The Verbacsum season will soon be upon us. Rather than trying to remember all the traits for all 13 species, it’s much better to remember the few things that you need to look for, and to note these carefully for each plant you encounter. 
The most important thing is that you should look very carefully (x10) at the anthers and their filaments. These anthers are all kidney-shaped (reniform).
The most important thing is that you should look very carefully (x10) at the anthers and their filaments. This is the only tricky thing: the way that the anthers are attached to the end of the filament. These 5 are all placed transversely (T-shaped).
The most important thing is that you should look very carefully (x10) at the anthers and their filaments. This is the only tricky thing: the way that the anthers are attached to the end of the filament. Here, the lower 2 are placed obliquely, decurrent down the filament
Next is a really easy one: are the hairs on the filaments white (left) or violet/purple (right)?
A slightly tricky one has to do with the nature of the hairiness on the leaves. You need to rub it, and see if most or all of its comes off. Look (x10) after you've rubbed: is it persistently densely hairy (left) or not (right) ?
Now look at where the leaf joins the stem. Do the middle- stem leaf-edges run down the stem for a distance (decurrent, left) or not (right) ?
Another easy one. Count the number of flowers in the axil of (say) 3 bracts (the number can vary a bit). Is there 1 flower per node (left) or 2 or more flowers per node (right) ?
Next, have a look at a basal rosette leaf (it is best to remove one carefully from the stem). Is the base of the leaf heart-shaped (cordate, left) or not (cuneate (right), truncate, clasping or rounded) ?
The length of the flower stalk (pedicel) is important: is it longer than the calyx (left) or shorter (or even sessile = stalkless) (right) ?
Look very closely at the hairs (x20 ideally but x10 if that's all you have). You might see simple, stalked glandular hairs (left) and/or branched non-glandular hairs (right, as on 'woolly-looking' plants).
Slightly trick one now. The bottom two anthers are fixed obliquely (not T-shaped) and the filaments are hairless at the top (towards the anther). Are they hairless all the way down (left) or are the filaments hairy in their bottom half (right). Look closely (x10).
Finally, look closely (x10) at the stigma (you may need to clear some hairy anther-filaments out of the way). Is it club-shaped (capitate, left) or elongated (spathulate, right)
Once you have had a chance to digest all of these preliminaries, I'll tweet an illustrated key to Verbascum when most of them are in flower. Enjoy.
