Michael A. Dirr, Ph.D.
Copyright © 2012
Abelia―Abelia
Significant enthusiasm for A. ×grandiflora and cultivars with compact ‘Rose Creek’ among the best. Twist of Lime™ (‘Aghop’) with cream-yellow margined leaves is, to date, the most stable. ‘Kaleidoscope’, yellow/gold leaves, commands most market share. Currently, ~30 cultivars with more on the way. Genus is being taxonomically rejiggered into four genera (Abelia, Zabelia, Diabelia, Vesalea). Abelia chinensis, a parent of A. ×grandiflora, is larger in stature, with white flowers throughout summer. Hardy to Chicago. Abelia mosanensis with pretty red-budded, opening white, exceeding fragrant flowers has garnered attention. Performs better in cooler climates. Was told fall color is a pleasing red.
Aesculus parviflora―Bottlebrush Buckeye
Noteworthy native with adaptability from Zone 4 to 8. White, bottlebrush flowers in late spring-summer. Beautiful birch yellow fall color. Terrific shade tolerant shrub. No significant breeding with buckeyes. Attempted to hybridize red buckeye, A. pavia, with A. parviflora to produce a pink-flowered bottlebrush.
Berberis―Barberries
Invasiveness has tempered enthusiasm for this species. Still numerous NEW introductions with ‘Admiration’ with red-purple centers and cream to yellow margins on a compact framework, quite eye-catching. ‘Daybreak’ is a yellow leaf selection with excellent heat tolerance and orange fall color. Yet to set seed. From www.plantintroductions.com.
Red and yellow leaf cultivars are so abundant and indistinguishable that it is difficult to pin-point the best.
University of Connecticut evaluated cultivars for fruit production. Lowest seed per plant: ‘Kelleris’ (6), ‘Aurea’ (5), ‘Bagatelle’ (2), ‘Antares’ (1), Crimson Ruby™ (0), and ‘Golden Devine’ (0).
Buddleia davidii―Butterfly-bush
In and out of garden fashion like neckties. Resurgence with new colors, rose-red - ‘Miss Ruby’, ‘Miss Molly’; compactness – ‘Blue Chip’; and total sterility - ‘Asian Moon’. Trials at RHS Wisley, England, reflected visitors’ penchant for flower colors approaching red with ‘Miss Ruby’ top vote collector.
Buzz™ Series from Thompson & Morgan are compact, 3 to 5’ high and wide. Flutterby™ Series from Ball Horticultural umbrellas ~15 cultivars. I consider ‘Blue Heaven’ among the best.
My Georgia program introduced eleven with ‘Attraction’ (red-purple), ‘Guinevere’ (deep purple), and ‘Bicolor’ (lavender with butterscotch eye) the most popular.
Noted a double-flowered, violet-purple hybrid that had the appearance of a double-flowered African violet. First double I have observed. From Peter Moore, England.
Sterility is a large issue with Oregon banning cultivars that produce greater than 2% viable seed.
Buxus―Boxwood
Voted number one shrub in a survey of 4,000 landscape professionals. See Nursery Management 27(8):2011. All are green, some blue-green, functional, utilitarian, deer-resistant, broadleaf evergreen shrubs. Observed no browsing in an Athens cemetery that is over-run with boxwood and deer.
Callicarpa―Beautyberry
Numerous species known for metallic-colored fruits in fall. Energetic breeding by Dr. John Ruter and Dr. Ryan Contraras, but nothing new materialized. Callicarpa dichotoma may be the most cold hardy. All flower on new growth. ‘Duet’ is a cream-margined, medium-green centered leaf, white-fruited selection that seldom reverts and does not scorch in sun/heat. ‘Early Amethyst’ and ‘Issai’ fruit abundantly. Treat as cut-back shrub to stimulate vigorous shoot growth.
Calycanthus―Sweetshrub
Great native (C. floridus) with sweet fragrance to flowers. Select ‘Michael Lindsey’, ‘Edith Wilder’, ‘Athens’ (yellow) for guaranteed fragrance. Hybrids from North Carolina State include ‘Hartlage Wine’ (3 to 3 ½”, red-maroon flowers), ‘Aphrodite’ (similar but fragrant), and Venus™ (white, akin to star magnolia). They embody the native species and C. chinensis, the Chinese species, with large, white tinged pink, non-fragrant flowers. These hybrids are block-buster shrubs in flower! Venus™ is fickle with petal spot and short-circuited life span (Dirr experience).
