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Growing Lilacs in Montana
by Martha Mikkelson, plant disease diagnostician; John Maatta, Liberty County Extension agent; and Bob Gough, Extension horticulture specialistNow is the brief season of the lilac bush, modest and enduring
symbol of ... depth and permanence... To what strange and distant homes
have its roots been transplanted, there to grow, blossom and, in their
turn be abandoned again...Were it not for the steadfast lilac, there would
be nothing to mark that here once dwelt human souls who shared happiness,
sorrow, hope, and despair."
--Editorial, the Rutland Herald (Rutland, Vermont), 1929
Lilacs are the beacon of spring to many Montanans. The earliest homesteaders brought lilacs to the state and found them to be one of the few exotic flowering shrubs to thrive on the Great Plains. Even today, many of these hardy shrubs survive next to long-abandoned homesteads.
The most well-known lilacs belong to the common lilac species (Syringa vulgaris), which includes "old fashioned" lilacs and the many French hybrids. However, there are 23 lilac species and many hybrids, none of which are native to North America. Two species, Syringa vulgaris and S. josikea, are native to Eastern Europe; the rest come from Asia. Lilacs were brought to New England from Europe perhaps as early as 1620.
Today's lilacs are very different from the ones early settlers had. Flowers are larger and more diverse in color and form. Plant size ranges from five-foot tall dwarfs to 30-foot tall tree forms.
About lilacs
Hardiness of lilacs
Lilacs are adapted to USDA Hardiness Map zones 3, 4, 5 and milder areas of zone 2. They thrive in sunny sites with good air circulation. Although they can with stand -40 degrees F, they need protection from cold winds that can kill flower buds. Lilacs will not tolerate poorly drained sites where the roots freeze in blocks of ice during the winter. Do not plant them in "frost pockets" or against warm, light-colored buildings that force the buds prematurely. Lilacs on their own roots are far more hardy than grafted lilacs.Lilac blooms
Modern lilacs are divided into seven standard color groups: white, violet, blue, lavender, pink, magenta and purple. Colors are most intense during cool, damp springs. Often the buds and open flowers are of different colors. This "unfolding of the colors" is part of the captivating charm of lilacs.The date of common lilac bloom is a good predictor of whether spring is early or late in a particular year. Bloom date depends on average spring temperatures (growing degree days), although the different lilacs bloom in a fairly reliable chronological order (i.e., common lilacs always bloom before Preston lilacs). From the beginning of bud color to the fading of the flowers, common lilacs typically bloom for 16 to 20 days, the longest bloom period of all lilacs. If you plant several species in your garden, various lilacs can be in full bloom for at least six weeks.
Lilac fragrance
Lilac fragrance varies considerably with species and cultivar. Even on the same shrub, fragrance depends upon stage of bloom, time of day and temperature. Lilacs are most fragrant on a warm, sunny afternoon when the florets are fully open.The lilac scent familiar to most Americans is that of the common lilac. However, cultivars of common lilacs differ widely in amount of fragrance, with many newer cultivars having little fragrance. Small, dark, single florets generally are more fragrant than larger, lighter, double ones, although there are exceptions. Some of the most fragrant cultivars are noted in Table 1.
Table 1. Sequence of lilac bloom
Early: Hyacinthiflora lilacsMid-season: Meyer lilac; common lilac and French hybrids; Chinese lilac, Manchurian lilac ('Miss Kim')
Late blooming: Hers lilac, Preston Hybrids, Late lilac
Very late blooming: Japanese tree lilac, Pekin lilac
Asian lilacs have different fragrances, many of which are heavy, "spicy" and pleasant.
