How To Grow Delphiniums

DelphiniumsThe delphinium is a genus of fully to half-hardy perennials, biennials and annuals so much admired particularly in the cottage garden setting.

Delphiniums are tall, majestic plants with showy open flowers on branching spikes. Each flower has 5 petal-like sepals with 2 or 4 true petals in the centre called a bee.

Delphinium species include all three primary colours, blue, red, and yellow. Hybridisation of delphiniums has resulted in many new colours and attractive flower forms and growing heights. Flower colours range in shades of blue from palest sky, through to gentian and indigo; rich purple, lavender, pink to purest white.

The modern delphinium is the result of hybridisation of delphinium species from wide and varied parts of the world. Crosses made by growers keen to improve specimens they were able to acquire have resulted in the modern plants, which are truly spectacular. In England Blackmore and Langdon, nurseryman and leading breeders of Delphiniums continued this work from early in the 20th century producing named varieties of large well-formed delphiniums. Others have also added their skills and developed the most dramatic and eye-catching plants to grace our gardens.

Many are varieties that are disease resistant, offering protection against both powdery mildew and black spot. They are good cut flowers and will last about 6 to 8 days in a vase.

Delphiniums give their best performance in well-drained soils in full sun but they can be grown in dappled or part shade. They are heavy feeders and therefore quire extra nutrients particularly in early spring and later in the season. Mulching with manure at the start of the season and a liquid feed every two to three weeks throughout the summer will giving them a rich diet for the development of their flowers. Stunted growth and yellow foliage are signs the plants need additional fertilizer. 

Some varieties of delphiniums may grow up to 6 ft. (180cm) tall, making them excellent background plants. The Chinese delphinium (Delphinium grandiflorum) grows 2 to 3 ft. (60cm-90cm) tall and therefore suited to the middle or front of the perennial border. Just like most other varieties, it will bloom throughout the summer if the plants are cut back regularly.

The Belladonna delphinium (Delphinium x belladonna) is a cross between D. elatum and D. graniflorum. These plants produce multiple flower stems that grow to 3 to 4 ft. (90cm-120cm) 'Dwarf Pacific' hybrids grow only 2 ft. (60cm) tall. 'Blue Butterfly' is one of the shortest delphiniums maturing at 15 in. (34cm).

PESTS & PROBLEMS: Slugs unfortunately relish delphiniums. At the first sign of spring your plants will be bursting into life - and they will need protection from these slimy creatures otherwise they may not survive to grace your garden. Many people will automatically reach for the slug pellets, these do work of course but they are life threatening to other creatures in the garden, other creatures that we would like to encourage because they work with us, helping to eliminate other pests around our gardens. Beer traps work well, just put some beer into a shallow container that has steep sides, the slugs love the stuff and when they have drunk their fill they simply drown. Seeing their stranded brethren does not seem to deter their fellow snails from slithering into the same fate.

Another good trap is to place a half of either grapefruit or melon skin with the flesh removed. Place them upside down on the ground; here the slugs gather over night; these can be collected the next morning and disposed of.

Mildew can be a problem if the plants are situated in a spot where they get poor air circulation or in areas that have high humidity; but it is easily remedied and in general only affects plants that have been neglected. Growing a disease resistant variety may be an option too. If you encounter Black spot, this is easily treated with a fungicide, especially if it is done early.

Waterlogged soils in winter will cause the crown to rot and that is why excellent drainage is absolutely essential. Also, any damage done to the crown by a slug or snail, or by a hoe when weeding, could cause water to enter the crown and cause it to rot during the winter. It is also thought that repeatedly heavy applications of nitrogen may add to crown rot. Plant breeders and exhibitors consider it good practice to spray their delphiniums with a good systemic fungicide in early June.

CULTIVATION: Select your site carefully, because mature delphiniums do not like to be moved. The location must be sunny and the soil as deep and rich as possible. The soil must be well drained, as they will not tolerate 'wet feet' especially during the winter. If the soil is heavy or does not drain well, add plenty of compost and grit to loosen it up. If your soil is prone to water logging then the raised bed method is a good way to over-come these conditions. The edges of the beds will require some form of retention either wood or edging bricks will do the job well. Stones, rubble, pebbles can be placed at the base of the bed, then build up the soil level to a suitable depth with either loam or a good quality compost.

