There are many suitable companion plants for cacti and succulents. Here are details on a few of those.
There are many forms of rosemary available. Some are tall and bushy. Others are low growing, and can be used much like ground covers. Like hardy cactus, rosemary can suffer if the soil remains too wet over the winter. Hardiness can vary somewhat, depending on the cultivar. While some are reportedly hardy to zone five, Iíve had them die here in western North Carolina (zone six) over the past couple winters. Rosemary needs a dry, well drained, rather infertile soil just like desert cactus and many succulents.
For warm climates the bromeliads are a wonderful choice for planting with cacti and succulents. These lend a tropical-like appearance to a bed or border. The bromeliads are tolerant of drought.
The ornamental pineapple is a very good choice. For those in cold climates, try growing this in a pot. Plant ground-cover type sedums around the edges of the container.
Companion Plants for Hardy Spurges
Many hardy spring flowering bulbs make good companions for the hardy spurges or euphorbias. As the leaves of the bulbs begin to die back in late spring, the spurge stems will conceal these from view. Some of the suitable bulbs for mixing with the spurges include tulips, hyacinths, grape-hyacinths, and feather-hyacinths.
There are both annual and perennial verbenas available. The annuals seem to be much more common. Generally many of the perennial species are only hardy to zone six or seven, while others are even more tender and suited only to zone eight or so. Most verbenas tend to be heat and drought tolerant. However, some suffer from mildew, especially if the humidity becomes high, which makes them less suitable for the South and Southeast.
The verbenas prefer a well drained soil in full sun. They would be good companion plants for cacti and succulents in beds and borders, especially in warm climates. For cold climates, create mixed container combos mixing annual verbenas with cacti and succulents.