Caryopteris―Blue-mist Shrub
Overrun with cultivars but a worthy choice in sandy, well-drained soils, full sun exposure, for summer into fall blue flowers. Flowers on new growth and has been hardy as a subshrub at the Chicago Botanic Garden. Many yellow leaf and white-margined variegated cultivars. ‘First Choice’ maintained erect, compact habit, deep blue flowers, and best foliage in Georgia trials. ‘Kew Blue’ and ‘Longwood Blue’ are worthy.
Ceanothus ×delileanus, C. ×pallidus
Blue and pink flowers on compact plants from old and new wood. Typically summer flowering. ‘Henri Desfosse’ (blue), ‘Gloire de Versailles’ (powder blue), and ‘Marie Simon’ (pink) prospered in Georgia trials. Most are West Coast, Mexican species, but the above hybrids incorporate C. americanus and C. herbaceus. Terrific breeding opportunities in this genus. Adaptable to Boston. Ceanothus americanus, New Jersey Tea, is a remarkably disease-resistant, summer white flowering, 3 to 4’ high native with adaptability from Zone 4 to 8.
Cephalotaxus―Plum-yew
Important Taxus, yew, alternatives where deer are problematic. Long dark green needles separate them from Taxus. Cold (Zone 6) and heat tolerance (Zone 8-9), deer resistance, and shade tolerance are attributes. Many cultivars with ‘Duke Gardens’, ‘Norris Johnson’, and ‘Prostrata’ leading the pack.
Chaenomeles―Floweringquince
Who cares? Old-fashioned, antiquated, creaky shrubs. Refreshing in winter-early spring in myriad colors on naked stems. Dr. Tom Ranney introduced ‘Scarlet Storm’, ‘Pink Storm’, and ‘Orange Storm’, all with 1 ¾ to 2 ½” diameter, double flowers, fruitless, and thornless. Observed abundant fruit on ‘Scarlet Storm’ in Oregon. New and time will arbitrate their greatness. Estimated to 6’ high. Noticed leaves present in good condition in November (Athens) which is not the norm for C. speciosa. At PII, sprawling habit in containers although advertised as upright.
Clethra alnifolia―Summersweet
Nifty native from Maine to Florida and Texas in moist to wet habitats. White, pink, to rose-red, spicily fragrant flowers in summer. ‘Compacta’, ‘Sixteen Candles’, ‘Ruby Spice’ among the best. Sugartina® (‘Crystalina’) and Vanilla Spice™ (‘Caleb’) are newer white-flowered selections. Growing both and not particularly impressed. Larger than advertised.
Cornus amomum―Silky Dogwood
Cornus sanguinea―Bloodtwig Dogwood
Shrubby, stoloniferous to clump-forming rather aggressive shrubs. Discovered a red-stemmed form of the typical purplish brown species C. amomum. Excellent orange-red fall color in 2011. Cornus amomum is native to most of the eastern U.S. and grows into Florida. This red-stemmed selection allows for successful culture in the South (and North) where canker is a problem on C. alba and C. sericea.
Several yellow-orange-red stemmed C. sanguinea cultivars including ‘Anny’, Arctic Sun™ (‘Cato’), ‘Midwinter Fire’, etc. Beautiful in the winter landscape.
Corylopsis―Winterhazel
Delightful yellow-tasseled flowers on naked stems dance to the yawning of a long winter. No cultivar glut here but the species C. glabrescens, C. pauciflora, and C. spicata are all worthy. ‘Gold Spring’ (‘Golden Spring’) is a yellow leaf type of C. spicata. Needs shade in the South. Have a compact, blue-foliage, soft yellow flower selection at PII.
Corylus―Filbert
‘Red Majestic’ and ‘Red Dragon’ with contorted stems and red-purple leaves provide year-round interest. ‘Red Magic’ leaves turn green quickly. ‘Red Dragon’ displays complete resistance to eastern filbert blight; 10 to 12’ by 6 to 8’. The latter hybridized at Oregon State University. Rutgers University has an active Corylus breeding program. Initiated in 1996 with no introductions to date.