Fall color
Most lilacs have little fall color. Exceptions include the Manchurian lilac ('Miss Kim') with its burgundy fall color, and the Meyer lilac with its reddish-brown autumn leaves.Choosing which lilacs to plant
With approximately 2000 cultivars to choose from, selecting lilacs to plant is not easy. All of the lilacs described here are hardy in Montana, although the Preston hybrids generally flower more reliably in colder, windy areas than do the early-blooming Hyacinthifloras. By planting lilacs from several different groups in your garden, you can extend the blooming season and create more variety in flowering as well as in shrub form, leaf texture, and fall color.Hyacinthiflora lilacs (S. vulgaris x S. oblata)
Hyacinthifloras are very vigorous hybrid lilacs that look like French hybrids but bloom seven to 14 days earlier. The shrubs themselves are very hardy, but since they are early bloomers, they should be planted in sites where May frosts are less frequent. Most cultivars flower freely and are very fragrant. Some have good fall color. Many grow to 10 to 12 feet tall, but there are some dwarfs (denoted by *). Cutivars include 'Assessippi,' 'Bountiful,' 'Churchill,'* 'Dr. Chadwick,'* 'Maiden's Blush,' 'Mirabeau,' 'Pocahantas,' and 'Sunset.'Meyer lilac (dwarf Korean lilac) (Syringa meyeri)
The Meyer lilac is a spectacular and unusual spreading shrub that grows only four to five feet high. Suckers allow it to spread from six to 10 feet wide. This lilac flowers abundantly with heavy, spicy fragrance even when very young. Because the Meyer lilac flowers before the leaves are fully developed, it is beautiful when planted in a shrub border with an evergreen background. 'Palabin' is a popular cultivar for low hedges. It also can be used as a focal point in a rock garden or where space is limited. The fall color is a nice reddish brown. Meyer lilac is resistant to powdery mildew.
European common lilac and French hybrids (Syringa vulgaris)
The European common (or "old - fashioned") lilac is the best known lilac. Common lilacs have either purple or white flowers and account for the majority of lilacs planted in Montana. These lilacs are fragrant, very hardy, quite drought tolerant and inexpensive. In general, they sucker more than French hybrids, a characteristic that has allowed them to persist on abandoned homesteads. They are used commonly in shelterbelts and as hedges.The French hybrids are so-called because the Lemoine family of Nancy, France, initiated a lilac breeding revolution that began in the 1870s and has resulted in about 1,500 different cultivars today. These cultivars represent a wide spectrum of flower color, size, shape and fragrance, as well as differences in shrub size and form. As a group, S. vulgaris cultivars are susceptible to more diseases and pests than other lilacs, although these typically cause fewer problems in Montana than in wetter, more humid climates.
Table 2 summarizes characteristics of some of the best and most widely available French hybrids. (This chart is in PDF format, viewable with Adobe Acrobat software.)
Chinese lilac (Syringa x chinensis)
The Chinese lilac is an arching, spreading shrub growing 10 to 12 feet tall and six to eight feet wide. Compared to the common lilac, the leaves are smaller and the flowers are more delicate and abundant. Very fragrant flowers bloom with or just after common lilac. Chinese lilacs do not produce many suckers, so they stay "in bounds" more easily than do common lilacs. However, some winter injury can occur during very severe winters or in colder areas of Montana. Cultivars include 'Alba' (white to very light pink flowers) and 'Saugeana' (lilac-red flowers). This is one of the best lilacs for taller hedges.Manchurian lilac (Korean lilac) (Syringa patula) 'Miss Kim'
The Manchurian lilac is rare among lilacs in that it provides four seasons of interest. Abundant, deep purple buds open to uniquely fragrant, purple-blue flowers. Flowering occurs shortly after the common lilac season. Leaves are an attractive and glossy, dark green in summer and turn burgundy in autumn. The branches form a pleasing winter habit. The best cultivar, 'Miss Kim,' is a vigorous, upright shrub that grows from five to seven feet tall. This cultivar is excellent for massing or hedging. 'Miss Kim' resists powdery mildew.Hers lilac (Syringa julianae var. 'Hers')
The rare and unusual Hers lilac grows up to five feet tall and 10 feet wide in a weeping form. This unique lilac can be trained to grow on a trellis. It is hardy to -30 degrees F and generally avoids spring frosts, since it blooms later than common lilac. The small leaves and fragrant, lavender-purple flowers occur abundantly on delicate branches. It resists lilac borers and other pests.Preston hybrids (Preston lilac, Canadian hybrids) (S. reflexa x S. villosa)
Preston hybrids are very valuable in areas where spring frosts often freeze the flowers of earlier blooming lilacs. Prestons bloom about a week after common lilacs. Most cultivars have pink or lavender flowers with a unique scent. They grow quickly and have showy blooms and large, dark green leaves which are mostly free of diseases and insects. Among the best cultivars are 'Agnes Smith,' 'Donald Wyman,' 'Isabella,' 'James MacFarlane,' 'Minuet,' 'Miss Canada,' 'Pauline' and 'Ursula.'Left to themselves, Preston hybrids can become rampant growers. They do not sucker, but grow strongly to 10 to 12 feet high. As shrubs, Prestons are effective along backyard property lines, as corner accents or along garage walls or service areas. Due to their size, they should not be planted too closely together or planted in small areas. However, certain pruning techniques can allow them to be used where space is a concern. By pruning them to three to five upright trunks while still young, they become wonderful small, spreading trees. When planted near small patios or decks, pruned Prestons provide excellent shade and focal beauty.
