Tree Identification with a Winter Key

By Laura Gardner, Master Gardener

Plant apps can be useful identification tools but their accuracy often depends on the quality of the photos and the features that are being examined. They tend to work best with more unique features such as fruit or flowers. I find that as a learning tool they can be limited because they do not give the user details about how or why the particular species was suggested. There are some other tools that can lead to an identification as well as help hone identification skills.

There is a little book called Winter Tree Finder by May Theilgaard Watts and Tom Watts. Originally published in 1970 and at only 58 pages, it is the perfect size to take into the garden or on nature walks. Known as a dichotomous key, it covers deciduous trees of the Eastern US and Canada and provides maps that show the various species ranges. Dichotomous keys lead you along a path towards the correct species by asking a series of questions about the tree’s various parts. This particular key relies mainly on the appearance and arrangement of terminal and lateral buds on twigs, twig width, bud scales, and vein or vascular bundle scars for identification. Other features may also be considered such as lenticels (pores), bark colour, pith (tissue inside twig), fruit, thorns, etc. It also includes a ruler on the back cover that is used to measure the width of the twigs.

There is a tree that is growing on my neighbour’s property that I know is Acer negundo (Manitoba Maple) and I decided to see if this book could identify it. I pruned a twig that was hanging over on my side of the fence. If you wish to identify other trees that are not on your property, it is appropriate to take photos, take a guidebook or key with you, or get permission to take a cutting.

Step 1: I noted whether the leaf scars are in an opposite or alternate arrangement (phyllotaxy) on the twig and if they were in pairs or were more numerous (whorled). Leaf scars are the markings on the twig where the leaf stalk was attached before it dropped off in the fall. In the case of my tree, there was an opposite arrangement and paired leaf scars.

Step 2: I looked at the width of the twig—less than 0.25” or greater than .25” In this case, the twig was no greater than 0.25.”

Step 3: I examined the texture of the terminal bud and the shape and the number of old vascular bundles or vein scars. Old vascular bundles represent the xylem and phloem (ports or channels) where water had flowed to the leaves. Since the terminal bud was not rough and dry, is conical, and the leaf scars had three bundles and were somewhat V-shaped, the twig belongs to the genus Acer (Maple).

Step 4: I considered the colour of the buds. The key asked if the buds were red, reddish brown or not. These buds might be seen as reddish brown and so could lead one down the path towards a different species but they also were “whitish and woolly” and the twigs were “purplish.” The leaf scars also met at a point on the twig. These combined features pointed towards A. negundo (Manitoba Maple). Colour variability within a species might question an identification. Also bear in mind that these types of guides may not identify exact cultivars or hybrids. But since this exercise, I now see this tree everywhere on my winter walks.