Cotinus coggygria―Smokebush
Have yet to add the species to the Dirr garden but am impressed with ‘Daydream’, ‘Dusky Maiden’, and ‘Young Lady’. “Smoke” on the first and third smother the foliage. The middle has a wavy-surfaced, bronze-purple leaf, smaller stature and resulted from C. szechuanensis × ‘Velvet Cloak’.
Cryptomeria japonica―Japanese Cryptomeria
Many compact forms with Chapel View™, ‘Globosa Nana’, and ‘Gyokurya’ among the best. Dr. Ruter is breeding to eliminate the winter bronzing/browning.
Daphne―Daphne
All are potentially wonderful, if they live! Fan of D. genkwa, Lilac Daphne, with lilac-colored flowers. Unfortunately, not fragrant. Daphne ×transatlantica produces white, fragrant flowers throughout summer. Hardy in Maine (Boothbay); performing well in Athens. Daphne odora is the best choice for the South.
Diervilla―Bush-honeysuckle
A rather obscure native shrub with smallish yellow flowers in summer, bronze young shoots, deep green at maturity, yellow to reddish in autumn. Principle asset is durability. Have grown countless seedlings with several added to garden. Bonnie always asks. . . why? Cool Splash™ is a white-margined, green-centered leaf selection. Holds variegation in heat. Prone to reversion. Diervilla has been hybridized with Weigela.
Distylium―Isu Tree
Evergreen members of Hamamelidaceae with small red flowers in late winter-early spring. Zone 6 adaptability with D. racemosum. ‘Blue Cascade’, ‘Emerald Heights’, and ‘Vintage Jade’ are hybrids of D. racemosum and D. myricoides. ‘Vintage Jade’ grew 2’ by 5’ in 6 years and is being successfully grown in Louisville, KY. Superb alternatives to cherrylaurels, hollies, and junipers. See www.plantintroductions.com for in-depth information.
Euonymus alatus―Winged Euonymus
Great red fall coloring shrub but invasive and listed as such in 21 states. Banned in Massachusetts and New Hampshire. Yi Li, University of Connecticut, developed a sterile triploid from endosperm tissue of ‘Compactus’. Not yet on market but a major advancement when commercialized, if truly fruitless.
Fothergilla ×intermedia
Hybrids between F. gardenii and F. major are designated as such. ‘Mt. Airy’, introduced by the author, gave Fothergilla an identity. Still the most common cultivar in commerce because of reliable flower production and consistent yellow-orange-red fall color. ‘Blue Shadow’ from Gary Handy, has glaucous blue leaves and is a sport of ‘Mt. Airy’. Beautiful summer foliage and similar flowers and fall color to ‘Mt. Airy’.
×Gordlinia grandiflora
Remarkable semi-evergreen hybrid between Franklinia and Gordonia engineered by Dr. Tom Ranney. Flowers, 3 to 4” across, open on new growth in summer-fall. Older leaves turn brilliant red (purple). Shrub or small tree. Exceedingly fast, 5’ in a single season at PII. Easy to root from cuttings. Requires excellent drainage and a degree of good cultural fortune. Estimate Zone 6 to 8.
Gardenia jasminoides―Gardenia
Worked on cold-hardy (Zone 6) breeding with Pinwheel™, a single, white, fragrant, 3’ by 3’, performing well in Louisville. By degrees, the hardiest gardenia to date. Successfully grown in Louisville, KY. Heaven Scent™ is smaller in all characteristics and slightly less cold hardy than Pinwheel™. Both with single, 6-petaled flowers in May-June, reblooming into fall. Breeding continues with a cold-hardy, double-flowered reblooming selection, ‘Doublemint’, from PII, still flowering on November 28, 2011.
