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Identifying Plants by Their leaves

By Marty Wingate

"Do you just know it?  Or did you figure it out?"

That was the question presented to me several years ago when I took a plant identification class from Clem Hamilton, then director of the Center for Urban Horticulture. 

There are many plants that we memorize as themselves.  We may know Daphne odora 'Aureomarginata' because of the white edges to the leaves and the standout fragrant, pink flowers in late winter.  We may know bigleaf maple because there is one growing in the park where we walk.

But other plants we need to figure out, and plant identification can be a good detective game if you have the right tools.  Those tools are knowing what to look for whether you are examining a bare winter twig or a plant in full leaf. 

Taxonomists use a key to identify plants.  A key is a series of questions or qualifications that leads from a broad description to increasingly narrow descriptions of a plant.  For the persistent woody stems of non-coniferous trees and shrubs, the first place to start is the leaf arrangement.

Leaves usually are arranged along a stem in either an alternate or opposite fashion.  This is the best place to start your ID, because there are far fewer species with opposite than there are with alternate leaf arrangements. 

Go out now and look at any member of the rose family, including apples (Malus), cherries and English laurel (both Prunus).  You will see a leaf (or leaf bud, if there are no leaves yet) first on one side of the stem, then a little further along on the other side of the stem.  Members of the rose family have alternate leaf arrangement.

The most commonly planted trees and shrubs with opposite leaf arrangement include maples (Acer), dogwoods (Cornus), horse chestnuts (Aesculus), and members of the honeysuckle family, Caprifoliacaeae.  That family contains such genera as elderberry (Sambucus), snowberry (Symphoricarpos), Viburnum, Weigela, and, of course, honeysuckle itself, Lonicera.

But rules are made to be broken.  Within one of the most common opposite-leaved species, dogwoods, are two notable exceptions.  One is the pagoda tree, Cornus alternifolia, which has, as its name explains to us, alternate leaves.  The other is the giant dogwood, C. controversa; its binomial reveals the "controversial" nature of its leaf arrangement.

For those of you who appreciate a mnemonic aid, these most common opposite-leaved groups can be shortened to: MADCAP HORSE, which stands for MA(maples)D(dogwoods)CAP(Caprifoliaceae)HORSE(horse chestnuts).

The next most important feature to examine is whether the leaves are simple or compound. 

Simple leaves are not divided and each leaf is attached to the branch in some fashion.  The stem of a leaf is called the petiole.  Beeches (Fagus), hornbeams (Carpinus), and hawthorns (Crataegus) are examples of trees that have simple leaves.  Some well-known shrubs with simple leaves include mock orange (Philadelphus), Cotoneaster, and blueberries (Vaccinium).

Compound leaves are divided up into leaflets.  There can be a few leaflets, such as with Akebia trifoliata (three leaflets) and Akebia quinata (five leaflets) or there can be many leaflets, such as with Mahonia x media 'Charity' (up to 21 leaflets) and the tree of heaven, Ailanthus altissima (up to 30 leaflets!).

Compound leaves can be twice or three-times divided themselves.  In other words, a leaf divides and then divides again (and perhaps again).  The devil's walking club, Aralia spinosa, has enormous leaves 60 inches long that are two or three times divided; total leaflets can be more 160.  Nandina is another shrub with leaves that are divided more than once.

To tell if you are looking at a branch with simples leaves or a compound leaf with many leaflets, examine where the leaf attaches to the stem.  If it's a leaf attachment, you will see a leaf bud in the axil.  If you are looking at where a leaflet attaches, then you won't see a leaf bud.

Compound leaves can be arranged in a variety of ways.  The leaves of a Mahonia, look like a feather; they are bi-pinnate.  Horse chestnut leaves are palmately compound.

Perennials have simple or compound leaves, too.  Coral bells (Heuchera) have simple leaves.  Columbines (Aquilegia) have compound leaves.  Meadow rue (Thalictrum) has compound leaves, too, and one species, Thalictrum aquilegiifolium, even tells us that its leaves look like a columbine.

Next up is the shape of the leaf.  There are quite a few different shapes, but some of the most common are cordate, heart-shaped as in the linden Tilia cordata; ovate and elliptical, which are close in shape and often used together ("leaves are ovate to elliptical"); and lanceolate, such as the willowleaf pear (Pyrus salicifolius). 

Leaf margins are a big clue to identification.  The margin, or edge, of the leaf can be entire, which means smooth and uninterrupted by divisions.  The leaves of Skimmia japonica and Magnolias are entire.  Leaves can also be lobed as those of the flowering currant (Ribes sanguineum), sweet gum (Liquidambar styraciflua), and Japanese maple (Acer palmatum).  Some cultivars of Japanese maple, such as 'Linearilobum',  are so deeply lobed that they appear almost compound.

Here is a selection of other leaf margin descriptions and examples of each: notched, Ginkgo biloba; serrate, Prunus serrulata or Zelkova serrata; undulate, Elaeaganus pungens; dentate, like teeth, Osmanthus delavayi.

And as if that isn't enough, using flowers to "key out" a plant is even more involved.  Let's save that for another time.

There are many books that contain keys and more descriptive information on plant identification.  Look for these books and more at local bookshops, online or read them at the Elisabeth C. Miller Library at the Center for Urban Horticulture.

Dirr, Michael, Manual of Woody Landscape Plants: Their Identification, Ornamental Characteristics, Culture, Propagation and Uses, Stipes Publishing, 1998, 1250 pages, $52.80

Gilkey, Helen Margaret, Handbook of Northwestern Plants, Oregon State University Press, revised paperback edition June 2001, 494 pages, $34.95

Winter Twigs: A Wintertime Key to Deciduous Trees and Shrubs of Northwestern Oregon and Western Washington, revised paperback edition, August, 2001, 118 pages, $19.95