic
FoliageDeciduous
FlowersShowy, two-lipped flowers
When To SowApril, May
Flowering MonthsJuly, August, September
When To PruneSeptember, October
Sunlight

Preferred
Full Sun or Partial Shade

Exposure
Exposed or Sheltered

Size

Height
0.5 – 1M

Spread
0.1 – 0.5M

Bloom Time
July – September

Soil

Preferred
Loam

Moisture
Moist to poorly drained

pH
Any

One may think of Lobelia as a superlative bedding plant and surely it is – but there is much more to this genus.

Boasting plants in numerous shapes and sizes, it is remarkable for the ‘variety’ in its varieties.

This characteristic, in turn, provides a Lobelia ‘customised’ for virtually every garden purpose, with those delightful five-lobed blooms occurring in hues both pastel and brilliant.

Diverse, Versatile and Varied

Lobelia is an astonishingly diverse genus of herbaceous flowering plants. This genus numbers from 420 to 440 species and many more cultivars.

These include annuals, biennials, tender perennials, and hardy perennials.

Their habits are just as diverse and include upright, bushy, clump-forming and trailing. As for the flowers, they occur in solitary form, in panicles, or in racemes. 

These differences extend to presentation as the respective habit and blooms of some varieties combine to exude delicacy and refinement, for example ‘Waterfall White Sparkle,’ while those of other varieties present a merry and robust appearance, for example ‘Hadspen Purple.’

Members of this genus, however, do have a few unifying morphological characteristics. First, the leaves are simple and alternate.

Second, the flowers have five lobes (‘petals’) with the upper lip comprised of two lobes and the lower lip of three.

Genus Lobelia is classified under Family Campanulaceae which has many bell-like flowers.

Besides the word ‘diverse’ one may use the word ‘versatile’ to describe this genus. Because of the range in the varieties’ habits and the variety in the flowers’ hues, there’s a Lobelia to suit virtually every garden need though it must be said that many Lobelias are ideally suited for beds.

This is because most varieties’ heights fall between 12 and 80 centimetres, many bear brilliantly-coloured blooms, even more have prolonged blooming seasons, and almost all are floriferous and self-cleaning – no deadheading required. That’s quite a few ticks for Lobelia.

a flower bed of Lobelia erinus in various shades of purple
Floriferous Lobelia’s Characteristics Make Many Varieties Ideally Suited for Beds

So Varied it’s Both Curative and Toxic

Lobelia as a genus is both widespread and widely varied; at the same time it is very popular and widely cultivated.

The upshot is that it includes species that are little-known and rare, and grow in remote locations, and it also boasts cultivars that are available in many a neighbourhood nursery and seen in equally many gardens; for example, Lobelia erinus cultivars and Lobelia cardinalis.

Both kinds are easy to grow from seed and have the advantages of being pest-resistant and disease-free.

The variations within this genus are such that they give rise to an apparent contradiction: Lobelia plants are both toxic and medicinal.

Each of several species is actually classified as a toxic herb and a medicinal herb with Lobelia inflata at the top of the list. The compound at the heart of Lobelia inflata’s powers is the alkaloid Lobeline, though it is present in the plant in only trace amounts. 

Ingesting even a small amount of any part of the plant or even failing to wash hands after handling some Lobelias may cause adverse effects such as sweating, tremors, diarrhoea and vomiting.

The good news is that ample first-person reports indicate that Lobelia formulations control or cure asthma and bronchitis. Naturopaths, including indigenous peoples, have used Lobelia for centuries for further medicinal purposes.

Regardless of whether you are wary of toxic plants or whether you are an old-line herbalist, one thing all of us can agree on is that the many different plants and blooms of this astonishingly varied genus also invite a similarly widely varied list of adjectives: gorgeous, delightful, soothing, vibrant, cool, decorative…

garden flower pots with lobelia and daisies
Lobelias, Among Other Flowers, Make Wonderful Container Plants

Background and Origins

Genus Lobelia is nearly Cosmopolitan, being found in all climatic zones except polar and all biomes except tundra. Lobelia species are native to all regions except Greenland, Alaska, Northern Africa, and a swath of Central Asia and South Asia but have been introduced in many of those regions too!

