It is worth preserving the base of I each post with green Cuprinol, ensuring that this preservative extends at I least 6in(i5cm) above ground level-the point where wooden posts quickly I rot. Construct the tripod with an open top, linking the posts with short I cross pieces (see Fig. 42). Lead the climber up through the middle to I festoon down from the top. Such tripods can make a most effective feature I and add height to a border, emerging through dwarfer shrubs and I ground-cover plants growing around about them. Single rustic posts (at least 4 to 6in[io to 15cm] in diameter) positioned to 8ft(2 to 2.5m) apart can be linked by rails of 2 to 301(5 to 8cm) in diameter, perhaps supporting plastic or wire mesh trellis. Such an arrangement forms an excellent support for climbing roses and so on, and makes a good background for rose or shrub borders. SITE PREPARATION, PLANTING AND AFTER-CARE The soil against walls, particularly of new buildings, is frequently poor and sometimes heavily ! adulterated with builders' rubble; it is often hard-packed. In older gardens, growing conditions may be little better if the borders have long been occupied and the soil exhausted by decrepit climbing or wall shrubs. For the best results, it is worth trenching the entire border area i| ft(45cm) deep, removing the rubble, chalk, gravel or sub-soil encountered, together with roots or stumps from any previous planting, and replacing this with good fresh top soil. This top soil should consist ideally of top spit loam, particularly in those places where new specimens will be planted. At second best, individual site preparation in borders or paved areas should be at least ijft(45cm) deep and 2ft(6ocm) square. Well-rotted farmyard manure or good mature compost should be dug into the lower spit before any re-soiling is carried out to aid rapid establishment and growth. Equally generous site preparation is necessary when plantings are being made to clothe pergolas and tripods, and particular care is necessary when preparing sites for climbers to grow through trees. Here, due regard must be given to competition with the feeding roots of the tree. These are usually some distance away from the trunk, in the vicinity of the outer edge of the canopy of the tree. If fibrous feeding roots are encountered it is a good idea to seal them off from the new climber by means of an open-bottomed box of wood, tile or slate, say 2 to 3ft(6ocm to 1m) square, in which the new plant is established in a good depth of fresh soil and compost. However, it is best to select a host tree for this purpose which has a thin, narrow crown rather than one which is broad and dense so that adequate light and rain can reach the climber.PLANTING Apart from climbing or rambler roses, the majority of wall shrubs and climbers are provided by good nurserymen as pot- or container-grown plants which can be planted at almost any time of the year, providing that there is adequate attention to watering and after-care (climbing or rambler roses and open-ground shrubs are normally planted between the end of October and March during open weather conditions).