Hamamelis―Witchhazel
First order shrubs offering kaleidoscopic flowers from fall into late winter, brilliant fall colors, and ironclad adaptability. ‘Wisley Supreme’, H. mollis type, with sweet-scented, yellow flowers and spectacular yellow fall color is a beacon in the new Dirr garden. Numerous H. ×intermedia cultivars. Consider the native H. virginiana, with fragrant spidery yellow petals in fall and rich birch yellow fall color. This species grows almost anywhere. ‘Lemon Lime’ has leaves speckled yellow, light green, and green; delicately beautiful. Holds variegation in shade. ‘Harvest Moon’ and ‘Little Suzie’, 5’ by 5’, are worthy.
Hibiscus syriacus―Rose-of-Sharon
Too many. Most so-so. Chiffon™ and Satin™ Series have merit. Impressed with Blue Satin™ (‘Marina’), a deep velvety blue-purple with red-maroon center. Species is back in fashion. Doug Welty, who handles plant sourcing for QVC (TV), mentioned that H. syriacus is one of the most popular offerings. Potential to sell 300,000 per year is real.
Hydrangea (see separate handout)
Hypericum―St. Johnswort
Four-hundred species. I’m confused. Believe the U.S. natives are worthy. Love H. densiflorum as it grows in water (mountain tops also) with rich deep green, linear, fine-textured leaves that turn yellow-orange in fall. Hypericum frondosum ‘Sunburst’ produces 2” diameter bright yellow flowers contrasted with the blue-green foliage. Hypericum kalmianum Blue Velvet™ with smaller ¾” diameter flowers and glaucous blue foliage is worthy. At PII, Hypericum breeding is in high gear, with seedling selections of Blue Velvet™ more compact and richer glaucous blue in foliage.
Illicium―Anise
Fragrant foliage, shade tolerance, and pest resistance have endeared these evergreen shrubs to Zone (6)7 to 9 gardeners. Illicium parviflorum ‘Florida Sunshine’ sports soft yellow leaves. New purple leaf type, ‘Purple Haze’, I. anisatum, increases the color palette. ‘Pink Frost’ has white-margined leaves that turn pink-rose in cold weather. A selection of I. floridanum.
Indigofera―Chinese Indigo
The genus is immense with as many as 700 species. This species grows 1 to 2’ high, spreading by rhizomes/stolons to form large colonies. Pretty blue-green leaves, pink-rose flowers in 4 to 8” long slender, arching-pendulous racemes. Flowers profuse in May; still chugging along in October. In the new garden, this plant is asked about more than any other. Indigofera kirilowii grows slightly taller (3’) with bright pink flowers. Both flower on old and new wood. Estimate Zone 5 to 7.
Kalmia latifolia―Mountain-laurel
Many beautiful flower colors from Dr. Jaynes breeding research. Not an everyday garden center item. Buy Dr. Jaynes book and marvel at the variation.
Kerria japonica―Japanese Kerria
Can’t gain commercial traction. Early leafing and flowering (yellow), late yellow fall color, grass green stems, sun and shade tolerance add up to a worthy garden addition. Have tried to find fruits for breeding work without success. Help! Number of cultivars with variegated foliage (‘Chiba Gold’, ‘Geisha’, ‘Picta’), large yellow flowers (‘Honshu’, ‘Golden Guinea’, ‘Shannon’), and double flowers (‘Peniflora’). Variegated types will revert.
Lagerstroemia―Crapemyrtle (see separate handout)
Lindera species―Spicebush
Yellow flowers on naked-stemmed and evergreen species. Beautiful narrow-elliptic, dark green, glaucous below leaves turn yellow, orange, and red in autumn on L. glauca var. salicifolia. Undemanding, sun and shade tolerant, withstands heat and drought, hardy to Boston. Lindera obtusiloba and L. triloba produce delightful yellow fall color. Great shade tolerance as a genus.
Loropetalum chinense var. rubrum―Pink Fringe-flower
Not a single clone until 1989; over 30 in 2011. Changed the market for purpleleaf shrubs in Zone 7 to 9. Forget purpleleaf barberries. Reddish purple leaves with degrees of staying power (color) in South; pink, hot pink, to red flowers (Ever Red®) in spring. Sizes from 1 to 2’ (Purple Pixie®) to 25’ (‘Blush’). ‘Zhuzhou Fuchsia’ is the hardiest and I observed healthy plants in Cincinnati. ‘Crimson Fire’ is a new, compact-mounded spreading selection from Plant Introductions, Inc.