From these facts one may infer that Lobelia has been evolving and diversifying since a prehistoric period and such an inference would be correct, for Lobelia diversification scientifically has been traced from 35 million years ago. Obviously, therefore, Lobelias were already existing and diversifying at that time.

Fast-forwarding (by many million years!) to our time, Lobelias such as Lobelia inflata and Lobelia siphilitica were valued by America’s indigenous peoples, and thereafter, also by the early colonists. Lobelia inflata was used as a stimulant, narcotic, and a curative for various maladies, particularly pulmonary and respiratory ones, with considerable success. Lobelia siphilitica? Not so much!

Varieties

It is hardly possible to do justice in one section of a growing guide to the varieties of a genus that is both as large and as varied as Lobelia. We list a few essential species and outline some popular cultivars.

As mentioned above, Lobelia habits run the gamut and plant heights and spreads are similarly varied.

Some trailing varieties, such as Lobelia richardsonii hort., grow to only about 10 centimetres while some upright varieties, such as Lobelia × speciosa ‘Pink Elephant’, can attain a height of 1.5 metres. Admittedly, these are the extremes but that is also the range – which is a multiple of 15.

Lobelia X speciosa varieties nearly always have a clump-forming, upright habit while Lobelia erinus varieties nearly always have a bushy, trailing habit. The former are cold-hardy whereas the latter are frost-tender.

The majority of popular Lobelia varieties’ flowers occur in hues of purple and blue. Reds and pinks are also found with white somewhat less common.

magenta pink and white lobelia flowers in a hanging basket
Lobelias are Mostly Blue and Purple, Some are Red and Pink, and a Few are White

L. cardinalis or ‘Cardinal Flower’ is a species that is both very hardy and also heat tolerant. It is native to the high plains of the United States. It is a very tall species with an upright habit and bears big flowers on spikes. They are the deep, intense red hue of a Catholic cardinal’s robe, hence the name. They produce blooms well into autumn.

a 1991 postage stamp from Hungary
Hungarians Seem to Appreciate an American Lobelia Species!

L. erinus is a frost-tender species that is native to central and southern Africa. It is a short, almost a dwarf, species. Though its varieties come in both upright and trailing habits, the latter are better known and more popular. It is also called ‘Fan Lobelia’ because of the resemblance of its flower’s lower lip to a fan.

L. inflata or ‘Indian Tobacco’ is native to the plains of the United States and is famed for its medicinal value and also feared for its toxicity. It produces small, nondescript greyish-lilac flowers with a yellow centre. The roots are wholly poisonous though the foliage was used for tobacco by the Plains Indians. The main value of this species is that it is used in Alternative Medicine to treat a variety of disorders and is also used as a relaxant. Allopathic Medical science has long harboured an interest in it.

L. × speciosa is a hybrid of three species, including aforementioned L. cardinalis and L. syphilitica. It is cold hardy to Zone H5. It grows to about one metre and is clump-forming with an upright habit. It and its cultivars’ foliage differ in colour from other Lobelias, being dark with bronze or purple tints or flushes in autumn; as such, it is also valued for its foliage. Flowers come in all colours.

pink upright flowers of Lobelia x speciosa
L. × speciosa Flowers in Lipstick Red

L. Laguna Compact Blue With Eye grows to 25 to 30 centimetres with a similar spread. It has a semi-upright, bushy habit. It is very floriferous and flowers from summer to mid-autumn. It bears lilac-blue flowers with a prominent white eye. It is ideally suited for borders and also for containers.

L. erinus ‘Crystal Palace’ is a dwarf variety that reaches only 10 centimetres and has a bushy habit. This popular variety is a tender annual. Both foliage and flowers are striking: the leaves are of a deep bronze green shade and the blooms are of a deep, intense hue of blue, crossing over into purple. RHS Award of Garden Merit.

