It’s that time of year again. The time when I remind British gardeners of the value of the indispensible Royal Horticultural Society Plantfinder, and prompt North American gardeners to realize that it’s of enormous value to them too. The new issue is out today.

First the basics. A total of 67,603 different plants are listed, and the British or European nurseries that stock them are given for every plant. In all, 541 nurseries included and there are 3,380 new plants in this year’s edition. The book runs to almost 1000 pages.

The part that should interest North American gardeners, as well as British ones, is that this is the most up-to-date record of correct plant names in the world. There are many minor corrections and updates to cultivar names this year - last year's were more significant - while other changes are extremely minor, no dramatic changes to fire up gardeners’ ire although there are small adjustments in Crocus, Cytisus and Prunus while the rarely grown Ledum is now included in Rhododendron.

Of special interest in North America is that the species of Disporum, fairybells, from North America are now separated from the Asian species in the genus Prosartes. For example, Disporum maculatum, the nodding mandarin, is now Prosartes maculata (right, click to enlarge). It’s worth noting that it’s taken the Plantfinder botanists almost twenty years to accept this change to Disporum, it’s been adopted in the US already. So you can see these adjustments are not made lightly.

Experts all over the world are consulted before the team of horticultural botanists make changes, so it’s not just a load of Brits making pronouncements. And in recent years, as you can see, they’ve become quite conservative although the mass of new genetic evidence gives them a great deal of fresh information.

It’s unfortunate, of course, that under the system of naming devised by Carl Linnaeus, when new evidence reveals unexpected relationships between plants their names have to be changed to reflect that. But that’s just how it is.

What’s more puzzling is this. After an ill advised decision made long long ago, when perennial chrysanthemums were moved into a new genus Dendranthema, there was a huge outcry. They were rapidly moved back, Dendranthema was abolished. That was in 1999. So why, almost 20 years later, have some American nurseries gone off at a tangent and continue to use Dendranthema – but only for some unusually tough varietiess and not all the rest? Baffling.

The RHS Plantfinder is not yet available to order in North America, but amazon will email you and tell you when it's in stock.