Vitex agnus-castus, chaste tree, and the 249 species relatives have endured indignities from gardeners for eons. Although common in herbal and medicinal lore, seldom does it appear in gardening literature or retail garden centers. Typically, V. agnus-castus is simply too large, oafy, cumbersome and intrusive in the average garden. If, and that is a huge assumption, size and habit could be reduced, hardiness increased, foliage more refined and disease-free, compact inflorescences of rich saturated blue-purple, and perhaps color in the developing fruits, then landscape momentum might accelerate.
Vitex could do with a makeover as suggested above and breeding/selection has occurred at Chicago Botanical Garden, The University of Georgia, Stephen F. Austin University, Louisiana State University, and our company, Plant Introductions Inc.
Where do the genes reside to provide the attributes listed above? If Vitex improvement was easy, a plethora of cultivars would already exist. Those that do are chance seedling selections. To date, I know of none via controlled breeding, but that is about to change.
The most common species is V. agnus-castus L., Chaste-tree, which reaches 15 to 20 feet high and wide. The National Champion, 28 feet by 40 feet, resides in Lancaster, Texas. Variety latifolia Mill. is considered more vigorous and cold-hardy. I am not sure I could separate the two taxa, in fact, GRIN, the USDA Taxonomy Resource (www.ars-grin.gov), does not list var. latifolia. Does any arboretum or garden have the genuine article?
The dark gray-green, aromatic, compound palmate leaves are composed of five to seven leaflets that grow 2 to 4 inches long, tapering at the apex and base. The lower leaf surface is gray with a fine pubescence. Leaves impart a refined, graceful texture and are free of serious insects and diseases. Leaf spots can be troublesome, especially with overhead watering in production nurseries.
With the pretty foliage as a backdrop, the blue to pale violet flowers appear in 12- to 18-inch long panicles in June or July and continue into September. Spent flowers can be removed with subsequent rebloom in approximately six weeks. The flowering on new growth and the measure of remontancy (rebloom) without spent flower removal guarantees shades of blue from USDA Hardiness Zones 6-9.
The species succeeds from Long Island and Chicago, to Oregon and Washington, into the Deep South, Southwest and California. There’s a terrific breadth of landscape potential if Vitex can be developed along the concepts of Knock Out roses and Endless Summer hydrangeas. Flowers open in early June in Athens and mid-August in the Portland, Ore., area. Obviously, the greater the heat, the earlier the flower. It’s important to note that flowering is still potent in September on plants where old inflorescences were removed. In production, pruning could be manipulated to schedule flowers for late summer and fall markets. Flowers open from base to apex on the elongating panicle and this provides color during a two- to four-week period. Few people realize that the flowers are fragrant, provide great bee pasture (Vitex honey, especially from V. negundo and varieties, is highly prized) and attract hummingbirds and butterflies. The bullet points for marketing this species could fill a billboard.
Vitex negundo L. and the cutleaf types (var. cannabifolia and var. incise) are more cold-hardy (Zone 5 or 6) than V. agnus-castus, but simply do not possess the flower stature. The lavender flowers occur in wispy 5- to 8-inch long panicles on new growth of the season. Fruits are similar to V. agnus-castus, but smaller.
Habit is loosely branched, vase-shaped, airy and open with a cloud-like foliage canopy. The two cutleaf varieties offer unique incised leaflets and refined texture. It’s truly amazing that no cultivars have arisen from the species or varieties. At Plant Introductions, seeds of var. incisa yielded 100 percent cutleaf seedlings in a range of sizes with consistent lavender flowers. If compact habits could be bred to include showier flowers, I believe cultivar proliferation would result.
On a recent trip to China, Donglin Zhang, Dirr Professor of Woody Plants at the University of Georgia, noted entire mountainsides covered with V. negundo. I asked Donglin to be on the hunt for brilliant blue-flowered, compact selections.
Vitex rotundifolia L.f. probably has more detractors than supporters because of its invasive tendency, especially in beach habitats. It was observed at Wrightsville Beach, N.C., where it was out-maneuvering all other vegetation. This species is on the hit list of the Global Invasive Species Database.
