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This is one of the largest genera of flowering plants and comprises over 1,000 species, including the common pest of our gardens, the groundsel.
The generic name, which was used by Pliny, is derived from the Latin senex, an old man, and refers to the usually grey, hair-like pappus of the seeds. The senecios have alternate leaves and the flowers (or ‘florets’ as they are usually called) are, as in all the Compositae, crowded in ‘heads’. The florets are usually of two kinds: those in the centre of the head, of tubular shape and known as ‘disk’ florets; and those of the circumference, tongue-shaped, radiating, and known as ‘ray’ florets. (The daisy is the most familiar type.) But sometimes the florets are wholly discoid, as in S. elaeagnifolius and S. reinoldii. All those included in the following notes are natives of New Zealand, where there is the greatest concentration of species that are both woody and hardy enough for cultivation out-of-doors in the British Isles. S. argentinus Baker (syn. S. leucostachys Hort., not Baker), from Argentina and Uruguay, is grown for its much divided, silvery foliage, but it is scarcely shrubby enough to fall within the scope of this work.
Recently, B. Nordenstam in Opera Bot., Vol. 44, pp. 25–33 (1978) has transferred the New Zealand species dealt with here to the genera Urostemon B. Nordenstam (S. kirkii) and Brachyglottis J. R. & G. Forst. (the rest). There is no doubt that the latter are more correctly placed in Brachyglottis than in Senecio, although the validity of the segregation of Urostemon from Brachyglottis is open to question. Since, however, work on the establishment of more rational generic limits within the Senecio-complex is still in progress, the species are here entered under their Senecio names.
Hybridisation amongst the New Zealand species is widespread, both in cultivation and in the wild, and the majority of plants in cultivation in the British Isles are of hybrid origin. Failure to recognise this has in the past caused much misidentification of cultivated material. Fortunately D. G. Drury, in New Zeal. Journ. Bot., Vol. 11, pp. 731–784 (1973) has provided us with a reliable key to the New Zealand shrubby senecios and their hybrids, both cultivated and wild. His account gives full descriptions and illustrations of all the known hybrids, and the reader is referred to it for a fuller treatment. Most of the hybrids recorded as in cultivation in New Zealand have yet to be confirmed as in cultivation in the British Isles; some may eventually prove to be of garden merit.
Provided the climate is sufficiently mild for them, the senecios are of easy cultivation and succeed well in a light or sandy soil. S. compactus, S. greyi, S. laxifolius and the cultivated Dunedin hybrids succeed on lime or chalk, as probably do most of the others. The senecios are not sufficiently planted in sea-side gardens, for which places the toughness of the leathery leaves admirably fits them. Propagation is effected by late summer cuttings placed in very sandy soil and given if possible a mild bottom heat. (See Olearia for the distinction between that genus and Senecio.)
Revised by C. Jeffrey, The Herbarium, The Royal Botanic Gardens, Kew