Rock gardening FAQs

Do I have to build a rock arden in order to grow rock arden plants?

No. Plants usually grown in a rock garden may be grown in raised beds, sinks, or troughs; but they certainly look best in a well-planned and maintained rock garden. Rock plants require good drainage and, usually, plenty of light; and the better the setting the more spectacular they look—even in a small space.

I have decided to build a rock garden on sloping ground at present covered with a rough collection of shrubs. What should I do to prepare the site?

Remove all plants, including perennial weeds, before you start construction. Use the existing soil, adding extra drainage only if it is heavy clay. Start laying stone from the base in a series of terraces, of any shape you like, but preferably simple; the number of terraces is determined solely by the slope of the ground and size of stones, which should match in height at their tops and be of the same type. Make the ends disappear in the ground by ‘bending’ them into the slope: they should not stand out like sentinels but should lean into the slope for greater stability and better appearance.

What type of stone do you recommend for a rock garden?

Preferably any stone with angular faces, which are easier to lay and maintain in position than those with rounded faces. Whatever type of rock you use, stick to it and do not mix with other types. Avoid volcanic granite as this has no strata and different pieces are almost impossible to join together. Where possible use the stone native to your area—it will be cheaper in terms of transport costs—and the individual pieces should be as large as you can safely handle.

How large should the stones be for a rock garden about 3 x 3 m and 1.5 m high (10 x 10 ft by 5 ft)?

This site has sides sloping at an average of 45 degrees; it should ideally have three to four layers approximately 450 mm (18 in) high on each terrace. The depth from front to back of each stone will not need to be more than 300-400 mm (12-16 in); they should be as long as possible, but manageable. This may sound horrifically large; you can reduce them in size as long as you do not lessen their height, otherwise there would be nothing but stones on the site!

I recently saw a reference to planting rock plants on a scree. What does this mean?

To the geologist a scree is an accumulation of rock, broken up into stones by the action of heat and frost, which collects on the slope of a mountain. You can make a miniature version of this in the rock garden if the slope is not more than 20 degrees. Lay a rock-garden compost in the norma way and on top of this put a layer up to 200 mm (8 in) thick of chippings or shingle, with e scattering of larger stones. Planting is carried out directly into the chippings or shingle; the plant roots will soon reach down to the soil below.

I would like to build a rock garden, but I have available only a flat site. What do you suggest?

The site must be free of perennial weeds before any start can be made. Then you should consider whether a stone raised bed would be more suitable than a flat site. What I have in mind is essentially £ dry stone wall, consisting of two faces sandwiching a core of soil. The total thickness would need to be not less than 1 metre (3Vi ft) to prevent the soil from drying out too quickly. The stones would be laid on top of one another until the required height was reached; the faces of the wall would be built to I batter (leaning inwards) to increase stability. If you decide to build a rock garden instead, make sure that each of the terraces is level all the way around to retain the soil; the structure should be not more than three terraces high.

I plan to build a rock garden approximately 9 x 9 m and 3 m high (30 x 30 x 10 ft). Do I need to build steps into the structure?

Yes, I would recommend them for a site of this size, to make access easier and to enable you to show off the whole site to friends. Make the steps wide enough for two people to pass, and lay them on the same strata as the rock garden, remembering to use the same stone type. Make sure that the risers are not higher than those of house stairs.

Is there a best time to order stone and build a rock garden?

Any time of the year is suitable, bearing in mind the rest of the work to be done in the garden during the growing season. Ideally, I would suggest you order your stone during the summer and begin building in late summer, aiming to complete by the time of your first spring plantings.

I have been lucky enough to obtain, free, some supplies of sandstone and Westmorland limestone. I intend to use them to build two small rock gardens. Any tips about laying these rocks?

Sandstone should always be laid with the strata (the layers which formed the stone) arranged horizontally. Place the stones so that they either join end to end or overlap slightly; this will discourage soil erosion. Then build in a series of terraces.

The Westmorland limestone is more difficult. It should be laid with its largest surface area to the ground and in a series of irregular small terraces. It will need to be fitted like a jig-saw puzzle, with as little space as possible between the joins, into which plants are placed to prevent soil erosion.

Stratified rocks include a number of limestones other than the Westmorland type. Bear in mind that stratified rock, of whatever type, is best for rock gardens: it is easy to lay and holds soil.

Do I need help to plan and build a rock garden?

To plan I would always advise at least a second (and preferably an experienced) opinion. To build you ought to have at least one other person, possibly more: two people can deal with stones weighing up to 100 kg (2 cwt); for larger ones, three or four people will be needed.

What equipment do I need for building a small rock garden?

Apart from the usual spade and fork, I would recommend at least one crowbar about 2 m (6 ft) long; a sack truck or a builder’s barrow (depending on size and weight of stone and the help available); at least four scaffold planks to run the transport on; a baulk of wood to act as a fulcrum for the crowbar and as a stop for the truck; and, for a flat site, a spirit level. All these tools, apart from the spade, fork, and level, can be hired.

How do I know what to buy when selecting plants for a small rock garden? I obviously do not want rampant growers.

