Photo courtesy of Chicago Botanic Garden

Lavenders have long been cultivated for their broad herbal and medicinal uses and are enduringly popular as ornamentals in gardens and landscapes around the world. Famously, bountiful fields of lavender grown for its fragrant oil are the essence of France’s Provence region. Lavender derives from lavare, Latin for “to bathe or wash”, because the ancient Romans steeped bundles of this aromatic herb in bathing water. Lavender is familiar in everyday life, giving its distinctive scent and color to a myriad of personal, home, and culinary goods.

Lavenders (Lavandula spp.) are Old World plants, with 28 species native to the Mediterranean region, northern Africa, western Asia, and the Middle East. While technically a subshrub, the woody stems are often injured or killed in cold winters—L. angustifolia, L. × intermedia and L. × chaytoriae are typically listed hardy in USDA Zones 6-9. Consequently, lavenders are treated as perennials in areas where stem dieback is the rule rather than the exception. The mild climate of their native range is so unlike the temperate Upper Midwest that growing lavenders may seem impossible. Despite their generally tender aspect and the constraints of their cultural requirements, some lavenders grow successfully in northern regions.

While its name would imply otherwise, English or common lavender, Lavandula angustifolia, is a native of the Mediterranean region and represented in cultivation by a plethora of cultivars. Flower colors range from shades of lavender or violet to purple, pink, and white, usually with darker or contrasting calyces. All parts of the plant are tomentose and aromatic due to volatile oils in the inflorescences, stems, and leaves. The oppositely paired leaves are generally narrowly lance-shaped, about two inches long, silvery to gray-green, and evergreen in mild winters. English lavender has a mounded habit, 12 to 36 inches tall depending on the cultivar, and can grow quite wide with age under ideal conditions.

Lavandin, Lavandula × intermedia, is a natural hybrid of L. angustifolia and L. latifolia, another Mediterranean native. There are only slight differences related to flower size and bloom season between the two species and lavandin cultivars are plentiful, too. Lavandula × chaytoriae, a manmade cross between English lavender and woolly lavender, L. lanata, originated in England in the 1980s. It has the fuzzy silver leaves of woolly lavender and some of the cold hardiness of L. angustifolia.

The evaluation study

In June 2010, the Chicago Botanic Garden (USDA Hardiness Zone 5b, AHS Plant Heat-Zone 5) initiated a seven-year lavender trial, which included 40 commercially available selections of Lavandula angustifolia, L. × intermedia and L. × chaytoriae. The impetus for the trial was a common conception that lavenders were not reliably cold hardy in USDA Zone 5; the goal was to determine the potential for growing lavenders in northern regions. Beyond chronicling their ornamental attributes, assessments were made on winter hardiness, adaptability to average growing conditions, and longevity in the landscape.

Three plants of each taxon were grown in side-by-side plots for easy comparison of ornamental traits and landscape performance. The evaluation garden was openly exposed to wind in all directions and received at least ten hours of full sun daily during the growing season, which averaged 180 days per year for the 2010-2016 trial period. The clay-loam soil had a pH of 7.4 throughout this period, and although typically well-drained, the soil retained excess moisture for short periods in summer and winter. Trial beds were raised eight inches above the surrounding turf to improve soil drainage.

Maintenance practices were kept to a minimum, thereby allowing the plants to thrive or fail under natural conditions. Trial beds were irrigated via overhead sprinklers as needed, mulched with composted leaves once each summer, and regularly weeded. Moreover, plants were not deadheaded, fertilized, winter mulched, or chemically treated for insect or disease problems. Dead stems were removed each spring after winter injury assessments were completed.

All lavenders were evaluated for cold hardiness; cultural adaptability to the soil and environmental conditions of the trial site; disease and pest problems; and ornamental qualities associated with flowers, foliage, and plant habits. Final performance ratings are based on winter hardiness, flower production, foliage and habit quality, and plant health and vigor.

Top-rated lavenders

Seven taxa received the highest ratings for exceptional flower production, healthy foliage, vigorous habits, adaptability to the growing conditions, and superior winter hardiness. Based on cumulative evaluation scores, the top-rated lavenders in descending order were ‘Imperial Gem’, ‘Royal Velvet’, ‘Munstead’, ‘SuperBlue’, ‘Jean Davis’, ‘Niko’/Phenomenal and ‘Sharon Roberts’. Phenomenal was the only top-performing non-angustifolia type.

Summary

There were two noteworthy consequences of undertaking and completing the lavender trial. Most importantly, the outcome, whether positive or negative, is informative to gardeners beyond simply anecdotal. Additionally, discovering a variety of adaptable lavenders diversifies the offerings for northern gardens. Of the 40 lavenders trialed between 2010 and 2016, thirteen selections—representing a good variety of flower colors and plant sizes—received high ratings for their overall performances. Besides robust habits and superior floral displays, these lavenders proved winter hardy and adaptable to the challenging growing conditions of the trial site. The thirteen top-rated lavenders included ‘Imperial Gem’, ‘Royal Velvet’, ‘Munstead’, ‘SuperBlue’, ‘Jean Davis’, ‘Niko’/Phenomenal, ‘Sharon Roberts’, ‘Violet Intrigue’, ‘Dwarf Blue’, ‘Loddon Blue’, ‘Thumbelina Leigh’, ‘Oxford Gem’ and ‘Irene Doyle’.

Cold hardiness and adaptability to wet clay soils were the primary concerns at the outset of the trial. Both criteria were challenges to successfully growing lavenders, and at times, it was uncertain whether one was more detrimental than the other was. While not proven in all cases, the lavenders that struggled to survive the growing conditions were more susceptible to injury from cold temperatures. Longevity was not conclusively determined but the top-performers survived and excelled for the duration of their time in the trial, which in many instances was five or six years.

All lavenders sustained some level of injury in one or more winters, although damage varied by lavender and by year. There was a direct correlation between the degree of winter stem loss and habit quality and flower production; that is, lavenders with the least amount of stem injury looked the best. A number of lavenders, such as ‘Hidcote Superior’, Lavance Purple, and ‘Mitcham Gray’, were strong performers early in their trial, but were not able to recover fully after suffering significant winter injuries.

The flower shows provided by the lavenders were quite good overall. The majority of lavenders produced copious flowers, and were ornamental for a protracted period because of the long-lasting color of the calyces. Longer floral stalks created a more dramatic flower show, which was especially notable on lavandin selections such as ‘Gros Bleu’, ‘Niko’/Phenomenal, and ‘Provence’. Plants were not deadheaded in the trial, but it was clear that removing ageing flower stems would improve the ornamental display. Winter-dead stems were removed after new growth began in spring to improve habit quality.

Through formal comparative evaluation, we realized our goal of identifying a variety of lavenders adaptable to colder regions and average to challenging growing conditions. In the end, a third of the lavenders garnered at least four out of five stars; these successes will provide greater certainty when selecting a lavender for northern gardens.

Richard G. Hawke is the plant evaluation manager and associate scientist at the Chicago Botanic Garden, chicagobotanic.org. To see the entire evaluation report, visit http://bit.ly/CBG_lavender_trials.