Lilacs scarcely need to be introduced to Canadians. Their heady scent and lavish blooms assure us that summer is just around the corner. Most lilac species are native to mountainous regions from Afghanistan to Japan, but 500 years ago the ancestors of our common garden lilac (Syringa vulgaris) began making their way west along trade routes to France from their home in the Balkans. Once on French soil, new selections and crosses were quickly introduced. By the time John Parkinson wrote his herbal in 1629, lilacs had crossed the Channel to England, and from there to the New World.
It’s best to plant lilacs in the spring. Choose a sunny site (at least six hours of direct sunlight) with good air circulation. Dig a hole 50 percent wider and deeper than the root-ball of the lilac, and amend the soil with compost, plus a couple of handfuls of bone meal. Acidic soil with a pH of 6.5 or lower should be amended with horticultural lime. Once the lilac is positioned, firm the soil around the roots with your foot and water in well. Transplanting fertilizer isn’t necessary, but an organic mulch of shredded leaves or wood/bark chips 10 centimetres deep will retain moisture and keep the root zone cool.
Plan to prune in early summer after the flowers have faded but before seeds begin to form. Unless you plan to transplant them elsewhere, always cut away suckers from the base of the shrub. Lilacs are pruned to maintain a graceful vase-like shape, a loose framework for good air circulation, and to encourage new growth and larger flower clusters. Old, woody trunks should be removed just above the crown with a pruning saw; use secateurs to get rid of dead, broken or rubbing branches, remaining spent flowerheads, and to reduce the height of new shoots.
Tip: Make a day of pruning and cut back other spring-flowering shrubs, such as forsythia, Vanhoutte spirea and mockorange (Philadelphus cultivars) at the same time.
Diseases like lilac blight and mycoplasma are difficult to control but good air circulation as well as pruning out affected areas by cutting down to healthy growth (dip secateurs in a 1:10 solution of bleach and water between cuts) will help. If powdery mildew is prevalent in your area, be sure to plant mildew-resistant types like the cutleaf lilac (Syringa ×laciniata) or dwarf Korean lilac (S. meyeri ‘Palibin’).
Insects, such as lilac leaf miner, may cause unsightly damage, but they do no harm; removing the affected leaves and keeping the ground underneath the lilac free of infested leaves in the fall will provide good control.
Tip: Lilacs are susceptible to certain pests. Dormant oil spray is useful for combating borers and scale insects in early spring; when shrubs are in leaf, fight scale with a soapy lime-sulphur spray.