Blue flowers can be particularly prized in summer, where their cool nature contrasts with summer heat (if we are lucky). People’s appreciation of blue differs, with some being purists and others nonchalant about a little lavender, purple or violet in their “blue” flowers.
Asking the Royal Horticultural Society’s (RHS) Find-a-Plant facility on its website for blue plants brings up many bluish shades. These are precise colour definitions done by RHS botanists comparing flowers with defined colour panels under a north-facing window.
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Over twice as many “purple” plants are recorded in the Find-a-Plant database as “blue” plants. Many of the so-called blue flowers are described as purple-blue, violet-blue or silver-blue. While blue shades are probably involved in attracting pollinators, there seems to be no advantage in pure blue flowers compared with other bluish hues.
Blue plant pigment molecules are anthocyanins, which may be red, orange, purple or blue depending on the acidity or alkalinity of the plant cells or in combinations with metals. In hydrangeas, the anthocyanin binds with aluminium for blue flowers, with aluminium only being available to plants, and therefore blue flowers, in acid soils. Another example is the morning glory, whose buds open pink, turning blue when fully open as the cells become more acidic.
Cornflowers, easily grown in a few weeks from seed, derive their bright blue colour from a red anthocyanin pigment molecule, linked to a flavone combined with iron, magnesium and calcium.
The need for precise flower cell conditions to express pure blue may explain their relative scarcity.
Delphiniums contain the blue to purple anthocyanin delphinidin, also found in other blue or purple flowers and fruits, blueberries for example. Delphinium Blue Nile (1.5m tall), with semi-double white-eyed medium blue flowers, is one of the best and is propagated by cuttings taken in spring. Seed-raised blue delphiniums include Magic Fountains Dark Blue and Summer Skies (pale blue), both tall plants. Delphiniums thrive in good garden soil that is not drought-prone.
Salvia patens bring a lovely shade of blue to gardens in summer (Photo: RHS/Leigh Hunt)Other blue flowers suit dry soils, although they will need careful watering in their first year or if grown in pots. Eryngiums – or sea hollies – include the fabulously blue cone-shaped flowers of Eryngium × oliverianum Big Blue and the more thistle-like Eryngium bourgatii Picos Blue. Both have spiky foliage and attain 70cm.
Notably, blue agapanthus includes dark blue Midnight Star and Loch Hope, both 1-1.5m and hardy – although a covering of mulch for border examples and extra protection for potted ones in cold regions is wise. With strap-like leaves, these too need little watering.
Nepeta or catmint – beloved by cats and bees – includes some convincing blues; Nepeta grandiflora Bramdean with hairy leaves and grey-green scented leaved Nepeta × faassenii Blue Wonder.
Salvia patens have the clearest blue flowers and are easily raised from seed, but are tender and need winter protection. Salvia guaranitica Blue Enigma is another deep blue salvia. For a hardier blue, consider Salvia uliginosa (to 2m). Rosemary is now classified as Salvia, and has clear blue flowers – Miss Jessopp’s Upright is a fine example.
Caryopteris adds late summer blues, C × clandonensis Heavenly Blue being very widely sold, but yellow-leaved forms are deservedly popular – yellow foliage contrasts well with the blue flowers – Caryopteris × clandonensis Hint of Gold is a good example.
Again, this plant is very good for insects and tolerant of dry soil.
From September, Chinese plumbago, including Ceratostigma willmottianum Forest Blue (60cm), and groundcover Ceratostigma plumbaginoides have clear blue insect-friendly flowers just right for crisp blue autumn skies.
The RHS is a charity inspiring everyone to grow via its research, advisory, outreach, shows and gardens. For more information, visit: rhs.org.uk.
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