Rock Gardens, Alpine Garden Plants and Pavements


Alpine Pavements

plants and limestone pavement

An alpine pavement is any paved area with plants growing between the slabs. This article deals with both using an existing paved area and building one from scratch. An existing paved area is rarely suitable as it stands, because the paving often lies on top of poor soil, rubble and rubbish left over from the building of the property which it surrounds and it therefore hides a multitude of sins. If you are planning a new pavement next to a house wall be careful not to site it above the damp course.

The first consideration when planning an alpine pavement must be drainage. This will only be a problem in areas of high flooding risk but it must always be good to a depth of 60cm (2 ft). To address an existing drainage problem, will entail lifting and relaying the existing paving. If the soil is of poor quality it is best to lift all the paving and resoil the whole area to a depth of 45cm (1-1/2 feet). The operation will involve a lot of work but it can be done in stages, so long as you start at the lowest area to which the drainage will run, otherwise heavier soil might cause blockages and perhaps create further difficulties later on.


Shape of the Pavement

The pavement can be constructed in many ways. It can be built with variously-sized stone blocks laid in formal rectangular arrangements, linked like brickwork; geometric patterns can be made using squares of stone between 15 to 60cm (6in to 2ft); crazy, informal paving can be  made by laying stones of all shapes and sizes like a jigsaw puzzle.

The idea is to plant between the paving slabs, wherever there are spaces of at least 2.5cm (1 inch). Small spaces will occur where jigsaw slabs do not match up closely and larger ones can be made by leaving out one or more slabs.


Variations of the Rock Garden

The possibilities of this type of garden can be widened by incorporating sinks into the planting areas and, where there are slopes, by making split levels with steps and walling between. The walling may be built in the same way as the sides of a raised bed and the steps can be made more formally. However the greater formality of the situation should be reflected in the design, therefore the stones must be regular in shape and laid more formally on a mortar base. If you use really large, heavy slabs on flat areas there is no need to use mortar as a bond between the soil and slabs but small stones will need it to prevent movement when the path is walked on. Both steps and walling should be firmly fitted.

Make sure that the steps in particular are of a large enough size to remain stable when they are being used. If they are to be fitted without mortar, you should choose slabs which are each at least 8kg (38 lbs) in weight. Otherwise, the slabs will have to be mortared in position in the same way as paving, see below – Laying the Paving Stones.

Equally, if the wall stones are to be laid in the same way as for a raised bed, they will have to be large enough to retain the paving. If the stones are small, they will have to be mortared into position because of the pressure from the paving stones above.

Where bricks are used for this purpose the pointing, that is the mortar between each stone, should be done in exactly the same way as for a house wall. However, if the stones being used (whether natural or artificial) are larger than house brick size, it looks much better if the pointing is rebated further in from the front, so that as little mortar as possible shows and the construction is thereby given more the appearance of a dry stone wall.

If you plan to plant in the wall or steps, leave the mortar out of any planting position, and plant when you build, as for a raised bed.

If you are building a dry stone wall it is better to lay the paving flush with the top line of stone of wall. This will prevent the danger that the overhanging slabs will dry out the wall and it will enable planting in the wall to be carried out. The mortared stones or bricks can have an overhang of paving where planting in the wall is not envisaged.

It is debatable whether there is any need to use mortar between the slabs as well as below them. Mortar certainly keeps weeds down but if, later, you have to make any changes and have not allowed for them they will be difficult to implement without disturbing other slabs. If you do lay mortar between the slabs, avoid the common mistake of making it level with the surface of the stone. It looks much more impressive when the mortar stops at least 1 cm (1/2 inch) from the top of the slabs, otherwise you might just as well cover the whole area with concrete and draw the lines on top; what a ghastly thought!

An attractive variation for a larger site is to surround the paved areas with raised beds as well as sink gardens. This gives extra scope and offers a variety of conditions to a very wide range of plants. Take care that none of the plants gets too big, even 0n the largest site. Nor should any of the individual groupings be too big, so that you can reach all parts of the pavement in order to maintain it.


Maintenance of an Alpine Pavement

Once constructed an alpine pavement will give the greatest pleasure with the minimum of drudgery. The areas to be maintained by weeding, watering and pruning are small in relation to the site as a whole and will always be  easy to get at. I would suggest that you don’t contemplate highly ambitious plans unless maintenance is no problem. As a rough guide, one person should be able to look after an area of 200 sq metres (217 sq yd) comfortably, working weekends and occasional evenings.

If you have a flat area of which some portion is shaded all year round, a good division of the site would be 30% for raised beds, 60% for the paved area and seating, 10% for sinks. The costs and labour can be spread over a five year period, with the construction done mainly during the winter and maintenance during the summer. Of course not all areas are going to be that straightforward; but in general best achievements can  be made by tackling a small area at a time and learning as the years go by.