All is not lost, as the hybrids with V. agnus-castus are intermediate in foliage, with small blue-flowered inflorescences on a more refined shrub habit. When first witnessing the plants at the North Carolina Arboretum, I was impressed. Kris Bachtell, Vice President of Collections and Facilities at the Morton Arboretum in Lisle, Ill., mentioned the hybrid was successful without any dieback in recent years. Flowering initiated in late September. It’s usually listed as Zone 7, but I suspect it may be more cold-adaptable.
Last but not least from a breeding perspective is V. trifolia L., the specific epithet reflecting the three-leaflets/leaf. Leaves actually vary from simple (one) to the three leaflets, at least on ‘Purpurea.’ Foliage is gray-green, flowers lavender to light blue in up to 10-inch long panicles. The plant is essentially evergreen in Orlando, Fla., and a dieback shrub in Athens, Ga. My thinking was to hybridize the purple leaf type (lavender-purple on underside) with V. agnus-castus to flip the purple and develop a purple upper surface. In 2013, hybrids were consummated with purple, but only on the underside. The leaves were composed of five leaflets, intermediate between the parents. Flowers were deeper blue and inflorescences larger than V. trifolia. The individual fruits were large like V. trifolia.
Culture is clear-cut – well-drained soil, acid or alkaline (species is salt tolerant), and full sun then watch it thrive. Typical Zone ratings are 7-9 (10), but the species is successfully grown in Zone 6. Expect some dieback at -5°F to -10°F, but flowers will develop on the new growth. In fact, the species is extremely fast growing with a 3-gallon container ‘Shoal Creek’ planted in fall, reaching 7 feet 2 inches high and 5 feet 5 inches wide in October in Athens, Ga. Delta Blues, planted next to ‘Shoal Creek’ and at the same time, was 4 feet 9 inches high and 5 feet wide.
Leafy cuttings root well in May to August. A rooted cutting, spring transplanted to a 3-gallon container, will be ready for sale in late summer. Seeds require no pretreatment to germinate. One study reported a 24-hour hot water (180°F) soak proved best. I have used nothing, cold water soak, hot water soak, and 30 days cold-moist stratification. Germination occurred with all treatments, but I can’t state a clear-cut winner. Invasiveness is mentioned, particularly in Texas, and I have yet to find a stray V. agnus-castus.
Cultivars for different flower colors and compactness have been introduced.
The following list was cobbled from the literature and my observations.
‘Abbeville Blue’ – Produces deep blue flowers; selected in early 1990 by Louisiana Nursery.
‘Alba’ [f. alba (Weston) Rehder] – White flowers.
‘Arnold’s Cutleaf’ – Leaflets like a dissected Japanese maple, blue-purple-lavender flowers, grows 10 feet by 10 feet.
Blue Diddley – Lavender-blue flowers on a small, 3-6 feet by 3-6 feet shrub; Proven Winners 2015 introduction.
Blue Puffball – Unique to the world of Vitex, this compact, 3 foot by 3 foot, densely branched shrub, produces copious blue flowers in summer. Foliage is a rich glistening blue-green with resistance to leaf spot. Flowers develop on new growth, so prune spent flowers to induce another flush of bloom. Quantum leap forward in Vitex breeding and will be available in 2016. Bred by Plant Introductions Inc. and introduced by Bailey Nurseries in the First Editions brand.
‘Blue Spire’ – Mentioned without description in RHS Plant Finder 2013.
‘Blushing Bride’ – Also mentioned without description in RHS Plant Finder.
‘Blushing Spires’ – Soft pink flowers, growth more restrained; from Niche Gardens, N.C.
‘Carolina Blue’ – LSU, no description.
Cooke’s Blue – Blue-lavender flowers, 15-25 feet by 15-25 feet from L.E. Cooke.
Cooke’s Pink – Pink flowers, 15-25 feet by 15-25 feet from L.E. Cooke.
Cooke’s Purple – Purple flowers, 15-25 feet by 15-25 feet from L.E. Cooke.
Cooke’s White – White flowers, 15-25 feet by 15-25 feet from L.E. Cooke.
