A staple in cold-weather gardens, lilacs (Syringa) are deciduous shrubs with highly fragrant, cone-shaped flower clusters. The most common colors are purple and white. Lilacs are woody shrubs that thrive in USDA Hardiness Zones 3 to 7, but most varieties do best in the colder regions, because the flowers need cold weather to properly set. The lilac flower spike can be confused with other cone-shaped blooms, including several perennials.
Often referred as the Southern lilac, crape myrtle (Lagerstroemia) is the plant and bloom that most resembles lilacs. Crape myrtles are shrubs or bushes with woody growth. They produce cone-shaped flower spikes made up of tiny flowers ranging in color from dark pink to purple to white. Crape myrtle has light green leaves that can be up to 4 inches long and 2 inches wide. Lilac leaves are more rounded.
Crape myrtle thrives is zones 7 to 9, which are too hot for lilacs. Crape myrtle should be planted in full sun and get moderate water.
Delphinium (Delphinium) has tall, blue flower spikes that are often confused for lilacs, but delphiniums are perennials rather than shrubs. The delphinium's lobed foliage grows close to the ground, and flower stalks grow from the middle. The mass of blooms on the spike are round and larger than the tiny flowers on lilacs. The delphinium is also available in pink, red and yellow.
The delphinium blooms in summer and is often used as an annual. This plant thrives in full sun with regular water.
Though stock (Matthiola incana) does not grow on a tree or a shrub like lilacs do, the flower could be mistaken for lilacs. Stock is a perennial with a heavy fragrance and is available in pink, purple and white, like lilacs. But stock grows on a long, sturdy stem and makes a good cut flower. Leaves can be up to 4 inches long, and the plant itself can be up to 18 inches high.
Stock is often grown as annual. It thrives in full sun to light shade and requires regular water.