Vinca minor is called by many names among them are periwinkle and creeping myrtle. Although Myrtle is a misnomer because it is not at all related to myrtle.

And I wonder if the ones you are referring to, Carolyn219,since you have posted on the annuals forum, are Catharanthus roseus, commonly referred to as vinca or Madagascar periwinkle but not actually related to Vinca minor or V major, aka periwinkle, the hardy creeping ground covers. If you Google these plants by their correct botanical names you will see the difference. Common names are often endearing and fun but a minefield of confusion.

Catharanthus roseus is very commonly known as vinca in the Deep South. It just might be our very best flowering annual. The glossy green foliage rarely looks ratty or tired during our extraordinarily long and hot summer season, it reseeds reliably, and of course, the plants are blooming machines. The stronger the sun and the hotter the weather, the better it does. Vinca minor (myrtle) is an altogether different plant which grows in fairly full shade and is positively rampant in the south when its happy. How these two could be confused is beyond me, but of course, it happens all the time.

I think the confusion is due to the slight resmblance in the flower shape, Donna.

And furthermore I can't understand how Vinca minor or major ever got called Myrtle (It isn't in the UK) since Myrtus (small fragrant leaved shrubs with little white flowers ) look totally different from Vinca (grovelling unscented vines with blue flowers). Bizarre.