Article and photos (except where noted) by Katherine Wagner-Reiss

2003-062 Spiraea thunbergii ‘Ogon’ in bloom. Photo courtesy of Lucy Dinsmore.

Spiraea thunbergii ‘Ogon’ is a delightful early spring-blooming shrub in the rose family; come see it at the Morris Arboretum, and, since it is widely available at nurseries, you can consider it for your own garden. Spiraea comes from the Greek word speira meaning spiral or twisted: the flexible stems of spirea can be twisted into garlands and wreaths. The specific epithet thunbergii honors Carl Peter Thunberg (1743–1828), the Swedish physician who introduced this species to Europe and who named over 250 plant and animal species. He was one of Linnaeus’s so-called “apostles:” 17 men who travelled the world to send him natural specimens (and he is certainly one of the lucky ones, since seven did not return home alive). The first leg of his adventure was a successful three-year collecting stint in the Dutch Cape Colony in southern Africa, where he also learned to speak Dutch, essential knowledge for entry into his next stop, Japan. Offput by the zeal of European missionaries in the 1600s, Japan was closed to most foreigners, allowing only two Dutch East India Trade ships to enter the port of Nagasaki each year; Thunberg was able to pass as Dutch when he arrived as ship’s surgeon on one such boat in 1775. The travel, even of the Dutch, was restricted to a small island in the harbor. Thunberg got away with a few explorations around Nagasaki, and he was able to collect some plants as he accompanied the Dutch ambassador who visited the Shogun in Tokyo, but his main source of plant material was Japanese interpreters; some of these were physicians anxious to learn about Western medicine, including mercury treatment for syphilis. This naturalist is honored with a plant genus name, Thunbergia, and many specific epithets e.g. Allium thunbergii, Berberis thunbergii, Geranium thunbergii, and Pinus thunbergiana.

Thunbergia grandiflora in FL 

Thunbergia alata in FL

Thunbergia erecta in FL


2006-067*A Spiraea thunbergii ‘Ogon’ (Mellow Yellow Thunberg spirea)  

‘Ogon’ means gold in Japanese, and the willow-like foliage of the cultivar ‘Ogon’ is known for being golden instead of the usual green, especially when grown in full sun. Discovered in a Tokyo plant market by Barry Yinger, a Pennsylvania plantsman who made over 60 plant-hunting trips to Japan, it was introduced into the USA in 1993, and is now more popular than the green-leaved parent species whose common name is baby’s breath spirea. The trade name for “Ogon’ is MELLOW YELLOW Thunberg spirea. It is difficult to track down the origins of cultivar names, but a possible guess is that this one might have been inspired by the popular song “Mellow Yellow,” written and recorded by Scottish singer-songwriter Donovan in the 1960s.

How do you pronounce Spiraea thunbergii ‘Ogon’? Spiraea is pronounced spy-REE-ah; the common name, spirea, drops that silent “a,” making the pronunciation more straightforward. As far as thunbergii, the general convention is that a proper name be pronounced the way that the honoree pronounces or pronounced his/her own name; thus, Toon-BERG- ee- eye would seem to be the most authentic choice, although you will hear reputable sources pronouncing it as Thun-BERG-ee-eye and even Thun- BERJ-ee-eye. And finally, how is ‘Ogon’ pronounced? Since 1/1/1959, new cultivar names are given not in botanical Latin, but in the vernacular. ‘Ogon’ rhymes with shogun to my ear, but to hear the authentic pronunciation you need a Japanese speaker.黄金

Come see this shrub and many other species of Spiraea blooming at the Morris Arboretum this April, including three species native to the US: S. betulifolia ‘Tor’ from western NA, S. latifolia from eastern NA, and S. virginiana from southeastern US. A particularly historic specimen remains from the time of the Morris estate: Spiraea x vanhouttei, which you may know as good old-fashioned bridalwreath. If you are interested in seeing another plant named for C. P. Thunberg, there are three Pinus thunbergiana at the Morris Arboretum. Find exact locations of any of these plants at Collection Connection.

1986-079*G Pinus thunbergiana—new spring growth (candles), male and female cones

Berberis thunbergii ‘Rose Glow’. You won’t find this barberry at the Morris Arboretum: B. thunbergii is a Class 1 invasive species in Pennsylvania, although some cultivars produce less seed than the species.


Tomorrow is the Spring Equinox! It’s time to celebrate increasing daylight, warming temperatures, and new beginnings. Morris Arboretum encourages you to get outside and awaken your senses…breathe in the fresh air, listen for the birds, and discover what’s blooming in the garden.

Epimedium x warleyense 'Orange Queen' and Chionodoxa - Found in Widener Woods
Prunus subhirtella Higan cherry - Found XX
Helleborus niger 'Potter's Wheel' - Found in Hill and Water Garden

Information provided by Aaron Greenberg at West Laurel Hill Cemetery, and former intern at Morris Arboretum.

Morris Arboretum’s Spring/Summer Seasons newsletter erroneously referred to our Katsura tree as a ‘former’ state champion in the article about Secret Spaces. In fact, it is very much an active state champion, which brought to mind: ‘what constitutes a state champion tree?’ Here is how it’s measured.

Each tree nominated to the PA Champion Tree Program is assigned a point value. The tree with the most points for each species is the Champion! The Big Three Program takes into account three measurements:

Circumference in INCHES: 1 point for every 1-inch trunk circumference +

Height in FEET: 1 point for every 1 foot of tree height +

Spread in FEET: 1 point for every 4 feet of canopy spread =


The first measurement is Circumference at Breast Height (CBH).