Delta Blues – Rich purple-blue flowers in compact panicles on a more refined plant, 8 to 10 feet high and wide, discovered by Matthew A. Dirr. Grew two-thirds the size of ‘Shoal Creek’ in one year. Introduced by Plant Introductions Inc., and available through Bailey’s First Editions brand. This is a great garden plant, especially for the richness of the flower color and abundance of panicles on a smaller statured plant than the old oafy cultivars. Would make a pretty container standard tree for summer color.
‘Fletcher Pink’ – A pale pink to lavender-pink flowered selection.
‘Flora Ann’ – Pinker great-grandchild of ‘Salinas Pink,’ named for Flora Ann Bynum of Old Salem, N. C., introduced by Greg Grant of Arcadia, Texas. Grant continues to evaluate open-pollinated seedlings for richer, deeper pink colors.
‘Le Compte’ – Discovered and described by Greg Grant as the “most beautiful Vitex I ever saw.” Sapphire-blue flowers in long inflorescences. Found in “a little old yard in Le Compte, La.”
‘Lilac Queen’ – Lavender flowers, broad-spreading, multi-stemmed shrub, 20 feet by 18 feet, introduced by Plantation Tree Co., Selma, Ala.
‘Mississippi Blues’ – More compact selection yet still will grow to 12 feet; robust grower with deeper blue flowers similar to ‘Abbeville Blue;’ there are essentially no differences in flower color among ‘Mississippi Blues,’ ‘Abbeville Blue’ and ‘Shoal Creek’ based on comparisons of the three cultivars in Georgia trials.
‘Montrose Purple’ – Produces rich violet flowers in large inflorescences; strong-growing form, 8 to 10 feet by 8 to 10 feet, rated just after ‘Shoal Creek’ in Longwood trials; an introduction by Nancy Goodwin of Hillsborough, N.C.
‘Patton’s Pink’ and ‘Pink Sensation’ – LSU, no description.
‘Rosea’ – Pink flowers.
‘Salinas Pink’ – Light pink flowers; from a yard north of downtown San Antonio; Greg Grant introduction.
‘Sapphire’ – Not sure of veracity of the name, might equate with ‘Le Compte.’
‘Sensation’ (‘Sensational’) – Mid-blue, offered by Gossler Farms Nursery, Springfield, Ore.
‘Shoal Creek’ – Large, blue-violet flowers in 12- to 18-inch long inflorescences and leaf spot resistance, although have observed leaf spots on plants under overhead water.
‘Silver Spire’ – Cleaner white flowers than ‘Alba’ and good vigor.
‘Snow Spire’ – White flowers in panicles as large as the species.
‘Woodlander’s White’ – Another white-flowered selection.
Ask yourself, how many of the cultivars are being offered in everyday commerce?
Four new cultivars will be released through The University of Georgia Research Foundation and EuroAmerican Propagators -- Daytona Heat Petty Blue, Daytona Heat Danica Pink, Daytona Heat Dale White (all the previous listed as 4-6 feet high) and ‘Little Madame’ (delicate purple flowers, tight compact habit, 3-4 feet high).
All are Zone 6-9 adaptable. EuroAmerican has them under license/test, and I suspect they’ll be introduced, but I haven’t heard a release date. The Danica Pink at the University of Georgia’s Horticulture Farm was 10 feet by 10 feet when observed on June 24, 2013. At a January 2015 seminar, the breeder, Carol Robacker, mentioned a 2015 availability release.
Robacker also mentioned Pink Pinnacle, a compact, rounded, bushy selection, that grows 3-4 feet tall and wide with bright-pink flowers. This would be a worthy addition to the world of Vitex. It’s listed as a 2016 release.
Vitex offers a new frontier for controlled breeding. The four species discussed provide the best opportunity for reasonable cold hardiness. Based on the number of universities and private companies breeding or contemplating the same, I believe Vitex has an exciting future. I envision a 2 foot by 2 foot compact cultivar, smothered with vivid blue flowers.
Maybe not the next Endless Summer, but then again …
Dirr is a retired professor of horticulture at the University of Georgia; prolific plant breeder, author and consultant at Bailey Nurseries.