Rosmarinus officinalis should be a simple plant to explain. It's easy enough to describe: an evergreen shrub native to the Mediterranean with dark green, needlelike leaves that have a resinous aroma. From late winter through spring, the plant displays blue flowers. There are two basic types: upright forms useful as shrubs, and prostrate types that will spill down slopes and cascade over walls. So far, so good.
Care is also straightforward. Once rosemary is established, occasional deep watering is almost all it needs. Prune lightly to shape, if desired. Feed little, if at all. The plants will endure drought, heat, wind, and salt spray. Insects leave them alone (the aromatic oils in the foliage act as a natural repellent). And deer and rabbits don't like the taste.
Rosemary has only two weaknesses. It is somewhat tender―most varieties suffer damage when temperatures dip below the teens. And it is susceptible to root rot―usually a consequence of poor drainage, often exacerbated by overwatering.
Now comes the tricky part: distinguishing between the many varieties on the market. They don't look much different in nursery containers, and catalog descriptions sound similar too. The fact that one variety may be sold under several names (for example, 'Collingwood Ingram', 'Ingramii', and 'Benenden Blue' are all the same plant) adds to the confusion.
To sort out the differences, we spoke with expert growers. They recommended the varieties listed below.