Flora of North America species comparison

Philadelphus lewisii, the state flower of Idaho, is rarely cultivated beyond its native range; there is no evidence that it is naturalized in other parts of North America.

A broadly defined Philadelphus lewisii is here recognized with some reluctance; it includes all previously named northwestern entities of subg. Philadelphus. Hu S. Y. (1954–1956) recognized 16 entities at varietal and specific rank, arrayed in two subgenera. C. L. Hitchcock et al. (1955–1969, vol. 3) described P. lewisii as being extremely variable in both vegetative and floral characters and further stated that most ecological or geographic races do not merit taxonomic recognition. Most recent floristic treatments have followed Hitchcock et al. or in some cases recognized two or three entities. The treatment of Hitchcock et al. is followed here, and there does not appear to be a case for the recognition of the two possible exceptions they mentioned (vars. gordonianus and parvifolius). Philadelphus californicus, P. cordifolius, and P. insignis were separated by Hu from P. lewisii at a subgeneric level based primarily on exposed buds and many-flowered inflorescences. The exposed buds are seen sporadically in P. lewisii, including outside the distribution accorded P. californicus, and are not evolutionarily cognate with the exposed buds of subg. Deutzioides; based on examination of herbarium material, they seem to be an expression of extra-vigorous growth, but additional study of plants in the field is needed. Some plants from the Sierra Nevada in California, considered by Hu to be occupied solely by P. californicus, are indistinguishable from P. lewisii in flower number, bud exposure, or any other character. The elaborated inflorescences of some plants in this part of California are striking; for now P. californicus and relatives are treated as part of a broadly defined P. lewisii. In her phylogenetic analysis based on ITS sequence, A. E. Weakley (2002) also found that P. californicus was not distinguished from P. lewisii.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Species ca. 25 (9 in the flora).

Philadelphus has a relictual distribution in western and southeastern North America, Mexico, and Central America (from southwestern Canada south in the western cordillera to Panama); southern Europe (perhaps only by human introduction); the Caucasus; and eastern Asia. It is naturalized elsewhere, including most temperate areas of the western and eastern hemispheres, and in Hawaii, where P. karwinskianus Koehne is invasive.

Hu S. Y. (1954–1956) relied on vestiture of the leaves, twigs, pedicels, flowers, and fruits to distinguish species in Philadelphus. She extended and expanded the traditional use of such characters by C. D. Beadle (1902), P. A. Rydberg (1905), and C. L. Hitchcock (1943). However, with more specimens now available, it is clear that these characters are variable and poorly correlated with one another. Basing taxa on permutations of such characters often has resulted in sympatric taxa that lack geographic and/or ecologic coherence. Herein, the recognition of biologically meaningful taxonomic units is attempted; this means that many previously named taxa are being combined in general agreement with recent, more local or regional, assessments in North America, which generally recognize fewer taxa (C. L. Hitchcock et al. 1955–1969, vol. 3; W. C. Martin and C. R. Hutchins 1980, vol. 1; C. F. Quibell 1993; N. H. Holmgren and P. K. Holmgren 1997; C. K. Frazier 1999; B. L. Turner 2006).

Hu S. Y. (1954–1956) recognized four subgenera in Philadelphus: Deutzioides S. Y. Hu, Gemmatus (Koehne) S. Y. Hu, Macrothyrsus S. Y. Hu, and Philadelphus (as Euphiladelphus). A preliminary phylogenetic study of Philadelphus based on ITS sequences provided cladistic support for three of the four subgenera (with the reassignment of P. microphyllus from subg. Philadelphus to subg. Gemmatus) and no support for recognition of subg. Macrothyrsus (A. E. Weakley 2002). Guo Y. L. (2013) conducted a more detailed phylogenetic study of Carpenteria and Philadelphus utilizing both nuclear and chloroplast markers and found three lineages: two species sampled from subg. Deutzioides (P. mearnsii and P. texensis var. texensis); Carpenteria; and the remainder of Philadelphus (32 accessions, including P. hirsutus, which had been previously considered to be a component of subg. Deutzioides). These results suggest the potential inclusion of Carpenteria in Philadelphus and also that characters (such as buds exposed versus in nodal pouches) that have been used to distinguish subg. Deutzioides and subg. Macrothyrsus from subg. Philadelphus may be plesiomorphic. In this treatment, species one through four belong to subg. Deutzioides, the basal clade in Philadelphus, which is restricted to southeastern North America and southwestern North America into northern Mexico; species five (P. microphyllus) belongs to subg. Gemmatus, which occurs from southwestern United States south to Panama; and species six through nine belong to subg. Philadelphus, of eastern North America, northwestern North America, southern Europe, the Caucasus, and eastern Asia.

Within each of these three subgeneric clades, the morphology of Philadelphus is very conservative; relatively few morphological characters are useful in distinguishing taxa in each subgenus. Additionally, W. Bangham (1929) studied the chromosomes of about 40 taxa and found no variation in number and complete compatibility in all hybrids that he studied; E. K. Janaki Ammal (1951) reported some pairing irregularities in hybrids.

A number of horticultural forms have been developed, are planted in temperate areas, and may be found as local escapes. Given the morphological variability of Philadelphus species and the uncertain origins of some of these plants, the horticultural forms are difficult to deal with by conventional taxonomic means. An example is P. ×virginalis Rehder, which is alleged to be a hybrid of P. ×lemoinei hort. (itself alleged to be a hybrid of the Old World P. coronarius and southwestern North American P. microphyllus) and P. ×nivalis Jacques (itself possibly a hybrid of P. coronarius and eastern North American P. pubescens); C. A. Stace (2010b) considered this to be the most widely cultivated, persistent, and presumably established Philadelphus taxon in the British Isles. Philadelphus ×virginalis was reported as escaping locally in Lenawee County, Michigan, by E. G. Voss and A. A. Reznicek (2012). Neither reliable identification of cultivars of this kind nor determination of their genetic origins are currently possible. Some idea of the cultivated entities can be gained from A. J. Rehder (1940), Hu S. Y. (1954–1956), D. Wright (1980), G. Krüssmann (1984–1986, vol. 2), and M. H. A. Hoffman (1996).

Philadelphus tomentosus Royle, a native of the Himalayan region, has been reported as naturalized in the flora area (M. A. Vincent and A. W. Cusick 1998) based on specimens from Allen and Paulding counties, Ohio. Because of the close similarity of all Philadelphus taxa, those specimens cannot be definitely identified as P. tomentosus. They appear to be P. pubescens, a native North American species widely cultivated and naturalized in eastern North America.

In some species of Philadelphus, the axillary buds are enclosed in pouches of cortical and epidermal tissue at the petiole bases. The petiole abscises distal to the pouch so that the bud is hidden and proximal to the leaf scar, only becoming visible as it expands. In other species, the buds are exposed, and the petiole abscises proximal to the bud; these buds are distal to the leaf scars, which is the condition normally seen in most plants. Whether the axillary buds are exposed (species one to four) or hidden in pouches (species five to nine) is best observed at nodes of mature leaves on vigorous long shoots and may not be apparent at nodes of young leaves or on short shoots.

(Discussion copyrighted by Flora of North America; reprinted with permission.)