When to Prune Lilacs in the Pacific Northwest

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A beloved garden shrub, the common lilac (Syringa vulgaris) is valued for its beautiful blooms and unmistakable, sweet fragrance. Left to their own devices these shrubs can grow as tall at 20 feet and often outlive those that planted them. Unfortunately, this unchecked growth results in fewer and fewer flowers. Careful annual pruning and deadheading can assure a yearly supply of blooms.

Lilacs

  • Members of the olive family, Oleaceae, lilacs were introduced to Europe in the 16th century and to the American colonies in the 1800s. Collected and prized by colonial gardeners, the wily lilac soon escaped and naturalized in some areas, earning for itself the title of state flower of New Hampshire. Lilacs thrive in cold winter regions and do well in moist, humus-rich, well-drained soil with regular watering. They will tolerate light shade or full sun and prefer a pH close to 7. Common lilacs do well in US Department of Agriculture plant hardiness zones 5 through 9.

Blooming

  • Lilacs bloom in early spring, usually before roses and other early season flowers. Blooms last about two weeks and, depending on the cultivar, color can range from pure white to deep purple. Flowers are produced in densely packed conical clusters called panicles. The individual flowers are tubular-shaped and flare into either four petal-like lobes or eight for doubles that give the flowers a frilly appearance. In addition to their beauty, lilac flowers are irresistible to hummingbirds and butterflies.

Annual Pruning

  • Pruning should be done as soon as the blooms fade and fall off, usually late May or early June in the Pacific Northwest. Flowers are produced on wood at least 3 years old, so pruning too late in the season or too early in the spring may cause few if any flowers to be produced. All lilacs can benefit from a little pruning. The technique depends on the age of the shrub. For younger shrubs, a yearly pruning means healthy stem growth and a framework that encourages flowering. Cut diseased, misshapen or unproductive stems all the way to the ground. Thin out some of the remaining growth to keep the width of the plant under control and encourage properly spaced new growth. Remove some of the new shoots that arise from the roots to avoid overcrowding. Pencil-thick branches are the most productive so they should be kept if possible. Tiny, twiggy stems will not produce flowers and contribute to overcrowding. Deadheading should be done as soon as the flower clusters fade. Remove spent flower heads right above the two new shoots. These new shoots will grow over the summer and produce flowers the following spring.

Pruning Older Shrubs

  • In older plants, a more severe pruning may be necessary even at the cost of flowers for a year to two. Before doing a severe pruning, check to see if your lilac has been grafted. Grafting is a method of propagation where the branch of one species or cultivar has been attached to the rootstock of another. This union should be visible at the base of the plant a few inches above the ground. Look for a bumpy place on the trunk where the bark of the scion wood and the rootstock are different. If this is the case, do not cut stems down below this union as the resulting new growth will be that of the rootstock and not the desirable cultivar. When a severe pruning is done, fertilize the pruned plant with composted manure or balanced chemical fertilizer to encourage vigorous new growth and hasten flowering.

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