Gold standard

Another yellow/gold leaf Spiraea japonica and the garden world is rendered sun blind. With some 40 types, and the number increasing yearly, where’s the uniqueness? The overarching need is a yellow foliage selection that does not discolor or bake in the heat. Even in Northern climes, most yellow cultivars are more green or a mottled mosaic of green-yellow as summer wanes. Are there other options?

Certainly, for with 80-100 species of Spiraea, there is significant genetic and phenotypic variation to develop better “sun traps.” During sabbatical at Hillier Arboretum, I collected field notes on numerous species with S. betulifola var. aemiliana, S. chamaedryfolia var. ulmifolia, S. formosana (name questionable), S. fritschiana, S. hayatana and S. miyabei (most unique). Seedlings of the above were grown to flowering at The University of Georgia and the Center for Applied Nursery Research, and with the exception of a yellow leaf S. formosana, none passed the “wow” test. Unfortunately, second generation container-grown plants of the above faded in the heat. Most of this work was conducted in 2000 and 2001. The failure to develop a single respectable yellow leaf selection caused me to shelve the premise until …
 

A fresh start

I found myself in a new home on a difficult site. What to plant? I found 1-gallon S. thunbergii, picked the holes (similar to concrete), and three years later, I had a 4-foot high specimen forming a billowy, flowing matrix with twinkling smallish white flowers, and 90 percent leaf retention in late December 2012. Leaves typically emerge in March (in Athens) as a soft light green, developing yellow then bronze and at times red-purple coloration in autumn. Foliage is extremely frost resistant and is persistent to at least 25°F. The species is an heirloom plant in the South, extant in old homesteads, often weed-choked, but still breathing after decades of neglect. During a December 2012 trip to Charleston, S.C., I noticed the species in several gardens but most prominently in St. Philips Episcopal Church cemetery, next to a monument dated 1877. Longevity? Doubtful, but possible since introduction date is listed as 1863.
 

The next step

The above experiences ignited the idea to initiate a breeding strategy involving S. thunbergii and related species. Before detailing Plant Introductions Inc. initial experiences, permit me to showcase the species. Spiraea thunbergii Sieb., Thunberg spirea, is a wispy, feathery, loose-fitting amalgamation of fine-textured leaves and stems, maturing 3-5 feet high and wide. Typically, plants are wider than high at maturity with the largest observed by me at 5 feet by 8 feet. The delicate bright green leaves range from 1-1½ inches long, 1/8-1/4 inch wide, with margins finely serrate from apex to base. The refined texture provides elegant contrast to bold-foliage broadleaf evergreens and large leaf flowering shrubs.

Flowers, five-petaled and 1/3-inch across, are borne in three- to five-flowered sessile umbels along the length of the naked stems in late February, spilling into March in Athens. The entire plant is a fountain of snow. Fruit is a small follicle (splits along one suture line) and contains two or more seeds. Collect seeds in April-May when capsules turn yellow, place on newspaper at room temperature, and tiny seeds are easy to sieve.

If the plant is truly elegant, why are they so seldom available and, if available, would gardeners purchase it? Legitimate questions and, for a career, except in the Dirr garden, I never considered the species a commercial entity. Why? And there’s the question: where’s the color?

Enter ‘Ogon,’ a yellow-leaf cultivar that is remarkable for maintaining color in the heat. In 2011, with 80-90 days above 90°F, no appreciable rain, ‘Ogon’ remained pristine. Could this be used to breed with S. cantoniensis, S. prunifolia, and S. × vanhouttei, all well adapted to Zone 7 conditions, for a new generation of yellow flowering shrubs? These species and approximately 15 others are taxonomically sequestered in the section Chamaedryon and have similar flower and fruit characteristics, so they should prove hybridable.
 

Making progress

On April 2, 2012, seeds of ‘Ogon’ were sown without pretreatment and seedlings transplanted to cells on May 18, 2012. Unexpectedly, 75 percent of the hundreds of seedlings were yellow foliage. All yellow leaf seedlings and a few green were transplanted to 2-gallon containers and grown in full sun at Plant Introductions Inc. Groundcover, compact-mounded, wispy habits, along with pure yellow, gold, yellow-green, non-burning foliage were showcased. Many seedlings exhibited vibrant yellow/gold foliage on Dec. 22, 2012. What started as an exercise in hope, resulted in a healthy dose of exceptionally beautiful plants. Most have developed flower buds indicating an abbreviated juvenile period.

We have now selected yellow leaf forms for compactness and I’m shooting for a 2- to 3-foot mound that holds the yellow color. We are in the second year and have about 50 selections for these criteria. In another year we will have something. The remarkable thing is the seed produced about 70 percent yellow leaf seedlings. Seed was collected from an isolated planting of ‘Ogon,’ so it self-fertilized. I have yet to witness a yellow/golden leaf form of S. japonica than keeps its color in the Southeast. These ‘Ogon’ seedlings keep the color. The most difficult aspect has been to pick the best one, because they are all nice.
 

Beyond yellow

Is yellow foliage the best PII has to offer? Consider ‘Fujino Pink,’ with pink-red flower buds, opening pink, quickly maturing white. Does the potential exist to breed deeper pink, perhaps red flowers with repetitive crosses with the species, in effect stacking the pink genes?

‘Mount Fuji’ has cream-white variegated leaves and white flowers, but will revert. ‘Yatsubasa’ is a double white selection that I have not observed in cultivation. Recently, scanning botanical garden databases, I noticed ‘Compacta’ was described. It’s difficult to know if this should be attached to S. × arguta, a hybrid of S. thunbergii × S. × multiflora.

There is potential for successful hybridization among the related species. Spiraea × multiflora is a hybrid of S. crenata × S. hypericifolia. It appears there is breeding wiggle room. More in a few years — I hope.

Michael A. Dirr is founder of Plant Introductions Inc.; www.plantintroductions.com and retired professor of horticulture at the University of Georgia.

Photos courtesy of Michael Dirr