We have found 15 plants that meet your criteria.

African lily
Agapanthus africanus African lily Wonderful globes of blue flowers from midsummer £8.99 Buy
African lily
Agapanthus Queen Mum ('Pmn06') African lily Big bi-coloured flowers £12.99 Buy
African lily
Agapanthus 'Taw Valley' African lily A spectacular summer-flowering perennial £6.99 Buy
sea holly
Eryngium agavifolium sea holly Striking foliage £7.99 Buy
New
tall bearded iris
Iris 'Best Bet' tall bearded iris Tall and elegant £9.99 Buy
Russian sage
Perovskia 'Blue Spire' Russian sage Violet-blue flowers and silvery foliage. Great planted en masse £9.99 Buy
Chinese plumbago
Ceratostigma willmottianum Forest Blue ('Lice') (PBR) Chinese plumbago Foliage turns wonderful autumnal... £9.99 Buy
tree hollyhock (syn. Blue Bird)
Hibiscus syriacus 'Oiseau Bleu' tree hollyhock (syn. Blue Bird)... Late flowering shrub with large blue flowers £12.99 Buy
African lily
Agapanthus 'Northern Star' (PBR) African lily One of the hardier agapanthus £9.99 coming soon
African lily
Agapanthus 'Silver Baby' African lily A beautiful dwarf variety £9.99 coming soon
African lily
Agapanthus 'Taw Valley' African lily Deep blue flowerheads £9.99 coming soon
west country lupin
Lupinus 'Persian Slipper' (PBR) west country lupin Pretty lagoon blue blooms £12.99 coming soon
Russian sage
Perovskia atriplicifolia 'Little Spire' (PBR) Russian sage A shorter form of the popular Russian sage £9.99 coming soon
Spanish sage
Salvia lavandulifolia Spanish sage A great alternative to common sage £7.99 coming soon
African lily
Agapanthus 'Castle of Mey' African lily A tall, rich blue form £9.99 coming soon