Plant of the Month

Plant of the Month: February 2025

The most variable trees

We have heard much about diveristy, equity and inclusion. Trees vary in their appearance more or less. A handful of trees display astonishing diversity. My article this month is brief and hurried, but in March I hope to expand it. The four trees featured are exceptional for their named ornamental diversity; certain fruit trees such as the common apple tree, and the mango tree, have even more named selections.

Japanese Maple

Acer palmatum Th.

Native in Japan & S Korea.

One of 21 species of Acer native in Japan. No tree species has been more variable in cultivation. Perhaps 500 cultivars have been named. They can be dwarf, shrubs, or trees to 50 feet tall and wide. Leaves can be green, yellow, bronze, red, purple --or variegated. Leaves can be small, medium or large; lobed boldly or finely. Spring leaf color may differ from summer or autumnal color. Twig color can be olive, bright green, yellow, orange, red or purplish; some twigs are corky. Tree shape can be round, rigidly narrow, weeping, and more. Because of the diversity, gardeners prize collecting them and planting them in perfect settings with companion plants.

American Holly

Ilex opaca Ait.

Native from Texas to Connecticut & Florida.

Though over 1,000 cultivars have been named, they do not exhibit the diversity of appearance we might expect; few are notably distinctive; most are slight variations on a theme. The English Holly (Ilex Aquifolium) of Europe & NW Africa has fewer cultivars but they display far more visual diversity.

Lawson Cypress / Port Orford Cedar

Chamæcyparis Lawsoniana (A. Murr.) Parl.

Native in SW Oregon & NW California.

Introduced to cultivation in 1854 when seeds were sent from California by William Murray to Charles Lawson (1794 - 1873), then head of Peter Lawson and Son nursery of Edinburgh, Scotland. Offered for sale in 1858-59 by Golden Gate nursery of San Francisco. Commonly grown, easily propagated, and tremendously prolific of cultivars. My 1996 book North American Landscape Trees has more than 50 treelike cultivars known or supposed to have been planted in North America. Numerous dwarf cultivars, and trees grown only in Europe, were excluded. Only a tiny fraction of cultivars originated in North America. A deadly root-rot (Phytopthora) appeared in Seattle in 1923, was identified and named in 1943, and invaded native forests by 1952. This pathogen resulted in a dramatic decrease of the commercial availability of the species. But in recent years, disease-resistant rootstocks for grafting have been identified. Many cultivars lose their distinctive attributes gradually over the years, and become difficult to identify. The various golden cultivars become merely light green in shade. Certain cultivars that are dwarfed more or less if grown via cuttings, are rendered vigorous and treelike if grafted; 'Ellwoodii' is an example. The foliage has a distinctive resinous parsley odor whether juvenile or adult, green, blue or yellow. Staminate flower catkins are reddish-purple in spring. Round seed cones are the size of peas, about a third of an inch wide, but some cultivars make them smaller, and some make none at all. Tree shape is normally columnar but can be round, horizontal, or weeping. It is curious that in the wild the tree varies scarcely. I have no good number of how many cultivars of it have been named. The 2019 Hillier Manual of Trees & Shrubs book, lists 117, and likely another 100 or so are not in that book. This is definitely the most variable of all conifers. The 2nd edition of my book Trees of Seattle has 18 kinds, and some exist here which I cannot identify.

Croton

Codiæum variegatum (L.) Rumph. ex A. Juss.

Native in Malesia & E to NE Australia, Vanuatu & Fiji.

The most variable tropical plant, but grows only to about 15 feet tall, so is more a shrub than a tree. More than 500 cultivars are named. Other genera of shrubs boasting warmly exotic colorful leaves are called Croton sometimes by gardeners in the tropics, such as Acalypha, Graptophyllum and Pseuderanthemum. Croton the variable is in the spurge family, and not related closely to genus Croton of the same family. It's just that in 1753 Linnæus had called it Croton variegatum, and the common name persisted.

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