Polygala x Dalmaisiana is a superb choice for conservatories and cool greenhouses with beautiful purple pea like blooms throughout much of the year. These decorative flowers are produced almost continuously along the ends of lax stems of attractive grey green evergreen foliage. Not so much an impressive centre piece perhaps but excellent as the supporting cast flanking shorter lived more flamboyant plants. And a pot or three are good for filling in below tall lanky exhibits.
Coming from South Africa this Polygala is one of many sub-tropical shrubby relations of our native Milkworts and slightly tender so cannot be grown outdoors in most of the UK. But this does not require much warmth just frost-free conditions and not too stagnant a position, damp cold is more of a threat than dry. Although these are small shrubs they seldom become too large. (And even if one does it can be pruned back just before growth re-starts in spring.)
Indeed these are exceedingly easy subjects, not particular to position preferring dappled sun and leaf mould rich loam if you want them to perform well. They need little dead-heading as the spent blooms fall and likewise few pests seem to bother these plants. Thus Polygalas need little attention other than regular watering with rain water, ideally directly to their compost, and a light feed occassionally.
Its not difficult to find this, Polygala x Dalmaisiana, except it is often called by its other name of Polygala myrtifolia grandiflora, and without doubt this is the most reliable of the family. However the Victorian Collectors had up to three dozen other even showier species to enjoy most of which are now seldom seen.
You might search for other South African species such as P. capitata and P. virgata. Another, slightly hardier and more common, European species is P. chamaebuxus which now comes with the same beautiful pea-like flowers in red and yellow or white versions. These are delightful undercover in a cool greenhouse or alpine house or even outdoors on an acidic rock garden. (P. calcarea is another species better suited for a chalky rockery.)
Oh yes, and the common name for Polygala of Milkwort comes from our blue flowered native wildflower, the perennial P. vulgaris, as infusions of this were thought to increase milk flow.