Lavenders for Northern Gardens
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Plant Evaluation Notes | Issue 42, 2017 Lavenders for Northern Gardens Richard G. Hawke, Plant Evaluation Manager and Associate Scientist Lavender trial beds in the Lavin Plant Evaluation Garden. Lavenders have long been cultivated lia, L. ×intermedia, and L. ×chaytoriae, are to eight verticillasters, form compact termi- for their broad herbal and medicinal typically listed hardy in USDA Zones 6-9. nal spikes on stiff stems held above the uses, and are enduringly popular as Consequently, lavenders are treated as pe- leaves. Flower colors range from shades of ornamentals in gardens and land- rennials in areas where stem dieback is the lavender or violet to purple, pink, and white, scapes around the world. Famously, rule rather than the exception. The mild cli- usually with darker or contrasting calyces. bountiful fields of lavender grown for its fra- mate of their native range is so unlike the All parts of the plant are tomentose and ar- grant oil are the essence of France’s temperate Upper Midwest that growing lav- omatic due to volatile oils in the inflores- Provence region. Lavender derives from enders may seem impossible. Despite their cences, stems, and leaves. The oppositely lavare, Latin for “to bathe or wash," generally tender aspect and the constraints paired leaves are generally narrowly lance- because the ancient Romans steeped bun- of their cultural requirements, some laven- shaped, about 2 inches long, silvery to dles of this aromatic herb in bathing water. ders grow successfully in northern regions. gray-green, and evergreen in mild winters. Lavender is familiar in everyday life, giving English lavender has a mounded habit, 12 its distinctive scent and color to a myriad of Like other members of the mint family to 36 inches tall depending on the cultivar, personal, home, and culinary goods. (Lamiaceae), lavenders feature bilabiate or and can grow quite wide with age under two-lipped flowers arranged in verticillate ideal conditions. Lavenders (Lavandula spp.) are Old World or whorled clusters on square stems. While plants, with 28 species native to the Medi- its name would imply otherwise, English or Lavandin, Lavandula ×intermedia, is a nat- terranean region, northern Africa, western common lavender, Lavandula angustifolia, ural hybrid of L. angustifolia and L. latifolia, Asia, and the Middle East. While technically is a native of the Mediterranean region and another Mediterranean native. There are a subshrub, the woody stems are often in- represented in cultivation by a plethora of only slight differences related to flower size jured or killed in cold winters—L. angustifo- cultivars. The small flowers, clustered in six and bloom season between the two spe- Plant Evaluation Notes | chicagobotanic.org | 2 cies and lavandin cultivars are plentiful, too. The essential oil of lavender scents per- cold-hardy in USDA Zone 5; the goal was to Lavandula ×chaytoriae, a man-made cross fumes, balms, and soaps; dried lavender determine the potential for growing laven- between English lavender and woolly lav- sprigs are a staple in potpourris and sa- ders in northern regions. Beyond chroni- ender, L. lanata, originated in England in the chets, as well as flavoring for teas, condi- cling their ornamental attributes, assess- 1980s. It has the fuzzy silver leaves of wool- ments, and foodstuffs. By most accounts, ments were made on winter hardiness, ly lavender and some of the cold hardiness English lavender yields the finest quality oil; adaptability to average growing conditions, of L. angustifolia. nonetheless, ‘Grosso’ lavandin rather than and longevity in the landscape. English lavender is most widely grown for Lavenders prefer full sun and well-drained commercial oil production in France. As a Three plants of each taxon were grown in to dry, alkaline soils—gravelly or sandy soils garden plant, lavenders of all stripes are ex- side-by-side plots for easy comparison of are ideal. Good soil drainage, especially in ceptional in rustic to traditional landscapes. ornamental traits and landscape perfor- heavier clay soils, is crucial to year-round At home in flower, vegetable, and herb gar- mance. The evaluation garden was openly survival. Maintenance needs are few since dens, lavenders fit especially well in rocker- exposed to wind in all directions and re- lavenders do not require regular irrigation ies and xeric or waterwise gardens. Laven- ceived at least ten hours of full sun daily or fertilizer to flourish. Deadheading is the der is a versatile garden plant, whether as a during the growing season, which aver- gardeners’ choice but is helpful in keeping specimen in a border or pot, massed as a aged 180 days per year for the 2010–2016 plants tidy. Shearing lavenders by one-third groundcover, or hedged to define a space. trial period (see Table 1). The clay-loam soil after blooming removes spent flower stalks The silvery leaves are a wonderful counter- had a pH of 7.4 throughout this period, and and shapes the plant. Substantial pruning point to a variety of leaf and flower colors, although typically well-drained, the soil re- may be necessary on severely winter-dam- while the blossoms draw an assortment of tained excess moisture for short periods in aged plants; cut stems back after new pollinators. summer and winter. Trial beds were raised growth begins in the spring. Fungal root rot eight inches above the surrounding turf to can be troublesome in poorly drained soils improve soil drainage. The Evaluation Study and during periods of heavy rainfall and/or In June 2010, the Chicago Botanic Garden high humidity. Fungal and bacterial leaf Maintenance practices were kept to a min- (USDA Hardiness Zone 5b, AHS Plant spotting, stem blight, and wilt are also pos- imum, thereby allowing the plants to thrive Heat-Zone 5) initiated a seven-year laven- sible problems in warm, wet weather. Four- or fail under natural conditions. Trial beds der trial, which included 40 commercially lined plant bugs, caterpillars, and northern were irrigated via overhead sprinklers as available selections of Lavandula angustifo- knot nematodes are occasional pests. The needed, mulched with composted leaves lia, L. ×intermedia, and L. ×chaytoriae. The natural oils in lavenders repel most grazing once each summer, and regularly weeded. impetus for the trial was a common con- mammals such as deer and rabbits. Moreover, plants were not deadheaded, ception that lavenders were not reliably fertilized, winter mulched, or chemically Lavandula angustifolia ‘Loddon Blue’ Plant Evaluation Notes | chicagobotanic.org | 3 treated for insect or disease problems. Dead stems were removed each spring after winter injury assessments were completed. Lavender nomenclature in this report fol- lows the Royal Horticultural Society’s online plant finder as well as other horticultural ref- erences. While it was possible to verify most names, a few discrepancies remained unresolved. Most notably, variations of the cultivar and trademark names for Lavandu- la angustifolia ‘Walvera’ exist in print and on websites, where it is alternately referenced as ‘Silver Edge’, Walberton’s Silver Edge, and Walberton’s® Arctic Fox; we settled on Walberton’s® Arctic Fox for the trial. We also went with Lavance Purple®/L. angusti- The Performance Report ‘Walvera’ was retested with the same re- folia ‘Kielapro’ instead of ‘Lavance Purple’ At the start of the lavender trial in June sults but ‘Silver Mist’ was not replanted. All or ‘L’Avance Purple’. Finally, based on 2010, 19 taxa were planted in the full-sun lavenders were evaluated for cold hardi- several references to its hybrid traits, we trial garden. An additional 16 taxa entered ness; cultural adaptability to the soil and assigned ‘Grappenhall’ to L. ×intermedia the trial in spring 2011; the remaining five environmental conditions of the trial site; rather than L. angustifolia, which appears taxa were newer commercial introductions disease and pest problems; and ornamen- to be an equally prevalent designation. when added in 2013 and 2014. In all, 40 tal qualities associated with flowers, foli- lavender taxa were evaluated between age, and plant habits. Final performance June 2010 and November 2016. Twen- ratings are based on winter hardiness, ty-four of the 40 taxa survived four or more flower production, foliage and habit quality, winters; 38 taxa lived through two or more and plant health and vigor. Table 2 shows winters. Two taxa—‘Walvera’/Walberton’s® the observed plant traits and overall ratings Arctic Fox and ‘Silver Mist’—grew for one for all lavenders, regardless of number of season but did not survive a winter; years in the trial. Table 1: Weather summary 2010–2016 2010 2011 2012 2013 2014 2015 2016 Lowest temperature °F (°C) 0 (-18) -7 (-22) 2 (-17) -3 (-19) -16 (-27) -13 (-25) -4 (-20) Lowest temperature date 1/3 2/10 1/21 1/22 1/6 2/28 1/18 Highest temperature °F (°C) 97(36) 102(39) 105(41) 96(35) 91(33) 94(34) 95(35) Highest temperature date 7/23 7/20 7/5 7/18 7/17 7/17 7/24 Number of growing season daysa 177 192 165 185 186 176 181 Number of days below 0°F (-18°C) 16 5 0 3 24 17 13 Number of days above 90°F (32°C) 6 22 40 15 5 13 17 Last frost date 4/28 4/21 4/24 4/20 4/16 4/24 5/15 First frost date 10/22 10/30 10/6 10/22 10/19 10/17 12/12 Annual rainfall in inches (cm)b 35.6(92.5) 48.2(122.4) 31.1(78.9) 39.1(99.3) 42.5(107.9) 43.6(110.7) 36(91.4) Annual snowfall in inches (cm)c 51.8(131.6) 35.3(89.7) 23.4(59.4) 40.3(102.4) 66.4(168.7) 45.5(115.6) 27(65.6) aNormal growing season: 180 days bAverage rainfall: 37.7 inches (95.7 cm) cAverage snowfall: 36 inches (91.4 cm) Data collected at Chicago Botanic Garden weather station Latitude: 41°51’N.