Calibrachoa

Free-flowering and weather tolerant, this genus is ideal for baskets and containers, says Graham Clarke.

Calibrachoa Privileged 'Light Yellow' - photo: S&G Flowers
Calibrachoa Privileged 'Light Yellow' - photo: S&G Flowers

The genus Calibrachoa has not been a part of our industry for long. The first plants were taxonomically described in 1989 and the first hybrid cultivars released in 1992. But interest in the genus, and its free-flowering habit and weather tolerance, have quickly made it a popular choice for hanging baskets and containers. It has become a major crop.

The original plants were found in coastal areas clinging to rocks and surviving in relatively harsh conditions. They so much resembled petunias that they were originally included in that genus, and were referred to as "seashore petunias". But in the intervening years the woody nature of Calibrachoa, as well as slight differences to the flower buds, seed coats and chromosome number, have been sufficient to separate it from Petunia.

A new strain of Petubrachoa (Petunia x Calibrachoa) was launched this year bringing, among other attributes, larger flowers than those found on the traditional Calibrachoa.

The family tree of this genus reaches deep down into Argentina, Brazil or Uruguay. There are some 25 species, and although we have had less than two decades of breeding work, considerable achievements have been made. Even so, breeding was hampered by the fact that not all of the species were compatible with each other. Elements of disease, heat and drought tolerance, which are clearly found in the original species, have been retained, and desirable production characteristics (including floriferousness, growth habit and the potential of being day-length neutral) have all been achieved, making for a successful series of market releases.

Plants that are raised differ mainly in their habit, being either low-growing and trailing or weakly upright. The flowers, up to 3cm across, are borne profusely in the upper leaf axils from late spring to late autumn. Calibrachoa plants are usually grown as annuals in hanging baskets and containers, but are perennial in frost-free climates.

Viruses can be a problem with these plants, including the Calibrachoa mottle virus, which causes mottled, blotched leaves. As there is no cure (meaning that the plants should be destroyed), it is crucial that virus-free stock is obtained. Root diseases are also common, and most of the recently introduced strains have a reasonable degree of resistance.

In the 2003 RHS trial of Calibrachoa it was found that many of the plants (particularly those with yellow flowers) were subject to yellowing of the foliage. This was thought to be due to the neutral pH (7) of the compost. Most Calibrachoa (as Petunia) grow best at approximately pH5, with an extra feed of iron.

WHAT THE SPECIALISTS SAY

Bruce Harnett, managing director, Kernock Park Plants, Cornwall "We have certainly seen an increase in the popularity of these plants in the past two years. Gardeners are choosing the strong colours and plants with good habit. But from the point of view of Proven Winners (US-based plant breeder, for which Kernock Park Plants is the UK agent), we are also looking for plants that show a good resistance to the root diseases to which Calibrachoa is generally prone. Extensive trials have identified the types that are vulnerable to fungal problems like Thielaviopsis, Pythium and Phytophthora, and the grower fraternity is beginning to get there in terms of resistance.

"We have concentrated our efforts on the Superbells strain, which has a particularly nice habit and really strong colours. The growth is prostrate, which gives them a particular stature when planted alone or in combinations. The most popular forms are 'Red Devil', 'Magenta', 'Yellow' and 'Amarena'.

"We are also offering Petubrachoa, a cross between Petunia and Calibrachoa, which we're marketing under the SuperCal name."

Stuart Lowen, marketing manager, Ball Colegrave, Oxfordshire "Calibrachoa is susceptible to several viruses, so it is vital to begin with cuttings from clean stock. Plants are susceptible to root diseases if over-watered, so growers should allow the media to dry slightly between watering.

"The pH is also important. Use a well-drained, disease-free, soil-less medium with a pH of 5.2 to 4.8. A pH of 5.5 to 5.8 should be maintained throughout production. Monitor plants for early signs of high pH (yellowing on youngest leaves).

"Calibrachoa requires heavy fertilisation. Use constant feed with a balanced fertiliser, and an additional iron feed as needed. A full complement of minor elements should also be provided.

"Also, keep light intensities high in spring, as low light levels can cause stem stretch and poor flowering. They flower best under long days in spring and summer."

IN PRACTICE

Louise Burks, director, Castle Gardens, Dorset "We sell around 3,000 pots of Calibrachoa, or Million Bells as we still refer to it, in a range of colours. In total we sell between 20,000 and 30,000 hanging-basket plants, so Calibrachoa represents some 10 per cent.

