The dividing line between rock garden plants and the other sorts grown in the garden is extremely blurred. The reason is simple - nobody has been able to produce a satisfactory definition of a rock garden plant, which means that nobody can be sure just where the rockery list begins and ends.
There is no problem with the alpines, an enormous group which is at the core of any rock garden plant list. Unfortunately 'alpine' is often used to cover all plants recommended for the rockery and this is incorrect. This term has a specific meaning. Alpines are herbaceous or sub-shrubby plants which were originally collected from the mountainous regions such as the Alps, Andes, Himalayas, Rocky Mountains etc. The natural home of the True or High mountain Alpines is above the tree line. The Edelweiss of the Swiss Alps has become the classic representative of this alpine group, low growing and extremely hardy with a passion for sun and gritty, free draining soil.
Alpines make up a vital part but not the entire rock garden plant list. Rockery plants include species from the seashore and others from woodland regions. Many of the specimens you will find on the 'alpine' bench at your garden centre have no natural home they are man-bred hybrids or varieties.
We must go beyond the alpines if we are to cover the full span of rock garden plants. Alpine and many lowland herbaceous and sub-shrubby perennials are grouped together as the Rockery Perennials, and nearly all the plants you will buy and grow belong here. The ones chosen for inclusion are not necessarily the best or the easiest to grow, they are the plants you are most likely to find in the catalogues, garden centres and text-books. Be careful if you plant Cerastium (Snow-in-summer), Aubrietia (Rock Cress), Alyssum saxatile, and Saponaria (Soapwort). They are easy to cultivate but their invasive habit means that delicate types can be quickly overrun if these rampant species are not kept in check. The height and spread information given for a particular plant is what you can expect after three years growth under average conditions.
Dwarf Conifers are an excellent way of providing an evergreen skeleton to the rock garden. Species of Chamaecyparis (False Cypress) and Junipers are included here, but there are others. Look for Pinus mugo (Gnom), Picea mariana (Nana), Taxus baccata (Standishii) and Thuja occidentalis (Hetz Midget). Make sure that the conifer you buy is labelled as a 'dwarf' variety. Dwarf Shrubs are another useful group for providing a woody backbone to the rockery. You can also grow dwarf species of Azalea, Berberis, Betula, Pernettya, Prunus, Rhododendron and Rubus.
Dwarf Bulbs are an essential part of rockery planting and many varieties of the basic spring-blooming trio (Crocus, Narcissus and Tulip) can be found in any rock garden catalogue. Iris, Cyclamen and Oxalis are also included here, but there are many others to choose from - Allium, Chionodoxa, Eranthis, Galanthus, Leucojeum, Muscari, Scilla, etc. Ferns were all the rage in Victorian times and still make a useful addition for the rock garden of today.
So what is a rock garden plant? The definition - A plant which looks at home and is at home in a rock garden is too restrictive. Some need the protection of an alpine house and there are others which require the humus and shade of a peat bed to be successful. Another definition - A low-growing perennial grown by alpine gardeners, seems to beg the question. The definition adopted here is - A plant you can expect to find listed in some of the alpine nursery catalogues and some of the textbooks on rock gardening. Not really satisfactory, of course as it leaves out the annuals and biennials which are so useful to provide a splash of temporary colour. Perhaps the absence of a clear-cut definition is a good thing. A rockery is an art form and should not be shackled by tedious scientific restrictions.