Nearly every inexperienced gardener seems to hanker after a rockery. One wonders why, because the only really decent ones you ever see belong either to botanic gardens or to the real alpine enthusiast.
More often you see a hideous mound of poor soil, the surface punctuated at intervals with small, badly chosen pieces of stone, rather in the manner of almonds on a Dundee cake, where a battle ranges between every pernicious weed in creation and those invasive horrors euphemistically known as ‘easily grown rock-plants’ – Sedum acre (stonecrop), Cerastium tormentosum (snow-in-summer) and Sedum spurium.
Probably you were first tempted by the sight of attractive spring displays of pink or mauve aubrieta, yellow alyssum, blue Campanula carpatica, pink, white and red mossy saxifrages and white iberis which can look so effective planted together and hanging over a wall, but apart from a later half-hearted flush, the party’s over by mid-summer and the whole thing looks a bit dull.
Rockeries are about the only feature in the garden today that have not been made easier to maintain by modern science. They require regular weeding, because once weeds get amongst the rock-plants and under the stones they are almost impossible to get out, except by spot treatment with glyphosate or alloxydim sodium for weed grasses — a chore which is as fiddly as the weeding in the first place, and which can lead to accidents if you are careless. If you can’t spare the time to look after it properly, don’t have a rock garden.
Of course, rockeries are not the only places you can grow alpines, and in this category of plants I am including many dwarf subjects appropriate for this kind of situation, not only true alpines (i.e. those originating in mountainous areas). Suitable varieties can also be used to effect (and generally speaking, are easier to maintain) in sinks, troughs, pots and windowboxes; in raised beds; as edging plants at the front of borders; to hang down over walls; in dry stone and peat walls; in scree gardens; in between paving stones and other surfaces; as specimens in specially manufactured ‘rock pots’, which resemble large rocks with planting holes in them; in cold greenouses (‘alpine houses’).
However, if you are still determined to grow your rock-plants in a rockery, here are a few rules.
DO NOT site the rockery under a tree – alpines dislike dripping branches — or in the wind tunnel between two buildings.
DO NOT make the rockery too tall. For every foot of height, the base should be 4-5 feet wide.
DO make it as big as possible.
ALWAYS use light, free-draining soil. If yours is not naturally suitable, you will have to improve it, with organic materiai and sharp sand.
BE SURE to remove all perennial roots before you start. If you do not, you will probably have to dismantle the rockery before long and start again.
DO NOT leave holes and gaps where slugs and snails can make a home.
DO obtain professional advice when choosing your rock-plants. As a general rule, most alpines like an open, well drained, slightly alkaline, reasonably sunny site, but there are others which prefer cool, shady and/or moist conditions, and acid soils, or to be planted in a gravelly scree or in a ‘fissure’.
They can all be used to effect in different parts of the rockery. For example, it is damper at the base of the rockery than at the top. If you have a south-facing, free-standing rock-garden, you will also have a cooler, shadier north face. If you want to grow lime-hating plants, you can construct one terrace (not where alkaline water can seep into it from above, though) with an ericaceous mix of soil-based compost. However, there are quite a few alpines which, although they can stand any amount of dry cold, simply cannot tolerate the cold, wet conditions of the British winter and should only be grown indoors in an unheated alpine house.
DO NOT choose plants which are too rampant. If you have a big area to fill, plant, say, three of a slow-growing sort rather than something which covers the area quickly and gets out of hand.
DO NOT plant in winter, if it can be avoided. Although in theory you can construct and plant up a rock garden at any time of the year, spring and autumn are best.
DO plant your alpines and other dwarf subjects in the rockery with the same care that you would use elsewhere in the garden (see appropriate sections).