Plant Palette

Plant Palette

Lilacs

Photo of Jennifer Schultz Nelson

Jennifer Schultz Nelson
Extension Educator, Horticulture
jaschult@illinois.edu

One of the sure signs that warm weather is here to stay, and not just teasing us is when lilacs bloom. Their sweet scent conjures up thoughts of green grass and blue skies of a beautiful spring day.

Though lilacs are a very common sight around the U.S., they are native to Europe and Asia. Lilacs are members of the genus Syringa. There are about twenty different species of Syringa, plus many different hybrids of these species. They are closely related to another common landscape shrub, Ligustrum, the privet.

Lilacs are considered to be deciduous shrubs or even small trees. Their size ranges from about six feet to over 30 feet tall. It is possible for the stems of lilac to be over twelve inches in diameter.

Leaves of lilac tend to be heart shaped, but may be more lanceolate or pinnate shaped depending on the species.

Most lilacs flower once a year in the spring. Flowers are produced in large clusters called panicles at the end of branches on old wood. Each individual flower is tube shaped, up to about a half an inch in diameter, and an inch long, though smaller flowers may be present.

The most common color of lilac flowers is pale purple—this is where we get the color name 'lilac'. There are cultivars available with white or pink flowers, or even purple edged with white. Victor Lemoine, a French plant breeder is credited with developing double-flowered cultivars which are commonly called French lilacs.

Following the flowers, a fruit consisting of a brown capsule is formed. As these fruits dry, they split in two and release their seeds, which readily germinate.

Lilac wood is very hard, and in Europe it was used for constructing some musical instruments, and knife handles. The genus name Syringa comes from the word 'syrinx' meaning hollow tube or pipe. This refers to the pith in the center of the stems of some species, which historically were hollowed out to make pipes and flutes.

Growing lilacs is best done in a sunny site that has well-drained, slightly alkaline soil. The good news is alkaline soil is pretty easy to come by in central Illinois, since many soils are high in clay, which is alkaline. The bad news is that many sites in central Illinois have poorly drained soil, since many soils are high in clay, which does holds water and prevents it from draining. Hopefully you can strike a happy medium in your landscape with some careful amending and attention to site selection.

Everyone wants to know how to make their lilac bush flower more. They are an extremely hardy shrub, often living for decades. Some have reportedly lived for centuries! This time of year I know we will get a few calls from people who have had a lilac in their yard "forever" but it doesn't flower anymore. And they want to know why.

There are a couple of possibilities. Lilacs flower on old wood. This means that they form next year's flower buds on this year's growth. If you decide to prune your lilacs, it needs to be done right after they flower. If you wait until later in the summer or fall, you risk pruning next year's flowers off!

After lilacs flower, they will produce seed pods if the flowers are not pruned off. Producing seed pods takes a lot of energy from the plant, if the flowers are never pruned after they have finished flowering, you may develop a cycle of one "good flowering" year followed by a "bad flowering" year because the plant is trying to recover from producing all those seed pods.

A lilac that is never pruned will often flower poorly, if at all, and will become leggy and sometimes even die out in the middle of the shrub. Lilacs produce suckers readily from the roots, and while in theory you could "renewal" prune it right to the ground, and have suckers sprout from the stump, most people don't go with this route.

Usually lilacs that have reached this stage are quite large, and taking the entire shrub down to a stump would leave a large unattractive hole in the landscape. A better strategy is to take out one-third of the largest stems at ground level each year for three years. By the end of the third year the whole shrub should be renewed, and should be flowering once more.

Another reason to stay on top of pruning your lilac is the lilac borer. This pest burrows under the bark of lilac, and can be devastating. I've seen it very commonly in shrubs that have lots of old thick branches that are growing poorly. Keeping your lilac growing vigorously with young branches is a cultural method to keep this pest at bay. If you have further questions on chemical control, feel free to give our office a call.

Lilacs may also have oyster-shell scale, an insect that looks like brown or grey warts on the stems. It is controlled by pruning out the affected branches, and applying dormant oil spray.

In late summer, I guarantee you someone will call me to ask about why their lilacs leaves look like they have white powder on them. This is a very common fungal disease called powdery mildew. It doesn't hurt the plant at all, it just bothers homeowners. You can reduce the extent of powdery mildew by pruning your lilac to allow good air circulation, but it may still develop. Luckily it doesn't tend to develop until late in the season, about a month short of when the shrubs naturally lose their leaves before winter.

Many cities across the U.S. host lilac festivals in the spring to celebrate their beautiful flowers. Lombard, Illinois, a western suburb of Chicago will host its 'Lilac Time' from May 3 through 18, 2008, with lots of celebrations, dances, fairs, a 5K and finishing with a Lilac Parade on May 18th. Go to http://lombardparks.com/ or call (630) 953-7300 Ex. 17 for more information.

Lilacs are a great addition to your home landscape. There are even some dwarf cultivars available, which makes it easy to fit one in your yard without having to give up a lot of space. I have discovered a reblooming cultivar called 'Josee' that blooms at least twice each year, once in the spring, and once in the fall. I have this lilac in my yard, and it does bloom just like the catalog said—one big bloom in the spring, and one in the fall. It also sends out sporadic blooms here and there through the summer. Its maximum height is four to six feet, so there's lots of potential for use in the landscape. I highly recommend this one!

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