These beautiful perennial plants require cool growing conditions if first class specimens are to be produced, and are well suited to the North West, although it is possible to obtain reasonable plants in other areas by raising them from seed at frequent intervals. They are used most frequently in herbaceous borders, mixed borders, and in front of shrubs. Few flowers are so pure a blue; there are also many varieties with mauve, purple, violet, lilac or white flowers. Early in the 1960’s the first hybrid reds, pinks and yellows were bred by Dr R. A. H. Legro in Holland.
Generally described as hybrids of D. datum, modern delphiniums are a mixture of more than half-a-dozen species. D. belladonna and its varieties are not so widely grown today. They produce several spikes on each main stem. D. belladonna tolerates light shade, but other delphiniums demand open, sunny positions, sheltered if possible from wind.
Soil Although a good garden loam is ideal, sandy or clay soils can be improved by digging in plenty of rotted manure, peat-moss, leafmold, rotted compost or spent mushroom compost. Dig 18—20 in. deep, mix in 4 oz. per square yard of a general fertilizer and let the ground settle for at least a month before planting.
Planting Spring and fall are the best times to plant, except in heavy, wet ground or where slugs abound, when spring is preferable. A space 24 ft. across each way is sufficient in a border, but plants intended to produce spikes of exhibition quality deserve spaces 3-34 ft. across. Young plants knocked from pots can be set with a trowel; those from open ground often arrive with their roots ‘balled’ in soil and then the roots must be spread out, keeping the crowns of the plants at surface level. Firm planting is essential.
Propagation Delphiniums are increased by seed, division and cuttings. Although seed does not come true; that from a blue-flowered plant may produce seedlings with mauve, purple and white flowers. However, the best quality seed produces excellent plants cheaply. Preferably sow freshly harvested seed in August or early September, in seed flats filled with seed compost. Stand the flats in a frame, greenhouse or sheltered spot outdoors. By late April or May seedlings are big enough to plant 1 ft. apart in a nursery bed outdoors and are moved later to their permanent positions. Seed may also be sown in April under cover or in May in open ground.
Named varieties will come true only when propagated by dividing clumps in early spring, when the shoots are 3-4 in. high, or by cuttings. The roots are teased apart and the crown severed with a knife so each new portion has roots and one or two shoots. Cuttings generally produce sturdier plants. They are made by choosing healthy shoots 3-4 in. high, scraping soil from around them before cutting them off close to the crown. Strip off their lower leaves, dip the lower ends of the stems in water, then in hormone rooting powder and set them with a dibber, about five in a 4 1/2-in. pot of cutting compost. Bury one-third of each stem, water the cuttings, and place them in a propagating case, or on a shaded greenhouse bench. They root in six to eight weeks and then are potted up singly in 3 1/2-in. pots.
Cultivation In early spring crowns should be covered with ashes or coarse grit to keep slugs off the young shoots. Thin these out to leave the strongest four to six per plant, when 6 in. tall. Knock in a 6-8 ft. cane beside each shoot; tie the stems to the canes as they grow. Feed each plant with a general fertilizer in May, the same amount of general fertilizer in early June, and a tablespoon of sulfate of potash as the flower buds show color, watering in these plant foods. Keep the soil damp. Dead flower spikes should be cut back;
secondary spikes sometimes develop and bloom in early fall.
Ten Reliable Delphiniums
‘Blue Dawn’, 5 ft, blue and pink with black eye;
‘Butterball’, 5 ft, cream with yellow eye;
‘Skyline’, 5 ft, blue with white center;
‘Xenia Field’, 51/2 ft, lavender with lavender and white eye, a very strong growing variety;
‘Blue Tit’, 3 1/2ft, indigo with blue black eye;
‘Cupid’, 3 ft, sky blue with white eye;
‘Lord Butler’, 4 1/2 ft, flowers blue with white centers;
‘Peter Pan’, 3 1/2 ft, striking blue with dark eye;
‘Purple Ruggles’, 5 ft, purple double;
‘Swanlake’, 5 1/2 ft, white with black eye.
Image credit: Nigel Burkitt/RHS