Diascia (plant)

''Diascia'' is a genus of around 70 species of herbaceous annual and perennial flowering plants of the family Scrophulariaceae, native to southern Africa, including South Africa, Lesotho and neighbouring areas.

The perennial species are found mainly in summer-rainfall areas such as the KwaZulu-Natal Drakensberg; about 50 species, mostly annuals, are found in the Western Cape and Namaqualand, winter rainfall areas.

Their common name is twinspur, in reference to the two (usually downward-pointing) spurs to be found on the back of the flower. These help to distinguish them from the similar (and closely related) genera ''Alonsoa'' and '' Nemesia''. The spurs contain a special oil, which is collected in the wild by bees of the genus ''Rediviva'' (e.g. ''R. longimanus'') that appear to have coevolved with the plants, as they have unusually long forelegs for collecting the oil.Google Books:
Thompson, John N., ''The Coevolutionary Process''

In gardens, ''Diascia'' cultivars (mostly hybrids) have become extremely popular as colourful, floriferous, easily grown bedding plants in recent years.Horticulture Week:
''Diascia'', by Graham Clarke

Etymology

Surprisingly, the generic name (from the Greek (language), Greek ''di'' = two and ''askos'' = bag, pouch or sack) does not refer to the spurs, but to the two translucent sacs, or pouches, known as 'windows', found in the upper part of the corolla (flower), corolla of the original Type species#In botany, type specimen, ''Diascia bergiana''. ''Diascia'' species in the section ''Racemosae'' have similar windows, but in some species they merge into one. The windows may help oil-collecting bees to find the correct position within the corolla when gathering oil from the glands within the spurs.

Description

Most diascia species are short-growing, straggling plants, reaching no more than in height, although ''Diascia rigescens'' can reach , and the rather similar ''Diascia personata, D. personata'' (with which it is often confused) up to or so. Some ''Diascia'' species spread by means of stolons, while others produce multiple lax stems from a single crown. The flowers are borne in loose terminal racemes. The corolla is five-lobed, and normally pink or rose-coloured in the perennial species most commonly seen in cultivation. Dark purplish patches of oil glands may make the flowers of some species appear bicoloured.

Species

Around 60–70 species are currently recognised in the genus ''Diascia'':

*''Diascia aliciae'' Hiern
*''Diascia alonzoides'' Benth.
*''Diascia anastrepta'' Hilliard & B.L.Burtt
*''Diascia ausana'' Dinter
*''Diascia austromontana'' K.E.Steiner
*''Diascia barberae'' Hook.f.
*''Diascia batteniana'' K.E.Steiner
*''Diascia bergiana'' Link & Otto
*''Diascia bicolor'' K.E.Steiner
*''Diascia capensis'' (L.) Britten
*''Diascia capsularis'' Benth.
*''Diascia cardiosepala'' Hiern
*''Diascia cordata'' N.E.Br.
*''Diascia cuneata'' E.Mey. ''ex'' Benth.
*''Diascia decipiens'' K.E.Steiner
*''Diascia dielsiana'' Schltr. ''ex'' Hiern
*''Diascia diffusa'' Benth.
*''Diascia dissecta'' Hiern
*''Diascia dissimulans'' Hilliard & B.L.Burtt
*''Diascia ellaphieae'' K.E.Steiner
*''Diascia elongata'' Benth.
*''Diascia engleri'' Diels
*''Diascia esterhuyseniae'' K.E.Steiner
*''Diascia fetcaniensis'' Hilliard & B.L.Burtt
*''Diascia fragrans'' K.E.Steiner
*''Diascia glandulosa'' E.Phillips
*''Diascia gracilis'' Schltr.
*''Diascia heterandra'' Benth.
*''Diascia hexensis'' K.E.Steiner
*''Diascia humilis'' K.E.Steiner
*''Diascia insignis'' K.E.Steiner
*''Diascia integerrima'' E.Mey. ''ex'' Benth.
*''Diascia lewisiae'' K.E.Steiner
*''Diascia lilacina'' Hilliard & B.L.Burtt
*''Diascia longicornis'' (Thunb.) Druce
*''Diascia macrophylla'' (Thunb.) Spreng.
*''Diascia maculata'' K.E.Steiner
*''Diascia megathura'' Hilliard & B.L.Burtt
*''Diascia minutiflora'' Hiern
*''Diascia mollis'' Hilliard & B.L.Burtt
*''Diascia namaquensis'' Hiern
*''Diascia nana'' Diels
*''Diascia nodosa'' K.E.Steiner
*''Diascia nutans'' Diels
*''Diascia pachyceras'' E.Mey. ''ex'' Benth.
*''Diascia parviflora'' Benth.
*''Diascia patens'' (Thunb.) Grant ''ex'' Fourc.
*''Diascia pentheri'' Schltr.
*''Diascia personata'' Hilliard & B.L.Burtt
*''Diascia purpurea'' N.E.Br.
*''Diascia racemulosa'' Benth.
*''Diascia ramosa'' Scott-Elliot
*''Diascia rigescens'' E.Mey. ''ex'' Benth.
*''Diascia rudolphii'' Hiern
*''Diascia runcinata'' E.Mey. ''ex'' Benth.
*''Diascia sacculata'' Benth.
*''Diascia scullyi'' Hiern
*''Diascia stachyoides'' Schltr. ''ex'' Hiern
*''Diascia stricta'' Hilliard & B.L.Burtt
*''Diascia tanyceras'' E.Mey. ''ex'' Benth.
*''Diascia tugelensis'' Hilliard & B.L.Burtt
*''Diascia unilabiata'' (Thunb.) Benth.
*''Diascia veronicoides'' Schltr.
*''Diascia vigilis'' Hilliard & B.L.Burtt

