Lilacs

By Jan Cashman • Posted on July, 29th 2020

by Jan Cashman 6/13/20

Everyone loves lilacs…and what’s not to love? Lilacs (genus Syringa) are beautiful to look at and wonderfully fragrant. They are hardy in our climate and grow well in our local soils that tend to be alkaline. Lilac bushes are long-lived; they make good hedges.

If you choose carefully, you can have fragrant lilacs blooming in your yard for as long as 6 weeks. See the chart for different lilac species, their size and characteristics. Lilacs flowers aren’t always “lilac” in color—they also bloom in shades of pink (Montaigne), white (Beauty of Moscow), dark purple (Ludwig Spaeth), striped (Sensation), and blue (Wedgewood). On some lilacs the tiny flowerettes are double, others single. And let’s not forget tree lilacs, the last of the lilacs to bloom with their fragrant, white blooms in early July.

Planting: Plant lilacs in a sunny, well-drained spot in your yard and mix compost into the planting hole if your soil needs it. Lilacs can be slow to get started so I recommend a mild liquid fertilizer made for transplanting to give them a boost their first year. After that, fertilize annually with a water-soluble, well-balanced fertilizer. Too much fertilizer can inhibit flowering.

Pruning: Common lilacs sucker, which can be an advantage if you are growing a hedge that you want to spread, but can also be a nuisance. French and Canadian lilacs generally do not sucker. To renew older lilac plants, prune the thickest canes to the ground removing 1/3 of the bush each year. An old lilac can be totally cut down to just 6” or so and will grow back, but because lilacs bloom on old wood, the bush will not bloom the next year after this kind of drastic pruning.

Pests and diseases: Lilac bushes are susceptible to powdery mildew if planted too close together or in the shade. There are two more serious pest of lilacs. One is root weevils. These black beetles live in the soil and eat the roots of the plant. The adult stage of this insect crawls up at night and eats half circle notches on the edge of the plant’s leaves. Root weevils are hard to control but unless you have a serious infestation, they won’t kill the plant.

Another problem with lilacs is a bacterial blight similar to fireblight in apples. Worse in wet, cool springs, lilac bacterial blight causes the tips of the shoots to curl and turn brown. This disease rarely kills the whole bush, but damages its outer leaves and blossoms.  Pruning the infected branches out down past the damage is one treatment, sterilizing your pruners between cuts to avoid spreading the bacteria.

New lilacs: The dwarf lilac called Bloomerang (Syringa ‘Penda’) is a new lilac cross that reblooms in late summer. Deadhead old blooms to encourage reblooming. Bloomerang is slightly less hardy than other lilacs—listed as Zone 4 but should do fine in most locations around here.