A visit to specialist nurseries is the best course. Opinions vary as to eventual size attained by many rock plants, but if you give some indication of the life expectancy of your rock garden (15 to 20 years is reasonable) the nurseryman will at least be able to tell you what to avoid. If you cannot visit a rock or plant nursery, send off for catalogues and make your enquiries by post.

I have raised beds and a sink garden. What rock plants can I grow in them?

Taking the raised bed first, you can use the same plants as in a rock garden of similar size though you should avoid too many tall, spiky plants: one or two will give an extra dimension, but a lot of them will spoil the whole effect. The sink garden, on the other hand, must be restricted to the slowest growers, so that the display can last 10 or 15 years.

What is a crevice plant?

This is simply a plant that will grow in the narrow vertical or horizontal spaces between stones in a rock garden or paving. It is worth remembering that all crevice plants should be placed in position during construction, never afterwards, otherwise the plants will be severely damaged when forcing them in. Some good examples of crevice plants are sempervivums, ramondas, haberleas, Saxifraga longifolia, and Al^ssum saxatile.

Can I grow shade-loving plants in a rock garden?

Yes, you can, and very good they are too—but beware of rampant growers. Many ferns are ideal, as are haberleas, ramondas, gaultherias, vacciniums, and many others. Again, you should seek advice from specialist nurseries, which in this case will include those growing woodland plants, especially dwarf rhododendrons.

Are ground-cover plants suitable for a rock garden?

Yes, but perhaps not the types usually advertised as such, many of which are rampant growers. The cushion plants, the trailing types, and many herbaceous plants of a dwarf habit are suitable. Bear in mind, however, that rock-garden plants in general grow more slowly than their open-garden counterparts. Maintenance by weeding is still very important.

How dwarf is a dwarf conifer?

This is always a problem for the rock-garden enthusiast. Nurseries advertise dwarf conifers in good faith—but dwarf in relation to what? The following dwarf forms are all of a stature suitable for the rock garden: silver fir (Abies balsamea ‘Hudsonia’) and A.b. ‘Compacta’; Hinoki cypress (Chamaecyparis obtusa ‘Nana’); Juniperus communis ‘Compressa’; and the spruces Picea gregoiyana and P. mariana ‘Nana’. None of these exceeds 900 mm (3 ft) in height; most are 600 mm (2 ft) or less—but their width may exceed their height.

Can I use dwarf shrubs as rock garden plants?

Yes, but again be careful because ‘dwarf’ is a term used in relation to their larger relatives; they may well be too large for the smaller rock garden. Their eventual size should not be greater than what will be wanted in, say, 15 to 20 years. These shrubs, together with dwarf conifers and dwarf rhododendrons in the shade , form the backbone of the planting, especially in winter, and the specialist nursery is the best place to go if you are uncertain about what to select.

Are heathers suitable as rock garden plants?

They can be used there, but they would really be better planted elsewhere in the garden. Most heathers grow quite large, especially in spread; and, as there are so many other plants to choose from, the rock garden is better used for those more suited to their culture.

What size of plants should I plant in a rock garden?

Planting size is immaterial: it is the size a plant will attain in a given period of time that is important. I normally recommend 15 to 20 years as a good lifespan before the rock garden is completely renewed; though a few plants will be renewed within that time owing to death, or their growing too large, or there being better plants to replace them. The size of rock garden will also, of course, determine the size of plants. In a rock garden 3 x 3 m in area and 1.5 m high (10 x 10 x 5 ft) the conifers and shrubs should not exceed 450 mm (18 in) in height and spread. The remaining rock plants can vary up to 300 mm (12 in) high and of as wide a spread as you wish.

How long do rock-garden plants live?

Some plants live for up to 60 years, notably dwarf conifers and dwarf rhododendrons. Some by contrast, are monocarpic—that is, they grow until they flower, and then they die, having produced plentiful seed. Examples include Saxifraga longifolia and Townsendia grandiflora. The majority, however, have a useful life of 15 to 20 years, which is why I recommend renewing the site after that time.

How do I know how many plants to buy?

I would suggest a maximum density of 10 plants per square metre (slightly less per square yard), provided that rampant growers are not used. If dwarf conifers and rhododendrons and shrubs are planted, reduce the number to one to three per square metre. For sink gardens, the number should preferably be increased to 20 plants per square metre, but make sure that only very dwarf plants are used.

It is my experience, however, that after one has completed the work of planting one invariably comes across plants which one would like to have included. So it is a good idea to leave a few spaces in one’s general scheme to accommodate these late-comers.

When is the best time to plant rock-garden plants, and how do I plant them?

Almost any time of the year is suitable except mid-winter; but dwarf bulbs should be planted in autumn. Avoid very hot or very cold weather. If plants are bought during weather extremes, plunge them in their containers (most are pot-grown now) in soil up to the rims, and plant them as soon as the weather changes for the better. Plant them at exactly the same depth as they were in the pots; firm them in; then water thoroughly. If more than one of the type is planted, place them closer together within the group, but place the different groups farther apart.

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