Another important matter is that a pergola or green alley, in the usual sense, should never wind or go uphill. It is not intended by this that shading coverings cannot be used in such places, but that they would want especial design, and it is altogether a matter of doubt if these could not be much better treated in other ways.
The circumstances of different gardens are so infinitely various that it is impossible to lay down hard rules; only general rules can be given and exceptional circumstances dealt with by exceptional treatment.
Green alleys require some attention. In winter the oldest of the wood must be cut out to make room for the young growth, and when this is lengthening vigorously it must be carefully laid in.
If the alley has an iron framework, which is necessary when such strong growing things as Wistaria are used, this may be clothed during the first few years, until the Wistaria is growing strongly, with annual climbers such as Cobæa scandens, lophospermum, Mina lobata, and even varieties of the large-flowered Clematis, which must be removed when the Wistaria covers the alley.
OLD APPLE WALK (Helmingham Hall).
Very charming alleys are sometimes formed of fruit trees—Pear, Apple, Cherry, and Plum making delightful spring pictures, and almost as much so when in fruit in autumn. Where fruit and flower are desired every shoot must be exposed to sun and air. When densely shaded by other growths the wood does not ripen, and therefore flowers badly, if at all.