Lilacs (Syringa) are a genus of about 20 species of flowering plants in the family Oleaceae (olive), native to Europe and Asia. Lilacs range in size from large shrubs to small trees, 2-10 m tall. The leaves are opposite (occasionally in whorls of three), deciduous, and in most species simple and heart-shaped, but pinnate in a few species (e.g. S. laciniata, S. pinnatifolia). The flowers are produced in spring, each flower about 1 cm diameter, white, pale pink or more generally purple, with four petals. The flowers grow in large panicles, and in several species have a strong fragrance. Flowering starts after 80-110 growing degree days.
Lilacs are used as food plants by the larvae of some Lepidoptera species including Copper Underwing, Scalloped Oak and Svensson's Copper Underwing.
Lilacs flower on old wood, and produce more flowers if unpruned. If pruned, the plant responds by producing fast-growing young vegetative growth with no flowers, in an attempt to restore the removed branches; a pruned lilac often produces few or no flowers for one to five or more years, before the new growth matures sufficiently to start flowering. Unpruned lilacs flower reliably every year. Despite this, a common fallacy holds that lilacs should be pruned regularly. If pruning is required, it should be done right after flowering is finished, before next year's flower buds are formed. Lilacs generally grow better in slightly alkaline soil.
Lilac bushes can be prone to powdery mildew disease, which is caused by poor air circulation.
A pale purple colour is generally known as 'lilac' after the flower. Purple lilacs symbolise first love and white lilacs youthful innocence.
The wood of lilac is close-grained, diffuse-porous, extremely hard and one of the densest in Europe. The sapwood is typically cream-coloured and the heartwood has various shades of brown and purple. Lilac wood has traditionally been used for engraving, musical instruments, knife handles etc. When drying the wood has a tendency to be encurved as a twisted material, and to split into narrow sticks. The wood of Common Lilac is even harder than for example that of Syringa josiakaea.
Syringa vulgaris is the state flower of New Hampshire, because it "is symbolic of that hardy character of the men and women of the Granite State" (New Hampshire Revised Statute Annotated (RSA) 3:5).
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