Philadelphus is not an Egyptian genus, but it was named for the Egyptian King Ptolemy Philadelphus who reigned from 285 to 247 BC. It has a chequered history of naming: syringa and mock syringa, mock orange and even jasmine, all of which we can understand as the flowers have the scented qualities of lilac and the same style of flower and scent of jasmine. It has also jumped about in the family classification: Philadelphaceae, Saxifragaceae, Hydrangeaceae.
The use of syringa is common to both lilac and philadelphus. This comes about through John Gerrard in his herbal. He called Philadelphus coronarius, Syringa alba, ‘white pipe’. He called Syringa vulgaris (lilac) Syringa caerulea, ‘blue pipe’. Jasmine he named Syringa Arabica,‘Arabian pipe.’ Linnaeus chose to allocate syringa to the lilac and philadelphus was given a genus name of its own.
To appreciate a genus, with over 60 species, it is helpful to consider the natural habitat, and philadelphus are found mainly in the northern hemisphere, essentially North America and Asia – namely West Coast of North America, the South East States of North America, and across Tibet, China and Japan. There are some species growing wild in Italy and the Caucasus but it is uncertain whether these are native or escapees from cultivation. They are found in open sunny positions on screes and rocky hillsides, at the edge of and in open woodland and sometimes by streams. In cultivation the species seem tolerant of any soil conditions and whilst preferring sunny sites will grow well in semi shade. The same can be said of the cultivars, although they respond better to a richer soil. The exceptions to this are the golden leaved cultivars whose leaves get burnt if exposed to too much sun. All are hardy down to at least minus 15C some to minus 25C the exception being P mexicanus which enjoys the warmth of the Mexican climate.
In cultivation, Philadelphus are grown for their ornamental quality especially their scented flowers. The scent varies from the citrus scents of orange or lemon to pineapple and the degree of scent varies from species to species. The species vary in appearance differing in height and spread and flower form. Whilst all have 4 petals and sepals they do vary in size and the configuration on the stems-some singly others in clusters or racemes. The flowers too vary from cup shape to flat or reflexed petals. Further variation comes with the cultivars when semi double and fully double flowers have been introduced and the weight of these double flowers gives the branches an arched appearance. The flowers are remarkably consistent in their flowering period both in the wild and in cultivation. The range is from the end of May to the end of July regardless of climate and local weather conditions with June being the peak.
They are also valuable to regenerate unstable screes and in America land managers use natural species to plant such sites and P microphyllus or P lewisii, are also used to colonise dried up river valleys.
The habit of the genus is also variable according to the species. Some are very open, with strong sparse elongated growth, others are dense and twiggy. Consequently there can be no general rule for pruning. The best advice is to prune out some of the older wood, cutting back to new growth, and to keep the shrub to a sensible size by pruning back the flower stems immediately after flowering. The flowers are produced on the previous season’s growth so the shrub needs time to make this growth before the autumn and leaf fall or there will be few flowers the following season. With some species a major problem for gardeners is the tendency to produce suckers which makes for even more congestion.
This genus has few pest problems. In a dry season aphid infestation may be found in cultivation and in the wild they are browsed by deer and elk. Quail and squirrels will eat the seeds but do no other damage.
Species can be easily propagated by seed, but a sizeable plant is more readily achieved by either soft wood or semi ripe cuttings, hardwood cuttings, suckers, divisions or by layering. Cultivars must be propagated vegetatively.
Many of the hybrids in cultivation were raised by the French nurseryman Victor Lemoine in Nancy during the early 1900s. His original hybrid was between P. coronarius and P microphyllus and this crossing also produced a number of clones which he also introduced. P coronarius is the most commonly grown philadelphus. It is strong growing, of medium height and spread and quite tolerant of dry soils. It has been in cultivation a long time and its origin is obscure, although certainly European. It is found in southern Italy, Austria and Romania and it is probably because they grew well in Europe that Lemoine chose P coronarius for one of his hybrid partners. P microphyllus has small leaves and is a small twiggy shrub seldom more than a meter high. The flowers too are small and dainty.
He named this original P x lemoinei and introduced it to the public in 1884. It is a small shrub with many of the characteristics of microphyllus and coronarius but the flowers are much larger 2.5cm across and very fragrant. They are produced in clusters of 3-7 on the short side branches. Of the clones, Avalanche, Coupe d’Argent, Erectus, Innocence and Manteau d’Hermine are probably the best known and still available.
These clones show different degrees of characteristics from both parents. Erectus and Avalanche are very similar, both upright with long slender branches which arch beautifully under the weight of the flowers as these 2 cultivars are very floriferous despite being single. The flowers are 2.4 cm across and in racemes of 7 flowers. The shrubs reach at least 2 m in height. By contrast Manteau D’Hermine is dwarf and compact reaching a maximum of 1.2 m with double creamy white flowers. The single flowered Innocence has touches of white in the green leaves. Coupe d’Argent has large single flowers, almost square in form, placed individually at intervals along the stem.
There followed many introductions across the world: singles, doubles, large or small flowered, tall, medium or small shrub habit and all claiming to be the most heavily scented. Belle Etoile is a very compact shrub, albeit vigorous, with large single flowers flushed maroon at the base of the petals. It is a triploid hybrid and sterile so needs to be propagated vegetatively. Boule d’Argent has large double flowers in dense clusters Bicolore has a wide spread, often over a meter. Its flowers are solitary and cup shaped occasionally appearing in clusters of three. Also with a wide spread is Conquete a 1903 Lemoine introduction. It is variably double with long pointed petals interspersed with shorter petaloid stamens. Another almost square shaped flower is Sybille, introduced by Lemoine in 1913. The single flower is quite flat. Burkwoodii also has a distinctive petal arrangement, the long narrow petals are arranged like windmill sails, slightly on an angle.
Virginal is probably the best double flowered cultivar 4 to 5 cm across. It is a strong grower and can reach 2.5m in height. It is heavily scented. Burfordensis, a sport from Virginal, was introduced by Sit William Lawrence of Burford Court Surrey. This too is a tall robust shrub up to 3 m. It has dense columns of flowers 7.5 cm across with conspicuous yellow stamens. The Hon. Lewis Palmer introduced a hybrid of Sybille and Burfordensis pollen. He named it Beauclerc. It has a spreading habit with broad reflexed petalled flowers about 7.5 cm across. They are a milky white with a touch of pink at the base.
The purple stain either at the base of the petal or through the petal veins is most noticeable in the purpureo maculata group. Purpurascens, a native of Western China also has a purple calyx. Delavayi, discovered by Abbe Delavay in1889 has a purple form (delavayi f. purpurascens) where both twigs and calyx are purple.
There are other distinctive species. P. tomentosus, native to Kashmir and China is an upright shrub with hairy leaves, densely grey on the underside. Coronarius Aureus is a golden leaved form of coronarius, similar in habit and flower to the parent as is the variegated leaf form coronarius Variegatus which has been around since 1770.
Many more have been introduced but like other genera, cultivars have been lost in cultivation. I would challenge you to find the following. Monster, a vigorous shrub which quickly gets to 4.5m with flowers 4 to 5 cm across. Atlas a medium shrub with a loose habit of long arching branches, flowers 5 to 6 cm across and leaves with a faint yellow mottling. Favourite growing to 2 m with single large cupped flowers, oure white with serrated petals and a central cluster of stamens. Velleda whose petals are crimped at the edges.