Magnolia stellata―Star Magnolia
Many cultivars with ‘Centennial’, ‘Royal Star’, and ‘Waterlily’ among the best. Years past grew a seedling population of ‘Centennial’. One proved spectacular with up to 56 tepals per flower, pink in bud, fragrant, and strewn with numerous flower buds. 12’ by 6’. Bailey Nursery will introduced in 2011.
Nandina domestica―Nandina
Too many, yet still being introduced. Functional, tenacious, and resilient. Will always be a part of Zone (6) 7 to 9 gardens. Blush Pink® (‘AKA’), Flirt™ (‘Murasaki’), and Obsession™ (‘Seika’) are compact growers with emerging colorful reddish foliage.
Osmanthus species―Tea-olive
Where adapted, Zone (6) 7 to 9, superb broadleaf evergreen shrubs with fragrant white flowers in fall or spring. Sun, shade, heat, drought . . . they excel. Osmanthus heterophyllus is the hardiest.
Philadelphus―Mock-orange
Like Deutzia species, minimal to no interest from commercial growers, but fragrant, purple-eyed, white-flowered types like ‘Belle Étoile’ and ‘Sybille’ are outstanding. ‘Manteau d’Hermine’ (double, fragrant, 3 to 4’) and ‘Silver Showers’ (single, fragrant, 5 to 6’, refined foliage) are garden worthy. With the repeat flowering trait, the plant would find commercial and greater garden acceptance.
Physocarpus species―Ninebark
Observed many times in wild, always by water courses, but exceedingly well-adapted to drier situations. Purpleleaf types are numerous with Diabolo® (‘Monlo’) the first. Best in cooler climates where foliage color is not diminished. ‘Little Devil’ and Summer Wine™ (‘Seward’) are smaller. Burgundy Candy™ (‘Podaras 1’), Center Glow™, Coppertina™ (‘Mindia’), and Lady in Red™ (‘Tuilad’) are additional purple-leaf types. More are on the way. Stay tuned.
Punica granatum―Pomegranate
Health benefits are touted! What about ornamental contributions? Pretty orange flowers in summer, bright yellow fall color, full sun, drought tolerant. ‘Purple Sunset’ with orange flowers and one-inch diameter shiny purple fruit grows 3 to 4’ high and is hardy in Zone 7. ‘Orange Blossom Special’ is 2 to 3’ high and wide, extremely floriferous, with small apple red-green fruits. Zone 8. Great choices for containers, annual beds, shrub borders. Prospers in full sun, heat, and well-drained soil.
Rhododendron―Rhododendron and Azalea
Difficult genus to wrap one’s arms around. Current focus of breeding is REBLOOMING and the Encore® azaleas, 25 strong, flower in spring, late summer-fall. Primary Achilles’ heel is minimal cold hardiness.
Bloom ‘N Again® (10 cultivars, 3 to 4’ high and wide) and Bloom-a-Thon® blanket evergreen reblooming azaleas from McCorkle Nurseries and Bob Head (Garden Debut®), respectively. Both appear to have Satsuki genes. Bloom-a-Thon® is listed as Zone 6 to 9. [Double Pink, Lavender, Double Red, Red (‘RLH1 1P2’ PP21,562), White; 2 ½’ to 4’ by 5’]. The Double Red is a true red with lustrous dark green foliage. Quite impressive.
Southgate™ Series: Brandy™, Breezy™, Grace™, Divine™, and Radiance™, all with pink white to lavender-purple flowers, have the heat tolerant R. hyperythrum as one parent. Zone 6 to 9 adaptable. Rhododendron hyperythrum habit and foliage remind of R. yakushimanum without the velvety pubescence on the lower leaf surface.
Rosa―Rose
Oversaturation? Who knows the best? Knock Out® series continues to lead the race. Drift®, Earth-Kind™, Flower Carpet®, Oso Easy®, Easy Elegance® are huffing and puffing to catch up. As I pen this in November, the original Knock Out® is still flowering with foliage to the base. For those gardeners who do not spray, it is about as good as it gets. Home Run® (single red) is a steady disease-resistant performer in the Dirr garden.
