The Katie Osborne Lilac Garden

At the Arboretum

The Katie Osborne Lilac Collection in the Lilac Dell is one of the most diverse. A popular seasonal attraction, it provides visitors with weeks of delightful springtime colour and fragrance. Visit during the Lilac Festival (late May) and enjoy special tours and demonstrations.

Once located on the eastern shore of Cootes Paradise, the lilac (Syringa) collection comprising 230 plants was moved to the Arboretum in 1959 to accommodate the development of Highway 403.  By 1964 Colin Osborne of Hamilton established the Katie Osborne Lilac Trust to honour the memory of his wife with the intention of expanding and maintaining the collection for the future. As of today the collection displays over 745 plants meaning RBG’s Lilac Collection is one of the most diverse to be experienced anywhere on the planet

Lilacs are deciduous shrubs grown for their showy and fragrant blooms in spring. Plant lilacs in well-drained soil in a location that receives full sun for six to eight hours a day. Some cultivars (cultivated varieties) with dark purple flowers will appreciate some shade in preventing flower colours fading. Lilacs appreciate good air circulation as this is beneficial in combatting diseases such as powdery mildew which is non-life threatening but unsightly. Prompt removal of faded flowers (termed thyrses) before seed set is critical as this will increase bloom in the following year. Pruning immediately after flowering should remove dead, diseased and rubbing wood and encourage air flow through the plant. Maintaining lilacs at a height of six to nine feet keeps shrubs manageable and maintains the fragrant flowers at about head/nose height. To renovate an old lilac or one that is in bad shape remove one-third of the oldest wood down to the ground in year one, half the remaining wood in year two and the remainder in year three. Lilacs may easily be transplanted and moved. The best time to do this is August when the plants are dormant retaining as much of the plants root ball as possible.


At the current time it is thought that approximately 21 different species of lilac can be found in the wild. The geographic range of the majority of wild species includes eastern Asia particularly Japan and China with two lilac species native to Europe. Of the European species Syringa josikae inhabits parts of Hungary and Romania whilst Syringa vulgaris is native to the Balkan mountain regions. It is this latter species and its cultivars which have captured our imagination and become the most beloved lilac as a garden introduction. As a result, cultivars of Syringa vulgaris form the basis of RBG’s Lilac Collection.

Cultivars of common lilac (Syringa vulgaris) are often referred to as single or double flowered French hybrids. This is in reference to the prolific hybridizing work of Victor Lemoine and his sons who had a nursery based in the Lorraine province of France. They worked with many different plant genera creating spectacular new cultivars that have lasted the test of horticultural time. Lemoine’s work with lilacs is of particular importance hence the reference to single and double French hybrids. Look for examples of his plants in the Lilac Walk amongst the single and double French hybrids section. Common lilac cultivars are considered to be mid-bloom season lilacs.

Not content with single and double French hybrids Lemoine added the species Syringa oblata to his collection and began crossing this plant with Syringa vulgaris. The result was the introduction of the hyacinth lilac (Syringa ×hyacinthiflora and cultivars). Manitoba lilac breeder Frank Leith Skinner also contributed to hyacinth lilacs. In this section of the Lilac Walk look out for S. ×hyacinthiflora ‘Maiden’s Blush’ of which the fragrance is sublime. Hyacinth lilacs are considered to be early bloom season lilacs.

Preston lilacs (Syringa ×prestoniae) are Canada’s contribution to the lilac world. Isabella Preston, Canada’s first female hybridizer (after whom these lilacs are named) crossed Syringa villosa and Syringa komarowii to create a whole new group of late blooming (Villosae Group) hybrid lilacs. Preston hybrids differ from common and hyacinth lilacs being much larger shrubs and having noticeably different flowers, leaf shape and fragrance. As a result, these lilacs are extremely hardy and were bred intentionally with the Canadian climate in mind. You will notice when in the Preston Hybrid section of the Lilac Walk that she named many of her cultivars after the women of Shakespearian tales. Preston Hybrids are considered to be late bloom season lilacs.

Diversity and Colour

When planting a lilac garden, it makes good sense to combine a diversity of common, hyacinth and Preston lilacs. By combining these early, mid and late season bloom lilacs one can have lilacs in bloom for at least six weeks in spring in each year. When planting en masse make sure you include white lilacs at intervals as this will really make your lilac flowers pop!

The most important ornamental quality of lilacs is colour. Flower colour can be very subjective and the range of colour makes its description difficult. In essence not all lilacs are lilac! As a result, botanical garden curators, botanists, scientists, horticulturists and plant hybridizers around the world denote flower colour using a simple colour classification process termed the Wister Code. John Wister, an American horticulturist and International Lilac Registrar 1956-1975 proposed the following logical sequence of colour categories:

  • I. White
  • II. Violet
  • III. Blueish
  • IV. Lilac
  • V. Pinkish
  • VI. Magenta
  • VII. Purple

In addition lilac flowers are classified as being either;
Single – with four petals
Double - with more than four petals

For this reason if a lilac is denoted as being S III then it is internationally recognized as being a lilac with single blueish flowers. If it is D VII then it would be a double purple lilac.


When trying to identify or confirm the identity of a cultivar in precise detail or when a plant hybridizer introduces a new lilac cultivar colour description becomes even more critical as an identification tool. When precise colour descriptions are required staff at RBG refer particularly to the 1966 Royal Horticultural Society Colour Chart containing 808 colour chips which is used to accurately determine flower colour.


Although lilacs are known for their intoxicating scent not all lilacs are equally fragrant. Common lilac has a strong but not overpowering scent whilst some of the wild species from the far-east have a spicier fragrance somewhat reminiscent of cloves. The tree lilacs Syringa reticulata and Syringa pekinensis have a more musk like fragrance that is not always agreeable to some people. When planting a lilac garden consider planting cultivars of common lilac with some of the spicier smelling wild cousins to appreciate the diversity of fragrance. Character of scent may also change somewhat from season to season based on changes in annual climate. A cool spring and summer may yield different fragrance levels compared to a hot spring and summer. As with wine which may have vintage years in taste, the same can be said of lilacs with fragrance. Some years are just better than others!


Royal Botanical Gardens is the International Cultivar Registration Authority (ICRA) for lilacs. Since 1975 RBG’s Curator Emeritus Mr. Freek Vrugtman has maintained the International Register of Cultivar names in the Genus Syringa. As international registrar he registers new cultivar names in accordance with the International Code for Nomenclature of Cultivated Plants (ICNCP) under the guidance of the International Horticultural Society (ISHS) Commission on Nomenclature and Registration. Information on cultivar name registration can be viewed on the ISHS website;

The International Register of Cultivar names in the Genus Syringa can be viewed here:

RBG is proud to be the home of the International Registration Authority for new lilac cultivars. Click here to learn more.