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.)