Type of Paving Stones

The paving stones do not need to be of the same variety as those used in raised beds on the site but some similarity will enhance the appearance. Composition paving may also be used. All natural stones, except the really dark ones like York paving and slates, darken with age, so bear this in mind when ordering. Ask how the stone will look in a couple of years or try to see some yourself.


Construction

Before you begin building the sink gardens or laying the paving stones, draw an overall site plan, marking  approximate positions for the raised beds, sitting areas and sinks, then design the paving pattern. Plan to have the sinks near to windows or the seating areas, where they can be easily seen and enjoyed. Leave a large area for seating to begin with and fill it in later with plants if necessary. It is difficult to do this the other way round, especially if it involves moving some treasured plants.

Decide on which plants you would like – or at least the types – and note their flowering times before plotting their future positions, so that you can have  a balance of flowers over the site during several months of the year. You could also plan to have pockets of soil at the bottom of the raised beds; planting in these can add continuity between paving and beds.

If building raised beds and adding sinks to the site, begin by building at least one raised bed, then add the paving surrounds. Sinks can be added any time after the first section of paving has been laid but decide on their permanent position before you fill them with soil, stones and plants, after which they are very heavy. See details for constructing a sink garden.


Type of Soil

The type of soil suited to a site depends to a large extent on the amount of sunlight it receives. On a sunny site, or where the soil is heavy, use a gritty mixture, increasing the leafmould or peat content if you have a shadier garden or light soil. Gritty soil will make for better drainage, while the leaf-mould or peat mix will allow greater water retention. Plants in sunny situations usually require sharper drainage; those in shade need more spongy, woodland conditions. If the site is a mixture of sun and shade, try a mixture of the two soil types.

If the soil is quite good quality, that is well-drained and light, it can be incorporated into the total soil mixture. Where the paved area is bordered by clay and there is any fall in the ground, water will tend to drain from that clay producing a boggy area and artificial drainage would be essential.

Tread the soil in well to make a firm base and allow it to settle. Watering it afterwards will help – if it rains, so much the better. Soft pockets underneath will allow the slabs too much movement.


Laying the Paving Stones

When the soil has been laid to a maximum depth of 45cm (1-1/2 ft), pattern out the site, by laying the slabs in position on the site and then setting them aside in order. Remember to allow spaces for planting between the slabs. Make up the mortar from 5 parts (by bulk) builders’ sand to 1 part cement, mixing it with water to the texture of toothpaste. Drop five spots, at least 5cm (2 inches) deep and 7 to 10cm (3 to 4 inch) wide on the soil, to correspond to the four outer corners and the centre of each of the stones to be laid. Small stones only need one spot. Lay each stone flat on top of the spots and tap it down with a wooden block until the stone is level and even, using a spirit level for greater accuracy.

By laying the first paving stone in the middle of the site and moving out in all directions from it, you will be able to maintain a level, regardless of the regularity or otherwise of the individual slabs. The distance left between each slab will depend on whether or not you have used mortar between them. If you have, allow at least 1.5cm (1/2 inch) between each stone, otherwise they can be laid closer together, except where you are going to add plants.


Planting Between the Paving Stones

As always, choose plants to suit the soil and the climate. You will have to plant the cushion plants and bulbs in narrow spaces during construction, otherwise you will not be able to get them in at all but all other plantings can be made after the paving is completed. Remember that cushion plants should be planted with the  roots entirely underground, so it is advisable to put them in narrow gaps between the shallower slabs, which allows the tops of the plants to be above the slabs, with the roots in soil underneath.

The water-holding capacity of a pavement with vertical crevices is far greater than that of a rock garden and it will rarely need watering.

The conditions immediately under the slabs are ideal for both downward and lateral root growth and, with the top growths resting on well drained surfaces, the plants will grow compactly provided there is no competition from others. Choose plants that are naturally matched by consulting the tables. Some plants can be walked over but you will be able to plant a greater variety of plants if this is not a consideration.

If there are larger spaces on your site you can grow deciduous shrubs and bulbous plants. The bulbs will flower when the shrubs are dormant and receive shade when the shrubs are in flower. You can substitute trailing and mat plants for the shrubs. When the bulbs die down, the other plants come into their own.

As a general rule do not mix herbaceous plants with shrubs in a pavement, as they will swamp the shrubs.


23. September 2010 by Dave Pinkney
Categories: Gardening Ideas, Pavements | Tags: , , | Comments Off on Rock Gardens, Alpine Garden Plants and Pavements

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