The scent of a winner

Features - Flowering Shrubs

Better hardiness and double flowers are only two reasons to reconsider Gardenia jasminoides.

August 1, 2017

Sweet Tea is the result of a controlled cross between ‘Double Mint’ and Pinwheel.
Michael A. Dirr

Why Gardenia jasminoides? The answer is . . . why not? The proliferation of new and improved cultivars has heightened enthusiasm for this most recognizable and time-honored garden plant. Primarily a Chinese native, it’s found in 15 provinces, as well as Taiwan, where it grows in thickets and forests at stream sides, on mountain slopes or hills, in valleys or fields, near sea level to 5,000 feet. It’s also listed as native in Bhutan, Cambodia, India, Japan, North Korea, Laos, Nepal, Pakistan, Thailand, and Vietnam. The species displays significant genetic and phenotypic variation; a fact undergirded by the tremendous range of cultivars. The Flora of China notes sizes from 1 to 10 feet in height. I observed similar size variation on plants in cultivation.

Flowers develop from old and new growth with sporadic production into late summer and early fall. Many of the newer cultivars were selected for repeat flowering. Petals are white, occasionally yellowish, usually aging to yellow before turning brown. Self-cleaning cultivars would be a welcome contribution. Six petals are the norm, although five to eight may occur. The doubles have multiple whorls of petals, often as many as three to four. Petal size ranges from approximately ¾ inch to just over 1½ inches long by ¼- to 1-inch wide in the species, often considerably larger in the cultivars. Flowers open in May in Zone 8 and are effective for three to four weeks. Their fragrance is possibly the most alluring and sensuous of all flowering shrubs.

Fruit is an ellipsoidal yellow, orange to orange-red berry, ¾ to 2½ inches long and ½ to 1 inch wide with five to nine longitudinal thin ridges (wings). The small white seeds, meshed with the pulp, are easily extracted, and your fingers become a pretty orange in the process. Seeds require no pretreatment for germination. Depending on cold, fruits may persist into spring by which time they are a bit untidy.

The search for increased hardiness

Many references ascribe Zone 8 and warmer adaptability, but breeding opened the portal to increased hardiness (a solid 7, perhaps 6b), more restrained habits and notable floral traits such as single, double, and persistent reblooming. Earlier introductions like ‘Kleim’s Hardy’ (single) and ‘Chuck Hayes’ (double) proved that a dollop of cold hardiness was present in the genome.

For my University of Georgia program, the hardiness leap forward came around 1983 when I traded Illicium seed for wild-collected, cold-hardy Chinese gardenia seed. Certainly, I had no idea these seeds would morph into the more cold-hardy ‘Shooting Star,’ ‘Griff’s Select,’ Heaven Scent, and Pinwheel over a 34-year period.

In early April 2007, a field trial of seedlings from cold hardy taxa and known cultivars were severely injured or killed at the Horticulture Farm. Low temperatures were 28.6°F, 26.6°F, 33.6°F, and 31.6°F on April 7, 8, 9, 10, respectively. Taxa like ‘Radicans’ were killed; ‘Frost Proof’ had 50 percent injury, while seedlings from Heaven Scent and Shooting Star-01-03 (sister seedling of Heaven Scent, but not introduced) displayed variable injury. Only numbers 2, 8, and 18 from Heaven Scent were unscathed; not a single dead leaf. Subsequent testing in 2009 and 2010 by Mike Hayman in Louisville, Ky., with cold approaching 0°, showed number 18 with a few tip burned leaves; number 8 with all dead leaves, and number 2 with about 50 percent leaf kill. Number 18 became Pinwheel (‘PIICA-1’, PP22,510) and was licensed to McCorkle Nurseries by Plant Introductions Inc. (PII).

All the seedlings were singles, with abundant flowers in May-June, variable rebloom into September, with number 18 the most prolific. Single-flowered gardenias are not as highly prized as doubles. To be sure, there are many doubles on the market, so some breeder/selector figured it out. It left us wondering, how does/did PII move beyond singles?

Double delight

I discovered two fruits (morphologically a berry) on ‘Chuck Hayes,’ cold-hardy to about 5°F, double-flowered, typically sterile cultivar that originated in Virginia Beach, Va. (HortScience 29:829-830, 1994). Plants in the J.C. Raulston Arboretum had reached 5-6 feet high and wide. Seeds were cleaned and sown with several resultant doubles, two of which became ‘Double Delight’ and ‘Double Mint,’ the latter with lustrous, leathery dark green foliage, more compact habit, 2-inch diameter double flowers and persistent rebloom. Absolute hardiness is unknown, but based on no injury at 6°F on Jan. 7, 2014, a strong 7a is justified. The serendipitous double-flowered seedlings from open-pollinated ‘Chuck Hayes,’ as well as another read of the HortScience paper, portended a pathway to more doubles via cross-pollination.

Heaven Scent and Pinwheel (singles) were crossed with ‘Double Mint’ (double) in 2011. In 2013, eight double-flowered seedlings were selected, one with perfectly formed, 3-inch diameter, glistening white, fragrant flowers. (See Sweet Tea in the chart). The range of leaf shapes (rounded, ovate, linear) and habits (compact, rigidly upright, spreading) was remarkable. The majority of the seedlings and all the early double selections were not injured by the 6°F temperatures on Jan. 7.

The logical question to pose is, where can breeders probe for gardenia improvement? A true Zone 6 cultivar with double flowers and potent rebloom would prove a game-changer. I have yet to witness a trace of pink in seedling populations. Somewhere in the Gardenia jasminoides genome, I am hopeful that a pink gene is waiting to be expressed.

The leaves of Gold Doubloon begin golden and mature to green.
Mchael A. Dirr