Salvia uliginosa
Clear sky blue is scarce in a summer border and bog sage supplies it on stems fine enough to move in any breeze. Salvia uliginosa carries short spikes of small flowers, each one a bright pale blue with a white throat, at the tips of tall wiry branching stems above narrow toothed green leaves. Flowering runs December to May, and the plant dies back over winter.
The height and the see-through habit make it useful for weaving colour through the middle and back of a planting rather than blocking it. Use it among grasses, with late perennials, or beside water where the blue reflects. Bees work it constantly, and the stems cut well although they are short lived in a vase.
Full sun and soil with reasonable summer moisture suit it, though despite the name it does not need a bog and grows perfectly well in ordinary garden ground. The thing to know before planting is that it travels: it spreads by underground runners and can cover several metres in a few seasons. Give it space, sink a barrier, or grow it in a large container.
Reaches 1.2 to 2 m tall, spreading steadily at the root, and usually needs support.
Trimming: Cut all stems to the ground in late autumn once they brown off. In spring, put a support ring or a few stakes in early, because the stems are thin for their height and lean badly in wind once they reach full size.
Deadheading: Not needed for flowering, which continues regardless. Removing spent spikes tidies the plant and reduces any seed set.
Fertilising: A light general fertiliser in early spring is enough. Feeding harder makes the stems taller and floppier and speeds up how fast the runners travel, neither of which helps.
Watering: Water through dry summer spells. It holds up better than most salvias in moist ground and is one of the few worth planting at the damp end of a border.
Pest Control: Aphids on the growing tips and slugs on emerging spring shoots. Neither is a lasting problem on a plant this vigorous.
Disease Management: A grey film on the lower leaves in a humid late summer is mildew, and it is worse where the clump is congested. Thin it by lifting runners each spring and keep air moving through the planting.
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