Hardy Geraniums for Northern Gardens Richard Hawke, Plant Evaluation Manager
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Issue 22, 2004 Plant Evaluation Notes Hardy Geraniums for Northern Gardens Richard Hawke, Plant Evaluation Manager ardy geraniums are among the most Hardy geraniums can be trailing to mound- grow leggy during the bloom period, especially fashionable garden perennials today. ing in habit, and low growing to several feet tall. in overly fertile or moist soils. New basal leaves HTheir popularity is due in part to the Basal rosettes of leaves arise from thick develop just as the bloom period is ending, so many new and improved cultivars that have rhizomes or taproots depending on the species. plants can be rejuvenated and encouraged to been introduced in recent years. Garden writers The leaves are typically rounded in outline and rebloom if cut back hard. time and again extol the merits of hardy gerani- palmately lobed but exhibit a variety of shapes, Whether grown in formal gardens or ums—their versatility, landscape adaptability sizes, textures and colors. Shapes and sizes vary naturalized landscapes, hardy geraniums are and superior ornamental traits. Hardy gerani- from the small, deeply dissected stem leaves of versatile herbaceous plants for a variety of gar- ums have come a long way from the days when bloody cranesbill (Geranium sanguineum) to den settings. They can be grown as specimens they were consigned to the wildflower garden. the broad basal leaves of Armenian geranium or in small groups in the sunny border, or as Geranium is in the geranium family (G. psilostemon); while textures range from the ground covers under crab apples, large shrubs (Geraniaceae) with approximately 300 species wrinkled Oxford cranesbill (G. koxonianum) and roses with bare lower stems. Species widely distributed in temperate regions of the to the felty G. renardii. Shades of green to gray- such as Geranium kcantabrigiense, G. macror- world. Hardy geranium is a general term that is green predominate, but plum-purple and rhizum, G. koxonianum and G. sanguineum used to distinguish Geranium from its tender golden-green can be found; while some leaves make exceptional ground covers because they cousin Pelargonium, the colorful floral are variegated, blotched or banded with cream fill in quickly and smother competitors. Many or bedding geranium. Cranesbill, another or purple. As further ornamentation, many species, especially those with trailing habits, common name, refers to the resemblance of the hardy geraniums have purple, red, orange or can be used on slopes, retaining walls and in fruit to the beak of a crane. yellow leaves in autumn. Some species have containers. Several varieties can be grown Their saucer-shaped flowers come in aromatic foliage that can be described as minty, together for continuous bloom from spring to shades of blue, purple, pink, magenta, violet or medicinal or malodorous. autumn. Hardy geraniums have an informal white, and often with darker or contrasting Hardy geraniums are easy to grow in a quality that makes them ideal for naturalizing in veins. For example, the deep magenta flowers of variety of soils and light conditions. Most meadows and woodlands. Whether grown in Armenian geranium (Geranium psilostemon) species grow in full sun to light shade in moist, the sun or shade garden, they combine nicely have prominent black veins, and the lavender- well-drained soils. Many hardy geraniums with grasses and other herbaceous perennials blue flowers of ‘Blue Cloud’ feature distinct prefer morning sun and afternoon shade, like blue oat grass (Helictotrichon), prairie magenta venation. Many hardy geraniums self- especially in hot climates. Plant health can suf- dropseed (Sporobolus), lamb’s ears (Stachys), seed freely, but most are not weedy in the gar- fer in hot, humid conditions or in soggy soils. lady’s mantle (Alchemilla), catmint (Nepeta), den. Prolific seeders such as meadow cranesbill There are a number of species such as bigroot aster (Aster), wormwood (Artemisia), hosta (G. pratense) are best cut back after flowering to geranium (Geranium macrorrhizum) and (Hosta) and lungwort (Pulmonaria). mourning widow (G. phaeum) that will grow prevent the spread of seed. In many cases The Evaluation Project hybrid cultivars, such as Cambridge well in dry shade. Plants grown in dense shade Hardy geraniums have become increasing- geranium (G. kcantabrigiense), are sterile or may have reduced flower production and leggy, ly popular in the past decade, and each year new self-seed infrequently. open habits. Many hardy geraniums naturally or improved introductions are offered in nursery catalogs and garden centers. With so many choices possible to gardeners, it can be difficult to decide which ones to grow. The Jenny Lee Jenny Chicago Botanic Garden (USDA Hardiness Zone 5b, AHS Plant Heat-Zone 5) evaluated 111 species and cultivars of Geranium from 1997 to 