The Royalty cultivar (Syringa josiflexa) is part of the Preston series of lilacs and produces single reddish-purple blooms. They are common in garden stores throughout the U.S. and Canada, as they can tolerate areas that experience temperatures down to minus 40 degrees Fahrenheit. Royalty lilacs are similar in size and shape to the popular Miss Canada cultivar.
The Royalty lilac has large, showy blooms and dark green foliage. It averages 4 1/2 to 5 feet in both height and width, but can get taller in some circumstances. Care is minimal and, like other Preston lilacs, there is little sucker growth from the roots. Suckers developing from the base of the plant are easily pruned. Extensive research by the government of Alberta between 1985 and 1994 found that Royalty is a versatile grower well suited to northern climates.
Preston lilacs are a series of plants valued for their cold tolerance. Isabelle Preston of the Central Experimental Farm in Ottawa, Canada, developed them in 1920. Prestons share parentage with the lilac species Syringa villosa and Syringa reflexa, and start blooming about one week later than the common lilac species, Syringa vulgaris. Preston lilacs average 8 feet in height and 8 feet in width at maturity. The growth rate is 12 to 18 inches per year.
Royalty needs the same cultural care as other lilacs. The plants need a site with partial to full sun and moist but well-drained soil, and they benefit from an annual pruning. All stems are larger than 2 inches in diameter should be removed to encourage new growth and to promote a compact shape. Prune after blooming is complete for the season, in late spring.
If you have an older specimen of a Royalty lilac that is spindly or no longer blooming well, restore its vigor with heavy pruning. A mature Royalty can be severely cut back to a height of about 4 inches. Thinning of large, older trunks should be done at this time as well. The bush should reach a height of 2 to 3 feet in the following year with its burst of growth. Afterward, employ an annual pruning after the lilac blooms to control its shape and size.