Lilacs have been a staple of the home garden for hundreds of years, coveted by founding fathers such as George Washington and Thomas Jefferson. The history of this beautiful plant can be seen in many of its cultivars' names.
The common lilac (Syringa vulgaris) is also known as the French lilac because of important hybridizing work that was undertaken in France in the 19th and 20th centuries. This lilac, indigenous to Eastern Europe, is the image typically associated with lilacs. It has elegant, heart-shaped leaves and large, fragrant flowers, typically in white or purple.
Another popular lilac is the early-flowering hyacinth lilac (Syringa x hyacinthiflora). This is a cross between the common lilac (Syringa vulgaris) and an Asian species (Syringa oblata) that flowers up to 10 days earlier than the common lilac. It looks very similar to the common lilac and has fragrant single or double flowers.
Tree lilacs (Syringa reticulata) are wonderful disease- and pest-resistant plants which can grow up to 20 feet tall. They have creamy-white flowers that appear in late June.
The Meyer lilac (Syringa meyeri) and Manchurian lilac (Syringa patula) are two dwarf varieties that make excellent additions to garden beds and borders and are often used as screen or hedges. Ranging in height from 4 to 8 feet, they produce pale, mauve flowers in late May to early June. The popular cultivar 'Miss Kim' (Syringa patula 'Miss Kim') is one of the few lilacs that have a wonderful, fall leaf color: deep Burgundy. These dwarf varieties do not suffer from powdery mildew like the common lilac.
Lilacs grow best in sites with full sun and good drainage. Lilacs do not tolerate wet feet. If you have heavy, clay soil, amend the soil before planting by adding plenty of organic matter. When planting your lilac, dig a large hole that is 2 to 3 times as wide as the root ball and incorporate organic matter into the back-fill (the rule is to add ¼ to ⅓ of the amount).
Lilacs prefer soil with a pH between 6 and 7. If you need to raise your pH, lime your soil for the first few years in early spring or late fall, following directions on the label. Once lilacs are established, they are fairly indestructible, rarely needing supplemental water unless there is a drought. Fertilize once early in the season with a balanced organic fertilizer. Do not fertilize with a high-nitrogen fertilizer, otherwise you will get foliage at the expense of flowers.
Often lilacs do not flower for several reasons. The most common is not enough sunlight. Lilacs can survive in partial shade, but they will not be as floriferous or perform as well as they will with 6 hours of sunlight daily. Another reason is if the soil is too acidic and needs to be sweetened with lime. Poor drainage can also affect the health of the plant and its ability to flower. Finally, improper pruning is often a reason for poor flowering. If a lilac is pruned in early spring, many of the buds (and that year's flowers) will be sacrificed.
Lilacs flower on second year's wood. This means that shortly after the lilac flowers in late spring, the following year's flower buds start to form. Pruning to avoid loss of the following years' flowers ideally takes place immediately after flowering (within two weeks after the flowers fade).
Cut back some of the older stems to the ground (or as close as you can get). This will thin out the plant and encourage young, vigorous shoots to fill in. Shorten any tall stem back to a strong branch. Remember to cut back to an outward facing bud as with any pruning cut; your new branch will grow in the direction of the bud.
Many lilacs, particularly the common lilacs (Syringa vulgaris), produce a number of suckers and young shoots at the base of the plant. Selectively remove several of the shoots to decongest the plant and promote good air circulation. The smaller varieties such as Meyer lilac (Syringa meyeri), with its numerous smaller flowers, do not need deadheading.
To rejuvenate an old, overgrown lilac, you have two options. The drastic measure is to cut the entire shrub back to eight inches from the ground in late winter (February/March). The lilac will resprout in the first year. Thin out sprouts to form a good structure. It will take the lilac two to three years to regain its former glory and to flower. The cautious approach is to cut out ⅓ of the old wood of the plant every year for three years. Thin out older wood to create good circulation and an open structure. Shorten any large branches that are drooping or top-heavy by cutting back to a strong side-shoot. This gradual renovation pruning can take place either in late winter or immediately after flowering.
As long as lilacs have full sun, good drainage, and plenty of space to grow, they have few problems. However, some common problems you might find are lilac borers, oyster-shell scale and powdery mildew. For lilac borers (wilted leaves, swollen stems, and small entry holes) remove infected branches. Borers tend to attack old, thicker wood, so it is important to have stems of various ages on your lilac. Spraying with an insecticide that is labeled for the borer is helpful in the spring at the time the eggs hatch. Oyster-shell scale (brown scale-like bumps on lower stems of unhealthy plants) can be treated in the spring (when the crawlers are present) by spraying with horticultural oil. Powdery mildew primarily causes cosmetic damage and occurs late in the season when the blooms are long gone. This can be left alone, unless it creates an eyesore. Good cultural practices are the best way to prevent powdery mildew: ample sunlight, good drainage and good air circulation (i.e., proper pruning). Otherwise, try biologically friendly products such as Neem.