Breeding Delphiniums

In this, the first of a short series on delphiniums, David Bassett discusses growing the cultivated forms from seed.

A Russian botanist, N.I. Malyutin, recently distinguished 365 species of delphinium on the basis of characteristic features of their seeds. Even if this number may be a subject for academic debate, it is nice to think that one could have a different delphinium for each day of the year. It also emphasises that the genus includes a wider variety of plants than we see in our gardens at present. Some species are fascinating and growing them presents challenges that will be the subject of a later article. I concentrate here on the more straightforward matter of raising 'conventional' garden delphiniums from seed.

If we ignore larkspurs, which are annuals that are now assigned to a separate genus, Consolida, all garden delphiniums are perennials, although three basic types of plant can be recognised. The first group comprises hardy perennials deriving mainly from the tall alpine species D. elatum. The flowers of these familiar plants of the herbaceous border are arranged in a long spike at the top of each stem, with a few more on small secondary sideshoots. The best forms have longer spikes with more florets than the species D. elatum, and the flowers are semi-double with more than twice as many sepals and petals and larger size. The colour range includes blues and mauves, pure white, creams enhanced by yellow eyes, purples, mauve-pinks and all manner of pastel shades. They make ideal plants for the hybridist to work with, since they can be grown to flowering within one year from making a cross. It is an exciting activity for any gardener to undertake and will be the subject of the second article in this series.

Many individual cultivars of D. elatum have been named, but their complex hybrid origin means that they do not come true from seed. The nearest to true-breeding lines with the Round-Table series of Pacific-Hybrids were developed in California by Frank Reinelt. Each line came true to colour, as in the brilliant blues of the 'Blue Jay' selection or the purples of 'King Arthur'. None of these seed selections has been maintained properly since Frank Reinelt retired in 1970 and most stocks now bear little resemblance to the originals. More recently, seed companies have developed true-breeding seed selections giving short-growing plants.

The second group of cultivated delphiniums derives principally from D. grandiflorum var. chinense. These are dwarf growers with finely divided leaves and a branched inflorescence so that an individual plant provides a broad mound of colour. The plants have a thickened fleshy rootstock from which fine feeding roots and the new stems grow out in spring, but in my experience they are so susceptible to damage by slugs that it is better to raise new plants each year from seed. The colour range includes violet and white but the great attraction for me of these little fellows is the brilliance of the blues that are obtainable.

The third group is formed by the 'belladonna' delphiniums. These probably originated from crosses between cultivars of D. elatum and D. grandiflorum and they are intermediate in character with small single flowers and a more branched habit than cultivars of D. elatum. The first belladonna cultivars were sterile triploids but fertile hexaploids were introduced and seed is readily available, although I suspect much of the material labelled 'belladonna' is merely inferior forms of D. elatum.

Raising any of these garden delphiniums from seed ought to be straightforward, yet people often tell me they have failed with a packet of seed, whereas seed from their own plants germinated like weeds. The fault usually lies with the grower, although the seed commonly gets the blame. I can best emphasise the important points and some of the reasons for failure by describing the way I grow all my delphiniums.

To get representative flowers from seed in one season, I treat delphiniums as half-hardy annuals, sowing the seeds indoors in early spring and then keeping the seedlings growing steadily through to flowering.

Seed harvested in August must be stored until February, which is where the first problem may arise. The viability of seed from D. elatum cultivars declines quickly unless it is stored properly. Seed collected from plants in the garden should be cleaned and allowed to dry for a few days. It should then be stored in labelled, sealed jars or plastic boxes in the vegetable-storage section of the refrigerator to keep it cold and dry but not frozen. Stored in this way, delphinium seed remains viable for several years.

Delphinium seed germinates best at quite low temperatures 60-68F (15-20C) providing the seed is kept moist throughout the germination period. The germination time is typically 14 to 21 days but can range from 5 days for D. grandiflorum and very fresh seed of other delphiniums to 5 weeks or more. Seed from a single cultivar often has a narrow range of germination times but many dark blue cultivars seem to germinate very slowly. Seed mixtures can thus have a wide spread of germination times and this is rather a nuisance as it is easy to lose the slow germinators altogether. In this context it is worth noting that the seed coat is commonly contaminated with moulds that cause rotting if conditions are not ideal, particularly if the temperature is too high.

For sowing, I use a soil-less sterile seed compost which is thoroughly moistened before use. After sowing the seed on the surface of the pot or tray, it is covered with a thin (3mm) layer of fine compost or vermiculite and then the pot is stood in a shallow container of water so that water is taken up into the compost by capillary action. Once the compost is thoroughly wet, the pot is moved to a warm room 60-68F (15-20C) where it is kept covered with kitchen foil until germination starts, usually in about 10 to 14 days. The cover is then removed and replaced with a transparent plastic dome or propagator top so that the compost stays moist. To avoid spindly growth, the seedlings need to be in good light near a window or in the greenhouse but they should be shaded from strong sunshine. Drying of the soil surface at this stage usually stops further germination and is probably a common cause of failure. Once most seeds have germinated I take the tops off the pots to reduce the chance of damping-off and water sparingly when necessary. The seedlings start to grow their first true leaf about three weeks after germination and, when this is well developed for most of the batch, I prick them out into individual pots or boxes of soil-less compost.

Seedlings are kept in a cool greenhouse for two to three weeks after pricking out and then go outside under an open structure with transparent roofing panels that keep rain or snow off. Losses at this stage are minimal, so long as one keeps slugs and snails at bay by putting down pellets. The plants grow quickly and are ready for planting out by the end of May, or earlier in a warm spring.

I plant dwarf delphiniums in the rock garden or patio tubs where they are to flower but seedling elatum hybrids go into a nursery bed as it is necessary to select just the best ones to keep for planting in the border later on. The young plants produce one flower stem only and are grown 9in(23cm) apart in double row blocks with just 18in(45cm) between the rows, so a lot of seedlings need only a small space. I fork a teaspoonful of dried-blood fertiliser into the planting hole to get the seedlings growing quickly and apply a mulch of mushroom compost between the rows to conserve soil moisture. Watering is occasionally necessary in very hot dry weather to keep the plants growing without check so that they throw up flower stems every bit as large and strong as those of a mature plant by the end of July. These stems should be staked as carefully as those of mature plants because the plants can be ruined if wind or rain breaks the stem at the base when the flowers are open.

Sowing delphinium seed indoors in early spring is not essential and you can sow successfully at any time providing moist cool soil conditions can be maintained. You can, for example, sow in the open ground in May or June but choose a shady spot or use shade netting to keep the soil moist and the temperature low.

Pests are a problem with open-ground sowing and it is advisable to sprinkle the ground with slug pellets to avoid seedlings being eaten almost as fast as they germinate. The other disadvantage is that you also have to wait until the following year to see your seedlings flower.

The great joy of raising delphiniums from seed comes when the plants flower for the first time. It is then that you can walk along a line of plants and appreciate all the subtle variations of form and colour that are possible in one type of garden plant. When those variations are just a little bit under your own control through hand pollination, it is even more exciting. In the second article I will describe how crosses are made and the features plant breeders are looking for in new delphiniums.