This section is from the book "Commercial Gardening Vol2", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
For market purposes there is nothing to beat the fine hybrid forms that are now so well known, and have been raised from crossing and intercrossing such species as elatum, elegaiis, formosum, and grandiflorum. They grow from 4-6 ft. and more high, have finely cut ornamental foliage, and long erect spikes of flowers varying from pale Cambridge blue to the deepest of Oxford shades, with intermediate shades of purple, etc. The plants are valuable because the "roots" sell well in spring, and the cut spikes of flowers of unsold plants sell freely later on in summer. Seeds are sown in shallow drills in April and May, and the young plants are afterwards transplanted 18-24 in. apart in rich and deeply dug and well-manured soil - that is if the finest clumps are to be secured. They may be grown almost in any place in the open, or between rows of fruit trees not too closely planted. Specially line varieties are easily increased by dividing the rootstocks into as many pieces as possible not later than September or October. The hoe should be used occasionally between the rows to keep the weeds down.
Fig. 192. - Delphinium nudicauls.
Besides the hybrid Larkspurs, some natural species also sell, such as cardinale, 2-3 ft., scarlet; cashmerianum, 1½ ft, pale blue; grandiflorum, 1 - 1 ½ ft., rich blue; nudicaule, 1½ ft., bright red (fig. 192); and Zalil (or sulphureum)4,ft., sulphur yellow.