Hydrangea (common names Hydrangea and Hortensia) is a genus of about 70-75 species of flowering plants native to southern and eastern Asia (from Japan to China) and America. By far the greatest species diversity is in eastern Asia, notably China and Japan. Most are shrubs 1-3 m tall, but some are small trees. They can be either deciduous or evergreen, though the widely cultivated temperate species are all deciduous.
Hydrangea flowers are produced from early spring to late autumn; they grow in flowerheads (corymbs or panicles) at the ends of the stems. In many species, the flowerheads contain two types of flowers, small fertile flowers in the middle of the flowerhead, and large, sterile bract-like flowers in a ring around the edge of each flowerhead. Other species have all the flowers fertile and of the same size. In most species the flowers are white, but in some species (notably H. macrophylla), can be blue, red, pink, or purple. In these species the exact colour often depends on the pH of the soil; acidic soils produce blue flowers, neutral soils produce very pale cream petals, and alkaline soils results in pink or purple. Hydrangeas are one of very few plants that accumulate aluminium. Aluminium is released from acidic soils, and in some species, forms complexes in the hydrangea flower giving them their blue colour. Hydrangeas are popular ornamental plants, grown for their large flowerheads, with Hydrangea macrophylla being by far the most widely grown with over 600 named cultivars, many selected to have only large sterile flowers in the flowerheads. Some are best pruned on an annual basis when the new leaf buds begin to appear. If not pruned regularly, the bush will become very 'leggy', growing upwards until the weight of the stems is greater than their strength, at which point the stems will sag down to the ground and possibly break. Other species only flower on 'old wood'. Thus new wood resulting from pruning will not produce flowers until the following season.
Philadelphus is a genus of about 60 species of shrubs from 1 to 6 m tall, native to North America, Central America, Asia and southeast Europe. Most are deciduous but a few species from the south of the genus' range are evergreen. The leaves are opposite, simple, with serrated margins, from 1 to 14 cm long.
Philadelphus lewisii: The flowers are white, with four petals and sepals, 1-4 cm diameter, and commonly (but not in all species) sweetly scented. The fruit is a small capsule, containing numerous small seeds. The bark is thin and flaky, finely shredding in longitudinal strips.
Mock-oranges are popular shrubs in parks and gardens, grown for their reliable display of late spring flowers; the scented species are particularly valued. In addition to the species, there are numerous garden origin hybrids and cultivars available.
References : Wikipedia
Here is a collection of the hybrids i grow