L. erinus ‘Laguna White’ is one of those ‘classic’ cascading Lobelia varieties that one may see tumbling out of a basket. It has a trailing habit and attains a height from 25 to 30 centimetres with a spread twice as big. Unusually for Lobelias, this one is heat tolerant, at least more so than most. It produces masses of pure white flowers.

L. × speciosa ‘Monet Moment’ is quite tall at 80 centimetres and has a clump-forming, upright habit. It has a relatively short flowering season that starts in late summer. This somewhat under-rated variety has a robust yet beautiful appearance as spikes hold up spires of brilliant magenta-pink blooms.

L. × speciosa ‘Hadspen Purple’ grows to 50 to 60 centimetres and has an upright habit which attributes combine to make it very suitable for different garden needs. The alternate leaves are of a rich green shade and look particularly neat on this variety. The spikes bear racemes of brilliant purple-to-violet flowers. It is one of the most popular varieties and is easy to find.

stalks and flowers of Lobelia x speciosa "Hadspen Purple"
The Erect Stalks and Brilliant Purple Bloooms of ‘Hadspen Purple’

L. cardinalis ‘Black Truffle’ (or L. cardinalis ‘Chocolate Truffle’) is a spanking new American sport of Lobelia cardinalis. It inherits its flowers and habit from the parent species but the foliage provides a fresh twist. As leaves sprout and form they are nearly black, maturing into a deep, rich chocolate-maroon shade with the lamina exhibiting a distinct sheen.

Habitat and Growing Conditions

As a very diverse genus whose species are native to numerous regions of the world, Lobelias’ habitats have a lot of variation.

As a general rule, many Lobelia species in the wild grow in or near marshes and near waterways. They are found in most any sun-shade mix. 

Rich and moist soils suit most Lobelias very well, and they are not put off by heavy soils. However, many species also grow in sandy soils.

When one considers that Lobelia linarioides is native to the temperate and cool southern tip of South Africa while Lobelia oxyphylla is native to the tropical heat of (the lowlands of) Cuba, one can readily conclude that it is difficult to peg down such a varied genus’s growing conditions.

lobelia growing out of a stone plant pot
Lobelias Lend Themselves to Many Garden Ideas and Purposes

Where to Plant Lobelia

Lobelia is one of those genera that is so extensive and so widely varied that where a Lobelia is best suited to is entirely dependent on the particular variety, to which the flip side is that for almost every garden need there is a Lobelia plant.

If one Lobelia is ideal for a rock garden, another is equally well-suited to a mass planting in a bed. One variety may be great for a pot on the parapet; another one perfect for a basket hanging by the door.

But do keep in mind that that first variety may look seriously weird in a hanging basket and the second one may look dumpy and awkward in a pot on the parapet!

You will need to take into account the habit, height, spread, and the bloom and its hue to determine the optimal setting, or at least a very good setting. Here is one good option for each of the four garden needs mentioned above.

  • Rock Garden: Lobelia erinus ‘Crystal Palace’
  • Mass Planting: Lobelia × speciosa ‘Monet Moment’
  • Pot on Parapet: Lobelia × speciosa ‘Hadspen Purple’
  • Hanging basket: Lobelia erinus ‘Laguna White’

As it happens, this selection throws up a very nice assortment of colours: lilac-blue, magenta-pink, intense purple, and pure white.

blue edging lobelia and butterfly stone crop plants in hanging window planters
Lobelia Laguna Series Varieties are Perfect for Hanging Baskets

Feeding, Care and Growing Tips

As we know, Lobelia is an incredibly diverse genus with species originating in and native to widely varied regions. As a consequence, it is to be expected that the ideal growing conditions and care will also be quite variable.

Therefore, if you obtain an unusual or exotic Lobelia species you will need to ask specialists or do some research as to the growing conditions and care that it requires.

However, for the vast majority of the common species and popular varieties, such as nearly all the ones listed in section Varieties, the following tips will work out very well.

