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Lilacs- Selection & Pruning
Lilacs are one of the most recognized and best loved shrubs found in Nebraska landscapes. It’s also one example of a plant that performs wonderfully in northern gardens, hardiness zones 3-7 for most species, but lacks vigor and does not flower reliably in southern gardens (Zone 8 and higher).
Lilacs are easy to grow and tolerate much abuse, particularly S. vularis. As with most blooming plants, lilacs prefer full sun locations and well drained soil. Plants in shady locations frequently struggle with powdery mildew fungus each year. Old flowers should be cut off as soon as flowers fade. Annual pruning helps to keep plants vigorous and blooming profusely.
Common species of lilacs include the following:
- Syringa meyer , Meyer Lilac (often called Dwarf Korean lilac) - a dense, neat lilac with a good uniform outline. Violet-purple flowers produced in May are softly fragrant. Literally covered in flowers when blooming. Height 4-7 feet, width 5-6 feet. ‘Palibin’ is a compact form that grows 4-5 feet high and 5-7 feet wide with reddish-purple flowerbuds that open to whitish-pink flowers.
- Syringa microphylla , Littleleaf Lilac- smaller leaves result in a finer textured shrub, when compared to S. vulgaris. A handsome, broad-spreading shrub grows 6 feet tall and 9-10 feet wide. Fragrant, rosy-lilac flowers. Works well in a shrub border, groups or as a hedge. ‘Superba’ had single, deep pink flowers and is quite floriferous.
- Syringa persica , Persian Lilac- a graceful shrub with upright, arching branches reaching 4-8 feet in height and 5-10 feet in width. Dark green foliage and pale lilac, fragrant flowers produced in May. A nice small lilac with a mass of flowers in season.
- Syringa reticulata, Japanese Tree Lilac- a large shrub or small tree form of lilac reaching a height of 15-20 feet, and width 15 feet. Stiff, spreading branches develop an oval to rounded crown. Creamy, white, fragrant flowers are produced in June. An excellent specimen tree and small enough to plant under utility lines. If allowed to develop into a tree, annual pruning is not necessary.
- Syringa villosa , Late Lilac- blooms in late May or early June, later than other lilacs. A bushy shrub of dense, rounded habit with stout, stiff upright branches. Many cultivars are available with flower colors including pink, dark purple-red, violet, rosy lilac and white. Not as fragrant as S. vulgaris. Height 6-7 feet, width 6-8 feet.
- Syringa vulgaris , Common Lilac- the most common form of lilac grown in Nebraska, forms an upright shrub with a height of 8-15 feet and width of 6-15 feet. Blooms in May with extremely fragrant flowers. Well over 400 cultivars are available including a wide range of colors and flower forms.
If you would like to add a lilac to your landscape, it’s worth your time to visit the Lilac Collection at the Yeutter Gardens on the University of Nebraska’s East Campus. There you can view over 50 cultivars and species of lilacs to consider, while strolling through the gardens with an ice cream cone from the nearby UNL Dairy Store. The best time for viewing most lilacs in bloom is from late April through mid-May.
Pruning The Common Lilac
Common lilac, Syringa vulgaris, is one of the most popular blooming shrubs in the home landscape. And, as with other blooming shrubs, it is best pruned right after it is finished blooming- meaning the best time for pruning common lilac is now!
There are several reasons for regular lilac pruning. First, removing the faded flower heads keeps your shrub looking neat and tidy. Also, as young plants develop, they often require light pruning to develop an attractive shape. If left unpruned for many years, lilacs will develop in a way that shades out the lower stem sections resulting in large, overgrown specimens that are leggy and unattractive. These thick, heavy stems are very attractive to lilac borers, which will tunnel through the stems. Younger, thinner stems are more healthy and vigorous, and produce more flowers than old, overgrown lilac "trunks". Finally, maintaining your lilacs through regular, light, yearly pruning is less stressful to the plants than an infrequent, severe pruning.
Pruning Young Lilacs
To begin pruning young lilacs, first remove the spent flower clusters, then lightly prune long stems to create a pleasant shape. Always prune stems to an outward facing bud or shoot. No additional pruning is usually needed for the first 3 to 4 years.
When the shrub is approximately four years old, begin removing a few of the oldest, thickest stems each year cutting them down as close to the ground as possible. If additional height removal is needed after removing these large stems, then lightly prune the tallest branches down to the desired height.
Selective removal encourages the plant to send up new, young stems from the crown each year, and prevents the development of old, woody stems. This pruning technique will create a nicely shaped plant and prevents your plant from becoming overgrown.
Rejuvenating Old Lilacs
Old, neglected lilacs can be renewed or rejuvenated by pruning, and home gardeners can choose between two different pruning methods.
Method One requires a commitment to rejuvenate your old lilac over a three-year period. Since lilac wood needs to be 3 or more years of age before it blooms, this pruning method allows you to enjoy flowers every spring.
Begin the first year by removing one third of the largest, oldest stems at ground level in late winter. The following year, again in late winter, prune out one half of the remaining old stems. Finally, in late winter of the third year, remove all of the remaining old stems. Additional thinning of the new shoots should also be done as needed if the new growth is too dense.
Method Two is more severe and will prevent blooming for several years. Step 1- Renovate your overgrown lilac by cutting the entire plant back to within 6-8 inches of the ground in late winter, preferably March or early April. This severe pruning will induce a large number of shoots to develop during the following growing season.
Step 2- In late winter of the following year, select among last year's new shoots and retain several strong, healthy shoots to form the shrub framework. Approximately 6 to 12 stems per plant is ideal. Remove all other shoots at ground level. Cut back the retained shoots just above a bud, removing a few inches of growth, to encourage branching.
When properly pruned, an old, overgrown lilac can be transformed into a vigorous attractive shrub within a few years. Once rejuvenated, begin an annual maintenance program of pruning for your shrub by removing a couple of the oldest branches every year.