"In February each year we will get early plants from our local supplier (North Wing) and plant up baskets of single colours. These are then in flower by the main selling period in April and May, and we hang the baskets as living labels over the benches. This is the best way to show customers how the plants can look, and has proved to be very successful."

SPECIES AND CULTIVARS

- Calibrachoa Cabaret series features well-branched, free-flowering plants with a mounded trailing habit. Foliage stays green in high-pH conditions.

- 'Apricot' has terracotta-orange flowers.

- 'Pink' has pale pink flowers with a yellow centre.

- 'Purple' carries rich purple flowers, each with a yellow centre.

- 'Red' has bright, pillarbox red flowers.

- 'Yellow' has golden yellow flowers.

- Calibrachoa Callie series offers good outdoor performance and uniform habit. Recently introduced varieties include 'Painted Coral', 'Purple' and 'Purple Sunrise'.

- Calibrachoa Million Bells series features a wide range of cultivars.

- 'Cherry' has red flowers on a semi-upright bushy habit.

- 'Cherry Red' is deep crimson.

- 'Cosmos Pink' has pink flowers with a deeper-coloured throat.

- 'Crackling Fire' produces red/yellow flame-coloured flowers.

- 'Lemon' produces lemon-yellow flowers on a semi-upright plant.

- 'Pink Morn' has a trailing habit and pink flowers with a white throat.

- 'Red' has red flowers on a semi-upright bushy and strong habit.

- 'Salmon' has coral-pink flowers.

- 'Salsa' has bright pink flowers with a darker throat.

- 'Star Blue' is violet blue with deeper veining to the petals.

- 'Terracotta' is apricot-gold on a semi-upright bushy and strong habit.

- 'Trailing Blue' is indigo blue with a yellow centre and a trailing habit.

- 'Trailing Ice' has frosty-white flowers.

- 'Trailing Lavender Vein' has pale lavender flowers with deeper veining.

- 'Trailing Pink' produces pink flowers.

- 'White' has flowers of pure white and an upright habit.

- Calibrachoa Minifamous series includes 'Peach', 'Orange Red' and 'Yellow', as well as 'Light Pink Eye', 'White + Eye' and 'Yellow Lilac Star'.

- Calibrachoa Noa series features 'Orange Eye', 'Yellow', 'Peach Eye' and 'Ultra Purple'.

- Calibrachoa Privileged series contains dense, compact plants, all with a branching, trailing habit.

- 'Light Yellow' is a pale, lemon yellow.

- 'Trailing Peach Melba' is pink/yellow with a darker centre.

- 'Trailing White with Pink Ring' is white with a rose-pink eye.

- 'Cherry Red' has bright red flowers.

- 'Dark Blue' is a deep, violet-blue.

- 'Light Blue' is a light blue with a mosaic pattern to the petals.

- 'Trailing Antique Rose' carries flowers of deep, almost magenta pink.

- 'Trailing Blue Purple' has flowers of blue/violet.

- 'Trailing Electric Burgundy' is bright purple.

- 'Trailing Mid Blue' produces flowers of a medium blue.

- 'Trailing Rose' has rose-pink flowers.

- Calibrachoa Superbells series offers a range of colours.

- 'Amarena' is a peachy yellow with a smudge of pink around the yellow eye.

- 'Candy White' is white with pink blushes surrounding the yellow eye.

- 'Cream' is compact, with large cream flowers.

- 'Imperial Purple' is a trailing variety with marbled plum-purple flowers.

- 'Indigo' is deep purple-indigo with a yellow eye.

- 'Magenta' has cerise pink flowers with a yellow eye; very floriferous.

- 'Orange' has mandarin orange flowers.

- 'Pink' Award of Garden Merit has a cascading habit, with pastel-pink flowers and good weather resistance.

- 'Red Devil' has dark cherry-red flowers on compact dome-forming foliage.

- 'Royal Blue' has flowers of royal blue, each with a yellow centre.

- 'Strawberry Pink' is of a darker pink towards the centre of the flower, flushing to a lighter pink at the edges.

- 'White' is a clear white on a mound-forming, slightly trailing habit.

- Petubrachoa SuperCal series is the first mass-produced hybrid between Petunia and Calibrachoa, with a limited introduction so far.

- 'Neon Rose' has pink flowers with a yellow eye, and deep green foliage.

- 'Terracotta' carries apricot flowers with hints of peach and rose blush.

- 'Velvet' produces velvet-like purple flowers, each with a darker eye, on vigorous trailing stems.


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