2002. The goal of the comparative trial was to observe the ornamental traits, disease and pest resistance, cultural adaptability and winter hardiness of a wide range of hardy geraniums for sun and shade environments, and to determine the best hardy geraniums for Northern gardens. Many new cultivars from the United States and Europe were included with garden favorites like ‘Johnson’s Blue’ and Geranium sanguineum var. striatum. The hardy geranium trial was conducted in two distinct sites. The primary evaluation site received approximately 10 hours of full sun during the growing season and was openly exposed to wind in all directions. The Geranium ‘Nimbus’ well-drained, clay-loam soil was amended with 2 Plant Evaluation Notes Chicago Botanic Garden composted leaves and had a pH of 7.4 through- back to new basal leaves after flowering to reju- Top-rated Hardy Geraniums out the evaluation term. Turf grass pathways venate plant health and habits. Mulch consist- Six hardy geraniums received the highest surrounded the beds on all sides; and 92 plots, ing of shredded leaves and wood chips was ratings based on heavy flower production, each comprised of eight plants, were separated placed around the plants for water conservation strong plant habits and good health, cultural within the beds by mulched strips. The second- and weed suppression. No winter protection adaptability and winter hardiness. The top-rated ary site was located in a mixed landscape where was provided. plants were: Geranium ‘Blue Cloud’, G. most plants received natural, dappled shade ‘Brookside’, G. ‘Moran’, G. kcantabrigiense Observations from nearby trees throughout the day. The ‘St. Ola’, G. macrorrhizum ‘Lohfelden’ and G. clay-loam soil was well-drained with an aver- The hardy geraniums were observed for macrorrhizum ‘Minor’. age pH of 7.5. Three plants of each taxon were their cultural adaptability to the soils and con- The lavender-blue flowers of Geranium grown in beds that were sheltered from wind by ditions of the test sites, disease and pest resist- ‘Blue Cloud’ were etched with magenta veins fences and shrubs. The 19 taxa that were grown ance, winter hardiness and ornamental traits and produced in profusion on robust stems. The in the partial shade site are highlighted with including floral display, plant size, habit quali- habit remained upright throughout the bloom boldface in Tables 1, 2 and 3. Of these plants, ty and autumn foliage color. In addition, plant period, but stems began to fall in midsummer habits were monitored for their response to Geranium ‘Blue Sunrise’, G. kcantabrigiense and were cut back to improve habit quality. The ‘St. Ola’, G. macrorrhizum ‘Variegatum’, G. shearing after flowering. Ninety-seven of the deeply dissected leaves turned red in autumn. sanguineum ‘Connie Hansen’ and G. syl- 111 taxa completed a minimum of four years of The dark blue flowers of G. ‘Brookside’ were vaticum ‘Baker’s Pink’ received approximately the six-year trial. Plant traits and evaluation similar to ‘Johnson’s Blue’ but the stems a half day of full sunlight rather than a full day specifics for these taxa are shown in Table 1 and remained upright during the summer. The dark of dappled shade. Table 2. The remaining 14 taxa in Table 3 com- green, finely divided leaves turned red and bur- Maintenance practices in both plots were pleted less than three years of evaluation due to gundy in autumn. Although G.‘Moran’ had a kept to a minimum to simulate home garden poor health, cultural problems and identifica- fairly short bloom period, its flowers opened pur- culture. Water was provided as needed, and no tion issues. A summary rating was assigned to ple and changed to iridescent blue in early May. fertilizer was applied. Faded flowers and each plant based on flower coverage, plant The robust habit was retained throughout early lodged stems were routinely deadheaded or cut health, habit quality, disease or pest problems summer but stems were cut back to the ground and winter injury. after flowering to rejuvenate the plants. Table 1: Plant Characteristics and Performance Summary Ratings Overall Geranium Flower Color Flower Bloom Period1 Flower Autumn Foliage Color Plant Plant Cut Back Rating Size Coverage Height Width Information2 1 ★★★★★ ‘Blue Cloud’ lavender-blue 1 /4 in. early Jun-mid Jul 60-80% red 26 in. 40 in. Yes+ 1 ★★★★ ‘Blue Sunrise’ (‘Verguld Saffier’) lavender-blue 1 /4 in. late Jun-late Jul 60-80% none observed 24 in. 36 in. Yes+ 1 ★★★★★ ‘Brookside’ deep blue 1 /2 in. late May-mid Aug 60-80% red, burgundy 24 in. 38 in. Yes+ ★★★★ ‘Dilys’ light magenta 1 in. late May-mid Jul+ 20-40% burgundy 14 in. 48 in. NR 1 ★★★★ ‘Johnson’s Blue’ blue 1 /2 in. mid May-early Jul 40-60% red 24 in. 33 in. Yes+ 1 ★★★★★ ‘Moran’ violet-blue 1 /4 in. early May-early Jun 60-80% none observed 24 in. 26 in. Yes+ ★★★★ ‘Nimbus’ purplish blue 1 in. late May-late Jul 40-60% none observed 24 in. 36 in. Yes+ 1 ★★ ‘Patricia’ magenta 1 /2 in.