Syringa vulgaris ‘Sensation’ 1
What other plant captivates your senses and evokes fond memories of springtime more than lilacs? The intense fragrance of their large, beautiful flowers and their relative ease of care, make lilacs treasured throughout the temperate world. They bring us a few weeks of fabulous color and fragrance each year, but their loveliness and charm leave lifetime memories.
Syringa meyeri ‘Palibin’ lilac is considered one of the best dwarf lilacs on the market.1
Lilacs (Syringa spp.) come in all different shapes and sizes from dwarf forms only 4 to 5 feet tall, to lilac trees, 25 to 30 feet tall. Lilacs can be mixed in the border with other deciduous shrubs, conifers, herbaceous perennials, bulbs, ground covers and annuals to extend and enhance the season of color.
Tree lilacs make terrific, urban-tolerant street trees, which offer shade on hot summer days. Smaller, slower-growing lilacs are perfect for most residential landscapes. They can be used in shrub beds or as foundation plants. They can also be planted in containers and placed on decks and patios. Lilacs tolerate very cold winters, hot summers, humidity and almost any well-drained soil. They do require full sun for good flower bud set.
Care of Lilacs
Lilacs require at least six hours of direct sunlight a day during the growing season to properly set flower buds for the following spring. The amount of sunlight determines the plant’s appearance and quantity of flowers. Lilacs planted in too much shade will either flower poorly or not at all. Do not crowd lilacs because they will grow tall and leggy with sparse flowering. The planting site should be large enough to accommodate the full-grown root system and the mature height and spread of the plant.
Some lilacs are prone to diseases, especially powdery mildew, which appears as white powder on the leaves. Spores of the fungus are most active when the weather is hot and humid and the air is stagnant. Certain species and cultivars of lilac are more susceptible than others to powdery mildew. It is important when selecting and planting lilacs to choose resistant varieties and plant them in full sun with good air flow to minimize these disease.
Lilacs are tolerant of a wide range of pH and soil conditions, but require good drainage. Poorly drained soil will result in little growth, poor flowering and gradual deterioration. This decline occurs over several years. If the soil is poorly drained, consider improving it with the addition of topsoil or organic matter (peat, composted leaf mulch or compost) to the planting hole or plant in raised beds with good soil.
Water and Fertilizer
Newly planted lilacs should be watered two to three times a week for the first month. After the first month, they should be watered deeply once a week. During periods of hot, dry weather, watering may need to be done more frequently. Most trees and shrubs benefit from 1 inch of water a week during the growing season, but do not over water lilacs because root diseases may develop.
Do not fertilize newly planted lilacs. Plants first need to establish a developed root system to support further growth. After this time, perhaps for two or three years, fertilization may be needed if the plant does not begin to grow more vigorously. A soil test should be performed to determine if phosphorus (P) and potassium (K) are limited. If so, a complete fertilizer such as 5-10-5 or 10-10-10 may be applied at the base of the plant, following labeled application rates. Do not over fertilize or use fertilizers containing high levels of nitrogen (N). This can cause excessive shoot and foliage growth at the expense of flower bud development. Apply any necessary fertilizer after the spring- flowering period.
Add a 2-to-3 inch layer of loose mulch around the base of the plant to help retain the soil moisture, keep the roots cool and suppress weed development. Take care to keep the mulch away from the trunk of the tree or basal portions of shrub stems to enable good air circulation. Otherwise, disease infections, pest infestations and damage by rodents may occur where the mulch is piled to closely to the bark.
Pruning lilacs will depend on bloom period, growth pattern and location. Newly planted lilacs will not need much pruning for the first two to three years. Established lilacs are usually best pruned during the late dormant season, typically March or early April. This time of year allows for ease in pruning because you can see what you are doing, there is less insect and disease activity and pruning wounds close more quickly with the onset of spring growth. However, pruning at this time will sacrifice some display because you are removing flower buds. Pruning can also be done immediately after flowering, if you do not want to sacrifice any blooms. However, if you wait too long into the summer to prune, you will remove next year’s flower buds.
Syringa vulgaris ‘Frederick Law Olmstead’ 1
Lilacs that sucker readily, such as common and early lilacs (S. vulgaris and S. xhyacinthiflora) should receive renewal pruning about every two to three years. Older, larger diameter branches tend to have reduced vigor and produce fewer flowers concentrated mainly at the tops of the branches – too tall to enjoy their beauty or fragrance. Larger branches are also more prone to lilac borer infestation at their base. This insect makes its way into the branch cambium and wood. As the insect eats the wood, the branch becomes weaker, leaves yellow and the branch begins to die. Few, if any, flowers are produced the following years. The best way to control this insect pest is to periodically cut the infested, weakened branches out.
Renewal pruning involves removal of around one-third of the largest diameter branches down to ground level with a pruning saw or loppers. Removal of these larger branches greater than 1 inch in diameter promotes new shoot development at the base of the plant.
Renewal pruning allows the lilac to continue to flower vigorously each year and maintains the size of the plant. Vigorous young growth generally produces larger and more numerous flowers compared to the older, larger diameter branches.
Prompt deadheading of faded blooms will improve a plant’s appearance and help the lilac concentrate its energy into flower bud formation and not on seed production. For smaller lilacs that do not sucker, renewal—pruning is unnecessary, only annual shaping of the plant may be needed.
For those interested in learning more about lilacs, consider joining the International Lilac Society (ILS) at internationallilacsociety.org.
1. Photo courtesy of Laura G. Jull.
2. Photo courtesy of Knight Hollow Nursery.
From State-by-State Gardening May/June 2013.