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Sunday, March 26 - Around the Bay Race Road Closure Notice: Please be advised that York Blvd. will be closed and a segment of Plains Rd. W. will be reduced to one lane for the Around the Bay Road Race. RBG Centre remains open but the Rock Garden and Rock Garden Bistro will not open until 1:30 p.m. Increased traffic congestion is expected during the race.


The Katie Osborne Lilac Garden

Peak Interest: Late May
Located in the Arboretum

Experience one of the largest and most diverse lilac collections in the world.

Every May, A heavenly fragrance drifts through Royal Botanical Gardens’ Arboretum, marking the official start of lilac season! RBG visitors can experience one of the largest, most magnificent and diverse collections in the world as it reaches peak bloom.

The Katie Osborne Lilac Collection in the Lilac Dell is a popular seasonal attraction, providing visitors with weeks of delightful springtime colour and fragrance.

Bloom Watch

With a number of different varieties blooming at different times, the lilacs often look beautiful for multiple weeks in late May through early June.

Check back during bloom season for updates.

Historic Bloom Dates at the Arboretum

Note: bloom dates can vary drastically from year to year. Check back for our “bloom watch” updates.

Year Date of Peak Bloom
2022 Week of May 22
2021 Week of May 23
2020 Week of May 31
2019 Week of May 23
2018 Week of May 27
2017 Week of May 14
2016 Week of May 22
  • Branch Of Purple Lilacs With Dell In Background
  • Purple Lilac Bunch In Bloom
  • Lilac Collection In Bloom Climbing Up Hill Arboretum
  • Two Tone Lilac Blooms
  • Bunch Of White Lilacs In Bloom

Where to Find Lilacs at RBG

Royal Botanical Gardens is made up of four distinct formal gardens contained within 1,100 hectares of nature reserve, across the municipalities of Hamilton and Burlington Ontario. See all our garden areas and start planning your visit at


16 Old Guelph Road, Hamilton

The Arboretum holds the Katie Osborne Lilac Collection, one of the largest and most diverse lilac collection in the world. The lilac dell is a must-visit during peak bloom!

Admission: paid admission is not required

Parking: reserved parking is required during bloom season (May 5 to June 1, $30/vehicle).

Hendrie Park

680 Plains Road West, Burlington

Find a small collection of lilacs around the Scented Garden in Hendrie Park. Take a deep breath and breathe in that lovely smell!

Admission: Garden admission applies. Free with RBG Membership.

Parking: FREE, available on-site

What’s in Bloom?

Blooms are ever-changing in RBG’s five cultivated garden areas and nature sanctuaries. Check back to learn what’s blooming now or see the blooms calendar for a rough estimation of what to expect in a particular season.

History of Lilacs at RBG

Once located on the eastern shore of Cootes Paradise, the lilac (Syringa) collection, comprising of 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. Today, the collection displays over 1,000 individual plants representing several hundred cultivars, meaning RBG’s collection is one of the most diverse collections of lilacs on the planet.

About Lilacs

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 to prevent flower colours from 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, halve the remaining wood in year two and the remainder in year three. Lilacs can be easily transplanted and moved. The best time to do this is August when the plants are dormant and retaining as much of the plant’s root ball as possible.


Currently, it is thought that approximately 21 different species of lilac can be found in the wild. The geographic range 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 while Syringa vulgaris is native to the Balkan mountain regions. It is this latter species and its cultivars that 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 stood the test of horticultural time. Lemoine’s work with lilacs is shown through 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 mid-bloom season lilacs.

Not content with just 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 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 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:

  • White
  • Violet
  • Lilac
  • Pinkish
  • Magenta
  • Purple
  • Blueish

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 are 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 while 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!

Support Horticulture at RBG

The care and growth of our horticultural collections are possible thanks to the generous support of RBG Members and donors. With a donation to Growing up Green, you can ensure an active, vibrant and healthy future for the children of today and tomorrow through our horticultural and conservation projects.

Royal Botanical Gardens (RBG) is the largest botanical garden in Canada, a National Historic Site, and registered charitable organization with a mandate to bring together people, plants and nature.