Late lilac (Syringa villosa)
Late lilac grows up to 12 feet tall and wide. Its pink flowers emerge a week after common lilacs and smell like cloves. Although used for backgrounds, hedges and mass plantings in large areas, its greatest value has been in hybridizing the Preston lilacs.Japanese tree lilac (Syringa reticulata)
The June blooms of the Japanese tree lilac bring the lilac season to a brilliant close. This small, vase-shaped tree has a refined beauty that is rare among trees that will survive in Montana. The form is graceful with and without leaves, and the showy cream-colored flowers smell like honey. The glossy, reddish-brown bark is similar to cherry bark. Japanese tree lilac has little fall color, but the seed heads persist into winter and compliment the branching pattern and bark.This hardy tree grows slowly and may reach 25 feet high and 15 feet wide in 30 years. Japanese tree lilacs can be grown as single-stemmed or multi-stemmed trees. The single-stem form makes an excellent small shade or street tree. The multi-stemmed form can be a fine focal point in larger areas. 'Ivory Silk' is similar to the species except the crown form is more narrow. This lilac resists powdery mildew, scale and borers.
Pekin lilac (Syringa pekinensis)
The Pekin lilac grows to a 15- to 25-foot tree form and differs from the Japanese tree lilac in its more vigorous growth and interesting peeling bark. However, Pekin lilac often does not flower for many years after planting.Obtaining lilac cultivars
Many of the lilacs and cultivars listed above and in Table 2 (PDF document vieweable with Adobe Acrobat) are available through Montana nurseries. If you are interested in obtaining a lilac that you cannot find locally, several U.S. nurseries specialize in lilac production. Contact the International Lilac Society at 11 Pine Street, Dixfield, ME 04224 or 1-207-562-7453 for more information.A book with excellent information and color photographs of many different lilacs is Lilacs: The Genus Syringa by John L. Fiala (1988, Timber Press, Portland, OR).
Lilac Cultivation
Planting location
Plant lilacs where they will receive at least six hours of sunlight each day. Shade greatly reduces flowering, causes plants to become leggy and increases powdery mildew problems. Dark-flowered lilacs seem to tolerate dappled shade better than lighter ones. However, do not plant dark-flowered lilacs on sites that receive hot afternoon sun, which fades the flowers quickly. Avoid windswept locations and warm areas near reflective, light-colored buildings where the buds will be killed or forced prematurely.Neutral soil is ideal, but lilacs will tolerate soil with a pH of 6.5 to 8.5 if it is well-drained. Soil type sometimes alters flower colors so arboretum specimens may not look the same in your garden.
For specimen plants, plant lilacs 10 to 15 feet apart. Spacing of plants for a hedge depends on the hedge height. For a three- to four-foot tall hedge, space plants 18 to 24 inches on center. A six- to eight-foot tall hedge requires spacing of two to three feet on center. For a very tall, informal hedge, spacing could be as far apart as six feet. Plant lilacs at least five feet from buildings, since their root systems can damage foundations over time.
Mulch an area from the base of the lilac to the drip line to reduce weeds, conserve moisture and prevent lawnmower damage. A two- to three-inch deep bark or wood chip mulch may be better than brick chips or stone, since the latter retain heat and may damage the roots in hot weather.
Watering
Lilacs need approximately one inch of water per week during the months of June and July. This is when buds for the next year are being formed. Begin decreasing irrigation in early August to encourage the shrubs to harden tissues for winter. New transplants need occasional water during this"taper off" period to prevent wilting. Deep water lilacs in late fall after they are dormant to ensure they have adequate soil moisture during winter.
