It’s lilac season in the cooler climes of North America. In fact, the annual convention of the International Lilac Society takes place this weekend — April 28-30, 2011 — at the Westin Hotel in Lombard, Illinois.

Lombard also is the home of one of the longest-running lilac fests, held (appropriately enough in Lilacia Park). It starts April 30, running through May 15, 2011, this year.

Other lilac festivals on the flowering calendar are:

Hulda Klager Lilac Festival, now through May 8, 2011, Hulda Klager Gardens, 115 South Pekin Road, Woodland, Washington.

Lilac Sunday, May 8, 2011, Arnold Arboretum of Harvard Arboretum, Boston, Massachusetts.

Lilac Festival, May 13-22, 2011, Highland Park, 171 Reservoir Avenue, Rochester, New York.

Lilac Festival, May 27-30, 2011, McLaughlin Gardens, 97 Main Street, South Paris, Maine.

Fourth Street Lilac Festival, May 29, 2011, Calgary, Alberta, Canada.

Lilac Festival, June 10-19, 2011, Michigan’s Mackinac Island.

And, Road Trips Gardeners who are lilac fans definitely should plan to get to the Royal Botanical Gardens, 680 Plains Road West, Burlington, Ontario, Canada, to see the Katie Osborne Lilac (Syringa) Collection (Lilac Dell and Lilac Walk) on one of the last two weekends in May. The Gardens are home to the largest lilac collection in the world. RBG is also the home of the International Registration Authority for new lilac cultivars, and a site for their display and evaluation. From the 26 wild lilac species native to eastern Asia and southern Europe, breeders have developed over 1,600 cultivated varieties.