Michael Dirr on Spiraea thunbergii

Departments - Mike’s Plant World

With time on your side, there are plenty of breeding opportunities with this species.

August 16, 2019

1) Dirr may not have found a selection with a true pink flower, but many provided lovely red-purple fall color.
2) A true pink from ‘Fujino Pink’ parentage never emerged, but this selection has lipstick-red buds that open to pink-white flowers that cloak the stems.
3) This seedling with ‘Ogon’ parentage is growing in the Dirr garden. After four years, it’s remained compact at 3 by 3 feet.
4) Dirr and his breeding partners, Mark Griffith and Jeff Beasley, were trying to improve S. thunbergii ‘Ogon’.

Photos by Michael A. Dirr

With 80 species of Spiraea worldwide, S. thunbergii is somewhat of an outlier in the nursery and landscape pantheon. ‘Ogon’, a yellow leaf selection, is available and has been renamed/branded as Golden Thread, Lemon Zest, Golden Times and Mello Yellow. ‘Fujino Pink’ occasionally shows on the commercial radar. More on these later.

The species is inherently adaptable from Minnesota, Maine to Georgia, solid Zone 4 to 8, thriving in acid and near neutral soils, tolerating heat and drought. Habit is upright-spreading, forming a rounded outline, 3-5 feet high and wide (have observed larger). The bright green, fine-textured leaves (each 1-1½ inches long, 1/8-1/4 inches wide with slight marginal serrations) emerge in March, still present in December (in Athens) in shades of yellow-orange-bronze. The white flowers, each 1/3-inch in diameter, clothe the stems, opening in (January)February-March, lingering into April after leaf emergence. Fruit is a follicle, opening along one suture line, ripening in April-May. I collect fruits at the yellow stage, place in a paper bag, with seeds dehiscing soon thereafter. Seeds are extremely small and abundant. They germinate without pretreatment and viability is high. The species is native to China and commonly cultivated in Japan, where it has naturalized. It does not appear to have invasive tendencies like S. japonica, which is often cited as invasive.

Yellow foliage shrubs have been hot commodities. Witness the ascent of Ligustrum sinense ‘Sunshine’, a yellow foliage, sterile privet. The many yellow foliage selections of S. japonica, now close to 50 by my count, simply lose the yellow coloration in the south. At the University of Georgia, I worked to develop a yellow foliage S. japonica which did not lose color or simply fry in Zone 8. I produced many beautiful yellow, gold, and orange foliage seedlings but none that held the rich colors. Eventually, at Plant Introductions Inc., we focused on ‘Ogon’ improvement, since it holds the yellow foliage color without diminution in the heat. Hundreds of seedlings later with 18 promising selections for foliage color and compactness, all were pitched, with most being added to the Dirr garden. One (four years old) has remained compact, 3 feet by 3 feet, has held the yellow foliage color and is possibly worthy of introduction.

Concurrently, we pitched the idea of a true pink ‘Fujino Pink’ selection and after numerous seedling populations ended up with pink to lipstick red flower buds, with the flowers opening light pink to white. Nary one remained totally pink. Compact, spreading (almost groundcover-like), rounded to fastigiate habits were expressed. Selections were made and observed with none fitting the flower color criterion. Several selections had gorgeous red-purple fall color. Again, as with ‘Ogon’, we pitched all with five moved to the Dirr garden. One seedling has been spectacular, now 5 feet by 5 feet after four years, upright-spreading, forming a rounded outline, and lipstick-red buds open to pink-white flowers that literally cloak the stems (see photo on page 34). There are precious few shrubs that offer glorious late winter-early spring flower color, require essentially no care, and prosper over such a wide geographic range. Spiraea thunbergii is the earliest spiraea to flower.

A major frustration in plant breeding is insufficient time to determine the true worth of a particular selection. I have enjoyed watching these throw-aways prove their mettle over time. Hopefully, we will find a commercial home for one or both.

For more than 40 years, horticulturist, breeder, and author Michael A. Dirr has impacted the green industry through research, teaching, books, and plant introductions. michirr@aol.com