Let Leaf Arrangement Lead the Way

 

Viburnum showing opposite arrangement

 

There are people who find plant identification intimidating. It might be you. Don’t let it be that way; use just a little bit of logic to quickly get you on the right path. Recently a photo was submitted to a plant identification group and the person said that a plant app had indicated it might be black cherry (Prunus serotina). Plant id apps are only as good as the photo submitted; they can’t always see the leaf arrangement and other important details from photos, so you’ll always want to double check their guesses.

With a closer look at this person's photo (not included here), one could discern that the leaves were oppositely arranged on the stem. Since black cherry has alternate leaves, it could not be that. When I talk to people about plant identification, leaf arrangement is my number one tip. I’ve written about this before, but the message can always be repeated (my previous blog is linked here and still worth reading with examples of woody plants).

While this concept is most persistent in woody plants (you can see the arrangement of tree and shrub branches in the winter), it also applies to herbaceous perennials. Herbaceous plants with well-known opposite leaf arrangement include plants in the mint family (Lamiaceae), but there are others like beardtongues (Penstemon), sunflowers (Helianthus), boneset (Eupatorium), Phlox, milkweed (Asclepias), and St. John’s wort (Hypericum). 

Opposite leaf arrangement

Alternate leaf arrangement

Leaf arrangement is visible in the first true leaves produced by dicot seedlings; dicots produce a pair of cotyledon leaves that often don't resemble the plants "true" leaves. For example, when they pop up in the spring, a maple (opposite) clearly has two true leaves compared to alternately arranged plants which produce only one. These cotyledon leaves are often very plain (although American basswood has fancy cotyledon leaves). Here are two examples:

Maple seedling with two plain cotyledon and two true 
leaves (example of opposite from Strand Herbarium)

Basswood seedling with two fancy cotyledon and
one true leaf (example of alternate)

While identifying the leaf arrangement can get you further on your plant identification path, it is not the only clue and you should consider other aspects of the plant. In particular, there are two cautions to consider:

• A leaf or branch may be missing due to damage, causing an oppositely-arranged plant to look like it has alternate arrangement. Check more than one place on the plant.
• Some herbaceous plants (that is, not woody) with opposite arrangement may transition to alternate arrangement toward the top of the flowering stem.

As in my earlier post, I encourage you to practice this skill on plants that you already know to see if you get the right identification. Put this concept at the top of your toolbox and see where it takes you!