Fertilization
In general, lilacs in Montana do not need fertilizers, although low rates of phosphorous and potassium sometimes are beneficial. Most soils contain enough nutrients for adequate growth and flowering. Nitrogen actually can harm lilacs for two reasons. First, nitrogen encourages plants to produce more leaves and shoots and less blooms. Second, succulent new shoots often cannot harden off sufficiently, and winter damage results.Fertilizing lilacs within two years of transplanting can harm the roots. However, if plant growth remains weak after two years, and soil tests indicate nutrient deficiencies, apply about one pound of 5-10-10 or equivalent fertilizer to each mature bush in early spring. Spread the fertilizer around the shrub in a broad band beneath the dripline. Use no more than a small handful of fertilizer on young plants.
Pruning and rejuvenation
New shoots that continually sprout from the roots allow lilacs to remain young and in prime flowering condition for many years. Suckers are such effective rejuvenators that lilacs planted centuries ago are still thriving today. These include lilacs planted in 1650 in Mackinac Island, Michigan and lilacs planted in Portsmouth, New Hampshire about a century later.Newly transplanted lilacs usually need no pruning for five years or until they are six to eight feet tall. After that, regular pruning is essential to maintain healthy flowering. An ideal lilac is one growing on its own roots with seven to 12 stems, all of different ages and thicknesses. Stems of one to two inches in diameter produce better blooms than stems that are three or more inches thick. The branches should not cross each other, and they should produce flowers all over the bush, not just at the top. If left to themselves, many lilacs will reach a height of 18 feet with a similar spread. However, a height of eight feet can be maintained with regular stem renewal and pruning.
The best time to prune is immediately after flowering, since the flowering buds for the next year are produced in June and July. Prune out all diseased canes, old and declining stems, thin suckers, and twiggy small branches.
To rejuvenate overgrown or declining lilacs, cut to the ground one-third of the largest trunks each year to encourage the growth of new shoots from the base. Over a three-year period, the lilacs will rejuvenate without a complete loss in bloom or canopy. Pruning in early spring when leaves are absent will make it easier to see which stems need to be removed.
Older literature recommends removing dead flowers (deadheading) immediately after flowering to prevent energy from going into seed production. On established lilacs, deadheading does not improve flowering significantly and is very labor intensive. However, we do recommend deadheading young lilacs since this encourages them to bloom well at an earlier age.
Propagation
When buying lilacs, ask how the shrubs were propagated. Lilacs grown on their own roots (through layering, cuttings or tissue culture) are superior to grafted or seed-grown lilacs. Grafted lilacs grow quickly at first, but as the stems thicken, they become difficult to rejuvenate. Sometimes the rootstock (which is commonly privet, but sometimes S. vulgaris) produces suckers that overtake the desirable cultivar scion, resulting in an entirely different shrub.If there is a non-grafted mature lilac you particularly admire, ask permission to remove several rooted suckers from the shrub. Plant these and the resulting shrub will be a clone of the mother shrub.
Why lilacs fail to bloom
Some lilacs begin to bloom within two years of planting, while others take five years. Heavy blooms normally occur only every other year as with many woody flowering plants. However, several practices can optimize blooming:1. Plant lilacs in full sun. Lilacs will never bloom well in the shade.
2. Lilacs need well-drained soil and will not tolerate standing water.
3. Prune lilacs immediately after blooms fade since buds for the following year are produced in June and July.
4. Overgrown lilacs with many thick, older branches and rampant suckers do not bloom as well as ones with a few well-maintained branches of younger ages. See pruning section.
5. Lilacs rarely need fertilizer. High nitrogen fertilizers favor leaves over flowers and may increase winter injury and bacterial blight.
6. Deadheading often improves flowering on young lilacs. See pruning section.
Diseases and insects
Bacterial blight (Pseudomonas syringae pv. syringae)
Bacterial blight causes brown leaf spots (often surrounded by yellow areas) and rapid browning of young shoots. Young tissues are more severely infected. The lilac pathogen also attacks pear, cherry, maple and other ornamentals.As with most bacterial diseases, mild, moist weather favors lilac blight. Factors that weaken or injure plants make infection more likely. These include wounds, frost damage, improper nutrition or other diseases or insects. The bacteria spread via wind, rain, insects, tools and infected nursery stock.
