For reasons that you’d have to extract from me over drinks, I’d have to say that the ash is my absolute favorite tree. I feel a very personal connection to them — particularly the common ash, Fraxinus excelsior. (Incidentally, this would have been the species of Yggdrasil, the World Ash of Norse mythology.) The only native ash in the Pacific Northwest is Fraxinus latifolia, the Oregon ash, so I feel obliged to look at that, too. In fact, I’d like to take more of a class approach in this particular “species” profile.
It is pretty easy to identify a true ash, which is to say, any member of the genus Fraxinus. They are unique in having opposite branching and compound leaves, meaning all buds and twigs arise directly opposite from one another, and each leaf is composed of several leaflets.
Telling the difference between a leaf and a leaflet can be confusing at first; simply put, wherever you find a bud, it will be at the base of an entire leaf, while leaflets do not ever have buds at their bases. In the picture at right, you can see in the distinct opposite branching (1); notice that the leaves are composed of five leaflets (2), while the buds occur only at the base of the compound leaf (3). Five to seven leaflets are typical of most ashes, while some will have the occasional set of nine.
Ash trees are also the only tree with narrow single samara fruits (4), which become brown and papery as they mature. The bark also has a distinct diamond pattern of fissures when older (5). Even in winter, they are easy to discern by their dark, velvety, deerhoof-shaped buds (6) situated atop a horseshoe-shaped leaf scar (7).
For the specific species in question, there are a few more diagnostic traits. The Oregon ash, F. latifolia, is commonly found on wetland edges and in saturated riparian zones; it remains windfirm in these situations via an extensive root system. It has tiny green flowers with no petals (1), its leaflets are very ovular (2) and the clusters of samara can be relatively thin.
On the other hand, F. excelsior has purple flowers (4), narrower lanceolate leaflets (5) and fruit clusters can get downright prodigious (6). It does better in drier, sunnier spots and is the most abundant and widespread species of ash in Europe — hence its other name, the European ash.
At any rate, telling them apart is fairly irrelevant since you’re only likely to ever see the two together in ornamental landscaping. Telling all the other species of ash apart would be more complicated, but with the information here, you should at least be able to spot an ash when you’re walking around a park or neighborhood.
There are a few lookalikes out there to confound you — especially if you’re in town rather than in the woods as you look — but these impostors are easy to rule out. Even in the woods, F. latifolia might be confused with: dogwoods (1), which have opposite branching and similarly-shaped leaves that are singular rather than compound (look for the buds!); or elderberry (2), which is opposite and compound, but produces berries and generally grows as a shrub.
Elsewhere, ash may be confused with hickory (3), mountain-ash/rowan (4) and walnut (5) due to their similar compound leaves; however, all of these have alternate branching rather than the ash’s opposite arrangement. Also, hophornbeam (6) has many single, alternate leaves close together on a branch, which can sometimes look like a compound ash leaf on first glance.
They’re easier to find than it sounds, and they’re everywhere in urban plantings. They’re also susceptible to some nasty diseases, so please take good care of them if you have them.