Sambucus―Elder
Black Beauty® (‘Gerda’) and Black Lace® (‘Eva’) with S. nigra genes are not comfortable in the South. Best as cut-back shrubs in most gardens. PII is breeding with S. canadensis to enhance heat tolerance.
Sarcococca―Sweetbox
Broadleaf evergreens with sweet cream-white flowers during winter-early spring. Ideal for shady habitats. Sarcococca hookeriana and var. humilis (hardiest) are most common but S. orientalis and S. confusa are better adapted in Zone 7 and 8. Nothing new except for two patented cultivars from Sidhu & Sons Nursery, Canada. Both are 1 1/2 to 2’ by 3’. Listed as deer resistant. I have grown seedlings of most species and experienced minimal variation. New species, S. bleddynii, from northern Vietnam, was described in 2011(see Plantsman 10(4): 232-237.
Spiraea―Spirea
Another yellow leaf spirea. Please, no! The pure folly is a handful of seed from yellow leaf cultivars produces numerous yellow leaf seedlings. Need one (1) that does not cook and discolor under high heat and drought. About 36 yellow types described in 2009 Manual. Magic Carpet™ (‘Walbuma’) is one of best. Highest rating in RHS trials.
Several “old” species that I have grown in the various Dirr gardens include S. cantoniensis ‘Lanceata’, Double Reeves Spirea, with blue-green foliage and terrific heat and drought resistance. Also, S. thunbergii with March-April twinkling single white flowers, bright green foliage, and yellowish tinged orange-bronze fall color. ‘Ogon’ (‘Mello Yellow’) sports soft yellow leaves; the color persisting in the heat. ‘Fujino’ Pink with pink-rose flower buds opens pink to white. Have contemplated breeding these iron-clad types, including S. ×vanhouttii, for improved habit, foliage, and flower.
Stewartia―Stewartia
Foliage, flowers, fall color, bark―first class. Stewartia monadelpha, S. sinensis, S. pseudocamellia are the most garden worthy. Can be frustrating for the nursery grower and gardener. ‘Scarlet Sentinel’ is a hybrid of S. ovata × S. pseudocamellia, white flowers crowned with red-pink (scarlet) stamens. Arnold Arboretum introduction.
Syringa―Lilac
~Twenty species and most disdain heat. What to do? Collect all and evaluate. Have done so at PII with S. pubescens subsp. microphylla, S. meyeri, S. ×laciniata, and S. protolaciniata ‘Kabul’ superior. Open-pollinated seedlings and controlled crosses afforded significant advances in reblooming traits. Bloomerang® (‘Penda’) is a rebloomer that suffers in Zone 7. Improved performance in North and was flowering in early October in Maine.
The joyous dividends of this breeding initiative are lilacs for the Deep South into north Florida yet with Zone 4 hardiness, and reblooming. Plant Introductions, Inc., has over 100 reblooming lilacs under evaluation with two currently in tissue culture.
Vaccinium ashei―Rabbiteye Blueberry
Vaccinium corymbosum―Highbush Blueberry
Gardeners can have their pie and eat it too. Many worthy cultivars. Blue Suede®, Bountiful Blue™ (‘FLX-2’), Summer Sunset™, and ‘Sunshine Blue’ (‘Blue Pearl’) are more restrained, self-fertile cultivars, with sweet fruits and orange to red fall color.
Viburnum―Viburnum (see separate handout)
Vitex agnus-castus―Chastetree
Beautiful lilac, violet, blue flowers in large panicles, summer into fall. Flowers on new growth so removal of spent flowers encourages new growth. ‘Shoal Creek’ rated best. Need smaller habit in 5 to 8’ range. Breeding at Georgia yielded nothing better than extant cultivars. Need good white, pink to rose, and deeper blue cultivars.
Weigela―Weigela
A common eyesore in midwestern gardens. Perhaps the ugliest pile of sticks in the winter landscape but numerous purpleleaf and variegated introductions provide a new lease on garden life. Yet to test any purpleleaf types that hold color in Athens (Zone 7). Flea beetles have done a number on ‘Spilled Wine’. Better in North. How many purpleleaf types can a garden stand?