Most common and French hybrid lilacs are susceptible, and some experts say that white-flowered lilacs are most susceptible. The following cultivars may be less susceptible: 'Edith Cavell,' 'Glory' and 'Ludwig Spaeth.'
Adequate plant spacing and pruning to allow good air circulation can minimize blight. Prune out and burn all affected tissues as soon as you see them. After each cut, sterilize pruning sheers in rubbing alcohol or diluted bleach (one part bleach to nine parts water). When spring weather is favorable for infection, Bordeaux mixture or copper fungicides can be used as protectant fungicides just before bud break. Always follow instructions on the pesticide label.
Powdery mildew (Microsphaera syringae)
Powdery mildew forms a grayish-white powdery mat on leaves. Infected leaves may turn yellow and fall off. New growth often is stunted. Hot, humid weather fa vors mildew. Generally, mildew occurs in late summer and does little harm. Most French hybrids are very susceptible, but Manchurian lilacs ('Miss Kim'), Meyer lilacs and Japanese tree lilacs are quite resistant to mildew.To prevent mildew, do not plant lilacs in the shade! Space and prune shrubs properly to allow good air circulation. High nitrogen fertilizers favor the disease, so minimize their use. Fungicides usually are not needed. However, if mildew becomes a yearly problem, contact your county Extension agent about fungicide use.
Lilac borer (Podosesia syringae)
Lilac borers are most common east of the Rockies. These moths resemble wasps as adults because of their brownish, clear wings. The moths fly to lilacs and lay their eggs on trunks and branches in late spring. Near wounds and cracks, larvae tunnel into branches, feeding on sapwood and heartwood. They cause branch tips to wilt in late summer, especially when weather is warm and dry. Lower branches swell and crack, and sawdust appears around the borer holes. Affected branches may die or break.Lilac borers are more common on stressed plants. Adequate watering and sufficient nutrients can reduce stress and future attacks. Fresh pruning wounds attract egg-laying adults. In infested areas, do not prune before the moth flight period. Instead, prune infested branches in the fall. Pheromone traps can be used to monitor flight periods using a standard clearwing borer lure to attract this species. Insecticides can be applied to the trunk two to three weeks after the first moths are captured. Reapplication may be needed if heavy flights continue for one month after application or if borers injured plants previously.
Lilac leafminers (Caloptilia syringella)
Lilac leafminers occur most commonly in western Montana. Leafminer damage first appears as light green blotches on the leaves in early summer. The outer cell layer of the leaf separates easily from other layers as a pale yellow larva up to 1/3-inch long grows inside. When infested heavily, nearly every leaf turns brown, causing the shrub to appear burned.There is some evidence that neem, a least-toxic option for leafminers, has some systemic activity. Diazinon and Orthene are labeled for leaf miner controls on lilacs. Spray when blotches are first noticed.
Scales
Scale insects cover lilac trunks and branches with scaly bumps that can be scraped from the branch. The hard covering protects the insect underneath. Scales extract fluids from the plant, resulting in stunting, leaf yellowing and premature leaf drop. Scales may infest lilac, ash, maple and many other trees.Depending on the species, scales lay their eggs on the bark in the fall or spring. Eggs hatch in late May or June. Young scales (crawlers) are small (1/10 inch), mobile and pale yellow or orange. Later in the season, their legs wither and a hard shell develops over the body. Beneath the shell, they continue to feed, and the females lay eggs.
A good planting site and adequate watering and nutrients promote healthy growth that helps lilacs resist scale infestations. Prune heavily infested branches. The most effective chemical controls are applied to the crawler stage of the insect. Carbaryl (Sevin), chlorpyrifos (Dursban), diazinon, orthene, bifenthrin (Talstar), and cyfluthrine (Tempo) are available as crawler sprays. Lilac is prone to phytotoxicity from some insecticides. Follow label directions carefully.
Insecticidal soap or oil sprays, applied at three- to four-day intervals during the crawler stage, can be effective also. Dormant oil sprays have not been as effective on oystershell scale because overwintering eggs are well protected by the hard scale covering. They work better when applied in spring after the egg covering has weathered.