Dr. Leonard Perry, Extension Professor
University of Vermont
Lilacs are ubiquitous in New England in the spring, great in landscapes as well as cut in vases. They are an historic plant, found around many old homes and even surviving around old foundations after the home is fallen down. They are easy to grow, require little care, have a famous fragrance, and come in variations of red, pink, white, and purple. Even though blooms only last a couple weeks, with a diversity of different plants from the various species you can have blooms over a 6-week period.
The two main requirements for lilacs to succeed are a well-drained soil and full sun. They will tolerate some shade, just won’t be a dense nor bloom as well. Once established, they will even tolerate dry soils and drought.
After planting you may want to water with some liquid fertilizer, or use fertilizer stakes you can buy as such. Once established, fertilize lightly each year if at all. In good loamy soils, or with some compost placed around plants, they may need no fertilizer. If fertilizing, do so right after bloom. Too early and flowers may abort with just leaves coming out. Too late and plants may not harden properly for fall. Around the fourth of July is about as late as you should fertilize, or for that matter prune.
You should plant lilacs where you can appreciate their informal upright natural shape. If you want neatly rounded shrubs, look for other plants with this habit. A couple of dense and rounded exceptions are the single purple Paliban Korean lilac (Syringa meyeri) that gets 4 to 5 feet tall and a bit wider, and the single violet ‘Miss Kim’ (S. patula) that is similar. If you need to prune branches that are obstacles, or crossing and rubbing on each other, right after bloom is best.
When it comes to pruning, experts have a couple of opinions you can choose from. Some (such as myself) only prune branches as needed, eventually removing about a third of old branches each year. This allows the plant energy to go into the new branches. Eventually most lilacs will get tall (maybe 8 to 15 feet), with most the flowers at the top. This makes them hard to see close up, but fine from home windows, the street, or a distance. Plus, with some pruning of lower branches you can appreciate the attractive stem architecture.
Others like to prune about a third of new growth off each year, but do so pruning back to sideshoots, not shearing as a hedge. This keeps plants and their flowers lower, but sacrifices the natural shape and effect of the stems.
Make sure when planting you allow plenty of room for the mature height (and width) of a lilac, otherwise you may need to “basal prune” all stems back near the ground, and then wait a couple years for new shoots to come up and start getting a few feet tall.
Lilacs are often seen near building foundations, especially good near corners. They make great specimens in lawns and borders, and planted in a line make a good seasonal hedge.
The main problem you may see with lilacs if your site has late morning dew and little air circulation, is the white powdery mildew disease on leaves. It is more an aesthetic issue, not causing enough harm to plants to really warrant treating with sprays. In some really wet springs some branches may all of a sudden wilt, with black tips. This is a blight which should only come once, with new buds emerging from stems in a few weeks. Occasionally a lilac may get small rounded brown bumps, or scales, which can be treated with a spray or infected branches cut off.
Most lilacs gardeners are familiar with are the common lilacs (S. vulgaris) and their cultivars (cultivated varieties) that bloom in mid to late May in the north. Some that bloom just a bit earlier are the hyacinthiflora hybrids, first bred by the famous Lemoine nursery in France in 1876. Examples of these are the single purple ‘Pocahontas’, the single white ‘Mount Baker’, the single blue ‘Blanche Sweet’, or the single magenta ‘Asessippi’.
As you see, lilacs come in more than the color “lilac” and white. Lilac specialists have grouped the over two dozen species and hundreds of cultivars into 7 flower colors: white, violet, blue, lilac, pink, magenta, purple. For each of these there are singles and doubles. In addition there is the single yellow ‘Primrose’, and the bicolor ‘Sensation’. The latter arose as a mutant in a Dutch greenhouse (from the lavender ‘Hugo de Vries’ that was being forced to flower at Christmas) in 1938, and has purple petals each in white.
The other large group are the late lilacs, mainly the Preston hybrids originally bred by a Canadian breeder by that name. These may not have the wonderful fragrance of the common lilacs, but bloom a week or 10 days later and tend to be larger in all respects– leaves, flowers, and wider plants. A few of my Preston favorites are the deep pink ‘Miss Canada’ and ‘Donald Wyman’, and the white ‘Agnes Smith’.
I often get asked what is my favorite lilac. It is hard to answer as so many, in fact most including the common species, are beautiful. The one that stands out for me and many though is ‘Krasavitsa Moskvy’, or as many know it Beauty of Moscow. It was selected by a famous Russian breeder in 1947 from an offshoot of ‘Belle de Nancy’– one of the French Lemoine hybrids. The pink buds open into creamy white flowers tinged with pink, a silvery opal color.