Coevolution with ''Rediviva'' bees

The two spurs found on the back of a ''Diascia'' flower (from which it gets the common name twinspur) contain a special oil, which is collected in the wild by at least 8 species of bees of the genus ''Rediviva''. The bees appear to have coevolution, coevolved with the plants, as the females have developed unusually long, hairy forelegs with which they collect the oil from ''Diascia'' spurs to feed their larvae (and sometimes to line their nests with). The spurs vary in average length from to as much as , mainly between species (although those of ''Diascia capsularis, D. capsularis'' can vary widely between populations); the bees' forelegs vary similarly. The spurs of ''Diascia longicornis'' are about in length, but the existence of a suitably equipped pollinator, ''Rediviva emdeorum'', with forelegs of the same length, was only confirmed in the 1980s. ''Rediviva longimanus'' has also been observed pollinating ''D. longicornis'' in the Western Cape.

''Rediviva neliana'', a widespread species, collects from at least 12 species of ''Diascia'', but in general, few different ''Diascia'' species grow together in the same locality. As a result, local populations of ''R. neliana'' have been found to differ from each other, as each has developed legs that match the spur length of the diascias that are available to them in that locality. This indicates that local populations of ''R. neliana'' are coevolving with the flowers on which they depend.

Garden uses

''Diascia'' cultivars have become extremely popular worldwide as bedding plants, suitable for hanging baskets, window boxes and other containers, as well as rockeries and the fronts of herbaceous borders. This explosion of interest is largely thanks to the breeding work done by the late Hector Harrison of Appleby, North Lincolnshire, England. From 1985, he raised hundreds of hybrid seedlings, from which several excellent cultivars have been selected and named. He increased the colour range to include shades of apricot, pink, coral, lilac, red and white. Other nurseries and breeders have continued to build on his pioneering work.

Several species and cultivars have been given the Award of Garden Merit by the British Royal Horticultural Society. The AGM includes a Hardiness (plants), hardiness rating: most have been rated as intermediate between H3 (hardy outside in some regions or particular situations or which, while usually grown outside in summer, needs frost-free protection in winter – e.g. dahlias) and H4 (hardy throughout the British Isles).

Cultivars

The species and cultivars commonly grown in gardens include the following (those awarded the Royal Horticultural Society's Award of Garden Merit are marked ):

*'Appleby Apricot'
*''D. barberae'':
**'Belmore Beauty'
**'Blackthorn Apricot' H3-4
**'Fisher's Flora'
**'Ruby Field' H3–4
*'Blue Bonnet'
*='Hecbel' H3–4
*'Dark Eyes'
*'Elizabeth'
*''D. fetcaniensis''
*Flying Colours Series
**'Flying Colours Appleblossom'
**'Flying Colours Apricot'
**'Flying Colours Coral'
**'Flying Colours Red'
*'Ice Cracker'
*'Ice Cream'
*''D. integerrima''
*'Jacqueline's Joy'
*'Joyce's Choice' H3–4
*'Katherine Sharman'
*'Lady Valerie' H3–4
*'Lilac Belle' H3–4
*'Lilac Mist' H3–4
*'Little Dancer'
*'Little Tango'
*'Pink Panther'
*'Red Ace'
*'Redstart'
*''D. rigescens''
*'Rupert Lambert' H3–4
*'Salmon Supreme'
*'Twinkle'
*''D. vigilis''
*Whisper Series
**'Whisper Apricot Improved'
**'Whisper Cranberry Red'
**'Whisper Pumpkin'
**'Whisper Tangerine'
**'Whisper White'
*Wink Series
**'Wink Garnet'

References

External links

PlantZAfrica.com ''Diascia integerrima''



A UK National Plant Collection holder's website

{{Taxonbar, from=Q2568982

Scrophulariaceae
Scrophulariaceae genera