Sea holly, Eryngium x zabelii Jos Eijking. Thistle-like flower heads surrounded by spiny, silvery blue bracts.

Sea holly, Eryngium x zabelii Jos Eijking. Thistle-like flower head surrounded by spiny, silvery blue bracts.

Yellow flowers of Clematis tangutica with fluffy seedheads in foreground against blue sky.

Common Water Hyacinth, Eichornia crassipes. Spikes of pale blue flowers of water plant considered invasive outside native habitat.

Viola cultivar. Group of flowers with delicate cream and pale blue petals and yellow eye at centre.

Viola cultivar with delicate cream and pale blue translucent petals and yellow eye at centre. One faced forwards, one turned away.

Virginia Bluebell, Mertensia virginica with branched stems of clustered, funnel-shaped blue and violet coloured flowers.

Orchid, Blue vanda with pale blue petals marked with checkerboard pattern at the 2011 Orchid Festival in Chiang Mai, Thailand.

Orchid, Blue vanda with pale blue petals marked with checkerboard pattern at the 2011 Orchid Festival in Chiang Mai, Thailand.

Orchid, Blue vanda with pale blue petals marked with checkerboard pattern at the 2011 Orchid Festival in Chiang Mai, Thailand.

Hollyhock, Alcea rosea cultivar. Flower spikes with pink flowers against blue sky. Thailand, Chiang Mai, San Kamphaeng,

Himalayan blue poppy, Meconopsis baileyi. Field of blue poppies with single flower in immediate foreground. Northern Ireland, County Down,