CBH in INCHES is 4.5' above the ground (this is the standard measure of "breast height" used by foresters nationwide)

If the tree has multiple stems (like Morris Arboretum’s Katsura), measure CBH of the LARGEST stem at 4.5', or above where the stems are conjoined

Each tree gets 1 BIG TREE POINT for every INCH of circumference.

The Katsura is 318" in circumference.

The Height of a tree is measured as the distance in FEET measured from the point where the tree emerges from the ground to the tallest living part of the tree.

Each tree receives 1 BIG TREE POINT for every FOOT OF HEIGHT

The Katsura is 64' tall.

Canopy Spread is a horizontal measurement, from leaf tip to leaf tip, of the extent of a tree’s canopy. This measurement is relatively easy to take with a long tape measure and a friend. One stands at one end of the canopy at the furthest end of a living branch, and the friend extends the measuring tape to the other furthers living branch in a straight line.

Because tree canopies are not perfect circles, it is important to get TWO measurements of Spread taken PERPENDICULAR to each other if possible. These two spreads are averaged, and that AVERAGE SPREAD is considered for estimating a point value. of the shortest spread, and the longest spread of the tree. Adding the two numbers together, and then dividing by two will give you the average crown spread.

Each tree gets 1 BIG TREE POINT for every 4 FEET of Average Canopy Spread. (Each FOOT of Average Canopy Spread adds ¼ POINT to the BIG TREE SCORE)

The Katsura has a 96' canopy spread.

Not only is Morris Arboretum's Katsura a state champion, it’s clearly a favorite with visitors. Its huge, spreading canopy and wide, exposed roots are awe-inspiring. The leaves provide a three-season show. They emerge a beautiful, light pink in spring before turning pale green. In summer, the leaves are blue-green and in fall, turn a yellow-apricot color. As the leaves fall and decompose, they give off an odor reminiscent of caramel or cotton candy.  The Katsura is a must-see attraction for any visitor!


Article and photos by Katherine Wagner-Reiss

As you enter the Morris Arboretum from the main parking lot, you will encounter a tall stainless steel sculpture, Two Lines, by the American artist George Rickey; it marks the former site of the Morris mansion, which stood there from 1889–1968. Straight downhill is a striking, large evergreen from the original Morris estate called Chamaecyparis pisifera 'Squarrosa'. ‘Squarrosa’ is a very old cultivar originating in Japan; first brought to Europe in 1843, it is not surprising that the Morrises had this cultivar in their collection.

1932-1127*A Chamaecyparis pisifera ‘Squarrosa’ downhill from the Two Lines sculpture (inset)

Deconstructed, this plant name is less daunting.

First: Chamaecyparis. Translates to “low-growing cypress”—Chamaecyparis is in the cypress family.

Second: pisifera. Means “pea-bearing”— the globose immature green cones resemble peas.

1932-1127*A Chamecyparis pisifera ‘Squarrosa’

Third: ‘Squarrosa.’ Chamaecyparis pisifera has mossy juvenile foliage with awl-like leaves that are squarrose i.e. spreading outwards almost 90 degrees from the stem; adult foliage has leaves that lie flat against the stem producing a scale-like effect. Since this cultivar was selected for a mutation that resulted in juvenile leaves, it was named ‘Squarrosa.’ Sometimes, however, the mutation reverts, and some adult foliage is seen on ‘Squarrosa,’ as in the photo below.

1932-1127*A Chamaecyparis pisifera ‘Squarrosa’ Dark green scale-like adult foliage on the left
contrasts with bluish-green squarrose juvenile foliage on the right. Each juvenile leaf is described as awl-like
i.e. tapering to a slender, stiff point at the end. (Inset photo shows an awl for comparison.)

Dimorphic juvenile and adult leaves also occur in some members of the cypress family besides Chamaecyparis e.g. Cupressus (cypress) and Juniperus (juniper). Physiologic studies have shown that juvenile-type leaves are advantageous for growth and adult-type leaves enhance stress tolerance.

The straight species, Chamaecyparis pisifera, grew on the original Morris estate along with the cultivars ‘Squarrosa,’ ‘Plumosa Argentea,’ ‘Plumosa Nana,’ ‘Filifera’ and ‘Filifera Aurea.’

Many of these old trees and/or established cuttings of them can still be seen gracing the Morris Arboretum. Their leaves distinguish them: the species has juvenile leaves succeeded by adult leaves (as nature intended), ‘Squarrosa’ produces primarily the juvenile leaves, ‘Plumosa’ produces semi-juvenile leaves, intermediate in length and transitional in form between juvenile leaves and adult leaves, and ‘Filifera’ describes branchlets bearing thread-like strands of adult leaves.

Over the years, additional cultivars have been added to the Morris Arboretum to include compressed, pygmy, and variegated cultivars, for a total of 13 cultivars; with more than one hundred cultivars of C. pisifera available, selectivity is required! Exact locations can be found at Collection Connection

P.S. If the thought of uttering Chamaecyparis pisifera makes you feel tongue-tied, go online to Missouri Botanical Garden Plant Finder, where you can hear the pronunciation of this and thousands of other plant names!

Katherine has her Certificate in Botany from the New York Botanical Garden and is a botanical tour guide and freelance writer. You can contact her with comments or requests for photos at