This section is from the book "Beautiful Gardens - How To Make Them And Maintain Them", by Walter P. Wright. Also available from Amazon: Beautiful Gardens: How To Make And Maintain Them.
A pergola, or series of connected supports, is a charming adornment to a garden when well made and adequately furnished. It may range in length from 20 or 30 feet to 150 or 200 feet, according to the size of the garden. It may also vary as to material. The magnificent pergola at Harlaxton Hall, in Lincolnshire, is built entirely of stone; the equally beautiful one at Overstrand, Cromer, is constructed of square brick pillars, connected by massive boles of timber; but the vast majority of pergolas are constructed entirely of wood.
A pergola with a broad grass walk beneath it should be one of the most delightful resorts in summer. It will give that delicious coolness, that play of light and shade, which are so refreshing in hot weather. There is no necessity to curve it; rather should it be straight, so that a clear vista of chequered shadow and dangling foliage may be obtained. It may end in an opening which reveals a beautiful piece of landscape, or in a rustic corner, with a summer-house or a shaded seat.
A pergola should not be either a low or a narrow structure. Seven feet of width, and seven feet of height, should be provided. Where poles are scarce there is a natural temptation to reduce in both directions, but it should be resisted. A flower gardener is always at liberty to tell himself that he can do without a pergola; he should never permit himself to contemplate a mean one.
A dignified and harmonious effect is produced if the main uprights are of Oak, some 6 inches in thickness near the ground. The bark may be left on if desired, but it should be removed from the part which is to be buried, and this should be well tarred, pickled in cold creosote, or charred. Durable as Oak is, it needs some protection where it is in constant contact with damp soil. The cost of such supports may vary from sixpence to two shillings each. If more perishable timber is employed the necessity for a preservative becomes, of course, much greater. The cross and top-pieces may be lighter, and they need no dressing, consequently the cost for them is proportionately far less. Even so, we have got to look the pergola fairly in the face, and recognise that it is not going to be put up for the cost of a simple arch.
The cross pieces may be nailed, spiked, or lashed with wire to the uprights, but in any case the whole structure should be firm and rigidly self-supporting. If there is such instability as to permit of considerable swaying in heavy winds, the plants which the pergola supports will not have a fair chance of thriving.
A well-made border should skirt the walk. We shall not only require twining plants for the uprights, but some dwarf flowers to grow between them, therefore the soil must be deeply dug and manured. The twiners in particular will want plenty of food, and to stint them would be folly. The ground should be prepared in autumn and winter, so that planting can be done quite early in the spring. This is a great advantage, as it insures the plants getting well established before hot, dry weather comes.