American highbush cranberry viburnum (Viburnum trilobum) looks very like a closely-related European shrub, V. opulus. They used to be considered, and some still consider them to be, geographical races of the same species, i.e. V. trilobum was named V. opulus var. americanum. They have always been considered taxonomically distinct, but being so closely related, they will interbreed, and the existence of hybrid individuals growing in wild places adds to the confusion.
There was some discussion about the differences between the two among my Facebook friends a couple of weeks ago, and I thought I would continue the discussion here, and post a few more photos. By just looking at the bright red fruits, which persist in winter, one cannot easily tell the two species apart, and this has led to some truly ghastly cooking experiments. American highbush cranberry got its common name because its sour fruits can be cooked up with sugar into a sauce that can be used like cranberry sauce. Fruit from the European look-alike cooks up into something that smells and tastes like a distillation of dirty gym lockers. The native American will vary in palatability from plant to plant, so to get the most reliable fruit for cooking it is best to plant named clones, but the native is just not foul-tasting in the manner of European species.
This matters not just to cooks experimenting with colonial recipes. Native North American birds prefer the American species and will leave the nasty, acrid fruit of V. opulus long after they have stripped the fruit from V. trilobum. There have been enough naive or starving birds that have eaten V. opulus (and pooped out the seeds) that the introduced shrub if now found in many wild places. If you are planting a garden and want to provide for the needs of birds, be sure to plant the native. Nurseries often get this one wrong, so there is no help for it, you will want to learn the differences between them and do the botany yourself before you buy.
I’ll get to the field identification in a moment. I want to stress something that is often overlooked: American highbush cranberry has beautiful red fall foliage colour, and this is an asset in the home landscape. In a really good year for colour, V. opulus can turn sort of red-green, so one has to be careful in using leaf colour to do identification. Yet when you see leaves from each species side by side, the difference is striking.
The easiest and most reliable way to distinguish between the two species is to look at the small glands that occur on the leaves just where the petiole (stem) expands into the broad leaf. In American highbush cranberry, these glands are stalked and look like little golf clubs. In V. opulus, the glands are flat, concave, and look a bit like octopus suckers. Leaves are not always typical, so you may have to look at two or three to get a good feel for what that particular shrub is presenting. Once the leaves are gone in the fall, identification is more difficult. Our native shrub will nearly always be found growing in wet, even swampy, places. Also, give the fruits a good sniff before you take them home and cook them; if they smell fetid when bruised, they won’t be improved by cooking.