Prepare a large planting hole, mixing in some peat moss, compost or well-rotted manure with a handful of bone or fishmeal your existing soil. Be sure the crown of the plant is placed right at ground level. Do not plant them under large trees or too close to a wall or tall hedges; too much shade will cause the flower spikes to become too tall and spindly. To get the very best results, at least six hours of full sun is required daily. Plant the delphiniums in groups of three or five each at least two feet apart; this will create a pleasing group picture. Surprisingly, odd numbers of plants work better and balance up the group and appear more natural.

Most delphinium hybrids bloom from early to mid-summer. The flower spikes are hollow and therefore brittle. In all most all locations other than the very sheltered spots, tall growing hybrids should be staked as wind and rain on the heavy, flower-laden spikes could cause the stems to snap and break. Using four dark green, thin canes, inserted into the ground, forming a square around the plant, is one of the best methods of staking.

The dark green colour of the canes gives some camouflaging qualities so that once the plant takes on some bushiness the canes will not be visible. The canes should be 4 ft. (120cm) tall. The first tie should be tied tightly to the canes, 10 to 12 in. (25cm-30cm) up from the ground. The second tie should be placed 24 in. (60cm) up from the ground, and situated just below the first bottom floret and tied loosely but securely. This method of staking permits the heavy flower spikes to move a little within the confines of the twine without breaking. If the flower spike is tied to the stake, it will break at the base of the tie. Immediately after flowering, cut off the old flower stalk to encourage additional flowering in late summer or even early autumn.

The spikes should be cut off under the first bottom floret on the spike and allow the green stems and the foliage to die down naturally. When they are entirely brown, cut them off at ground level. If you cut down the entire spike to the ground after it has bloomed and when it is still green, you will force the delphinium into an immediate second growth when the crown does not have sufficient energy, which will shorten the life of the plant.

The second crop of flowers will not be as large or as bountiful as those of the first, the new growth of a second bloom will be about 12 in. (30cm) high but never the less, they will still have much of the beauty and are of course most welcome.

The usual failures in germinating delphinium's seed are these:

1) Keeping the soil too wet.
2) Sowing the seed too deeply. Seeds should only be covered with approximately 1/8" of soil.
3) Trying to germinate old seed. Delphinium seed loses its vitality after one year.

Try the Pacific Giants variety; they are reputedly the easiest to germinate.

Many seedsmen recommend a period of pre-chilling for some varieties this greatly aids germination. Sow the seeds in pots or in a tray in September to April. After sowing, seal the container in a polythene bag and place on the bottom shelf of a fridge for 3 weeks. Check regularly to monitor germination. Then, move to an optimum temperature of about 15C (59F), keeping the surface of the compost moist but not waterlogged.

SOWING: You can start new delphinium plants from seed at almost any time. From April to September the seeds can be sown directly into the garden about 1/8 in. (3mm) deep. The seeds can be started early during the winter months of January, February and early March, indoors. Plants started from seed during the winter will generally flower a little the first year. Those started outdoors, do not flower until the following year. As the seedlings grow they must be thinned out to 9 in. (23cm) apart to give them the space to grow healthy and well.

AFTER CARE: When the tall growing varieties are about 2 ft. (60cm) high, stake with canes. During dry spells make sure that the plants are well watered. Keep the surface of the ground around the plants loose and free of weeds. Each spring apply a handful of bone or fishmeal around the plants and a good mulch of organic fertilizer such as well rotted manure. This it an important time when the plants are beginning to burst into growth. The added food will replenish the goodness in the soil for growth building requirements.
About every 3 years it may be necessary to divide the roots. This is an early spring job, just when the shoots are breaking through the ground. Dig up plants carefully; cut clumps into sections, each with plenty of fibrous roots and one strong stem. Plant immediately in well prepared fibrous soil, setting plants so the crowns will be 2 in. (5cm) below the surface.

THINNING: This is essential to promote proper growth and to ensure that the plant will be long lasting. Newly planted, first year seedlings should have only one flower spike. Second year delphiniums should only be permitted to have three flower spikes. Older and established delphiniums should be permitted to have only five flower spikes. When the new spring growth is two-to-three inches high, select the correct number of the strongest shoots to remain and cut off all other shoots at ground level.

A properly grown 8-year-old delphinium can have as many as twenty to fifty shoots in early spring. If all were permitted to grow on, the plant would literally bloom itself to death. When the delphiniums die down and become dormant in late autumn/ early winter, cut down all stems and foliage to ground level, and remove all leaves, debris, stakes, and weeds around the crowns. This will greatly diminish the hiding places where pests may over-winter. If you have heavy infestations of slugs and snails, late winter and early spring can be times when they cause excessive amounts of damage. Covering the tops of the crowns with coarse gritty sand helps to deter them.

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