Lavender (Lavandula) is a flowering herb that belongs to the mint family and blooms from mid to late summer. It prefers a "Mediterranean" habitat with well drained, somewhat sandy soil conditions.
Lavender is largely grown for its fragrant essential oils. In order to flourish and reach heights of 18 to 36 inches tall, lavender is a perennial that requires room to breathe and is best situated with companion plants that don't compete for nutrients and prefer the same sunny conditions that support lavender growth.
Cone Flower (Echinaceas)
Echinaceas, more commonly known as cone flowers, resemble daisies and come in a variety of colors including pink, yellow, bright orange and white. Echinaceas can reach heights of up to 40 inches and, like lavender, prefer sunny, hot conditions. These perennials begin blooming mid to late summer and continue blooming well into the fall. Their height and color contrast makes a vivid backdrop that skims just past the height of lavender's purple blooms.
Rosemary (Rosmarinus officinalis)
Rosemary is a culinary perennial herb that prefers dry, sunny conditions. Rosemary blooms with small pink or white flowers in late spring, just before lavender begins to bloom. It's not unusual for rosemary to reach heights up 5 feet, so providing young plants with plenty of space from the start will allow them to flourish as a bushy backdrop against shorter lavender bushes.
Pineapple Sage (Salvia elegans)
Pineapple sage's fragrant leaves and bushy tall branches tipped with crimson red tubular blossoms make it a standout against the purple blooms of lavender. Pineapple sage grows well in hot, sunny conditions, however it typically requires more water than lavender. Planting pineapple sage behind lavender creates a thick wall of leafy stalks that can reach heights of 30 to 40 inches. Blooms break out in September and continue through mid October.
Celosia (Celosia plumosa)
Celosia is an annual that blooms through the summer and into fall with soft, fuzzy plumes in bright colors, including red, purple, hot pink, gold and yellow. Celosia thrives in full sun and well-drained soils. Varieties can reach 24 inches, making them a prime choice for "bookend" plants at the sides of lavender bushes. Celosia is low maintenance and its continual blooms through fall add interest when lavender bloom begin to subside.
According to Janet Davis at Beautiful Botany, roses and lavender make excellent tried and true companions. Lavender looks particularly striking against deep red roses and the two grow well together despite the roses' requirement of more regular watering intervals. Lavender, when planted in front of rose bushes, creates a thick, shrubby wall of purple blooms at the base of the often "leggy" looking rose bush. Roses of the same color will be of more obvious contrast to the solidity of lavender's silvery foliage. Opt for roses of red, white, yellow or deep pink.