With respect to the popular varieties that are widely available and commonly grown, the fundamental difference is that Lobelia erinus cultivars are ‘hardy’ only to Zone H2 whereas Lobelia X speciosa cultivars are hardy down to Zone H5.

The upshot is that Lobelia erinus cultivars are grown and treated as annuals in British gardens unless you overwinter them in a sunny and warm spot indoors or in the greenhouse.

In strong contrast, Lobelia speciosa cultivars are treated in the United Kingdom for what they are: deciduous perennials.

Lobelia X speciosa varieties are often propagated from cuttings or bought as potted plants but are also grown from mixed seeds. Lobelia erinus varieties are typically grown from seeds.

Lobelia seeds are unusually small and so much so that some seed producers pack several seeds into a ‘pellet,’ especially in the United States. If you see a packet advertising Lobelia ‘pellets,’ keep in mind that one pellet will germinate into multiple seedlings. 

Start the seeds indoors about ten weeks before your region’s last projected frost date with a view to transplanting the plants about four weeks after the last frost.

Seeds can be started in small pots though Lobelias are very well suited to germination in growing trays. Put seed starting mix in the compartments and simple scatter seeds from above. Pat them on the soil, and firm up the soil, but do not cover the seeds. 

purple and white lobelia flowers
A Type of Purple Lobelia With Eye – a Brilliant and Eye-Catching Flower

Temperature and light play a big part in the germination of Lobelia seeds. The soil temperature will need to be kept between 16° and 20° centigrade. The seeds should get ample sunlight, otherwise set up a grow light over the tray. Keep the soil consistently moist but not wet. 

Seeds will germinate in about a fortnight. During the following four weeks you may thin them.

After the last frost, harden the seedlings by setting them outside for progressively longer periods during the day. Do so for a week to ten days. Then transplant them outdoors. Be mindful of the habit and spread of the variety and space accordingly.

A very good soil for Lobelias is a fertile loam of sand, chalk, and clay with some organic manure. The soil should drain very well though it should be kept almost consistently moist to get the best out of these denizens of marsh and bog.

Though the soil pH may vary from Moderately Acidic to Slightly Alkaline, a pH between 5.5 and 6.5 is ideal.

Most Lobelias do not withstand the dog days of summer very well; in hot sun they tend to wilt and lose their flowering energies. Therefore, in sunnier and warmer regions of the UK it is best to site them where they get morning sun and afternoon shade. In other regions they will do well in full sun. 

Water moderately but regularly. The key is to keep the soil consistently moist.

These plants are known to be heavy feeders. Feed them with a balanced liquid fertiliser about every 20 days until about early July and then switch to a liquid 0-10-10 fertiliser, feeding at the same frequency through the season to keep these plants growing and glowing.

As summer wears on and if the weather gets hot, many Lobelia varieties will start to flop and look as if they might give up the ghost.

If this happens you may cut back the plant by one-third to a half but continue watering and feeding as before. When the temperature decreases in autumn the plant will revive and will reward you with fresh blooms.

tall Lobelia Monet Moment plants in a rockery
L. × speciosa ‘Monet Moment’ is Tall and Robust with Brilliantly-Coloured Blooms

Common Diseases and Problems

Lobelias are remarkably disease-free and pest-resistant. 

On occasion some Lobelia × speciosa cultivars succumb to crown rot. If the disease is not spotted and treated early, which it seldom is, the plant needs to be removed and the soil treated and sanitised.

Crown rot can be avoided by ensuring that the soil is not overly heavy, drains well, and does not stay wet. Also, it is better to keep the soil line a little too low than a little too high.

Many Lobelias are subject to attack by slugs. They are less of a garden pest and more of a garden nuisance, and not very difficult to get rid of.

Lobelia erinus 'Crystal Palace'
L. erinus ‘Crystal Palace’ is Well Suited for Rockeries

Where to Buy Lobelia

You will find at least a few potted Lobelias in spring at most garden centres. Many of the popular varieties are seasonal favourites.

Seeds too are widely available, both at nurseries and online retailers.

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