This section is from the book "How To Make A Flower Garden", by Wilhelm Miller. Also available from Amazon: The Well-Tended Perennial Garden: Planting and Pruning Techniques.
We prune shrubs to regulate their growth and make them graceful, pretty bushes, to accentuate their natural character, to invigorate weak growth or check overluxuriance, or to increase the profusion or enhance the quality of their blossoms. We prune a privet hedge with a hedge-shears in a closely sheared, straight, artificial line or rounded form; but this sort of priming in the case of spireas, deutzias, weigelas, mock-oranges and other garden favourites, grown in shrubbery-masses or as isolated specimens for beauty of form or blossoms, would be desecration.
All kinds of garden shrubs may be pruned between the times when the leaves drop off in late fall and before the buds start to burst into growth in earliest spring, but I do not like pruning in very frosty weather. A stout, sharp pocket-knife, as Saynor's pruning-knife, or a pair of seven-inch, eight-inch, or nine-inch spring priming-shears, are the handiest implements for pruning; for cutting out the stoutest shoots and the bigger old wood a parrot-bill is excellent, or a pair of lopping shears with handles three feet long.
In pruning shrubs of any kind, have an eye to regulate the growth of the plant, and give it an easy, graceful, natural outline, always trying to keep the branches well down to the ground. Thin out old and gnarly stems and stunted or enfeebled wood, and endeavour to preserve a fair fullness of healthy shoots with plenty of firm, well-ripened spray twigs for flowers. In pruning twigs, always cut back close to an eye or joint, and in pruning branches, large or small, always cut close back to a joint or stem. Never leave a snag, and wherever you find an old snag cut it off close to the living wood. Never use hedge-shears on a shrub. We not infrequently see shrubs bare at the bottom and with tall stems and broad, spreading heads, but they are repugnant to the eye. When the shrubs begin to crowd each other in a bed, do not try to remedy matters by pruning; instead, thin the mass by removing a number of the bushes - dig them up carefully and plant elsewhere.
A bit of effective border planting.
If any of your shrubs get infested with bark scale - lilacs, primuses, euonymus, and some others frequently do - root them out bodily Vithout hesitation, and burn them. Do not try to cure the shrubs by pruning off the infested limbs. I once had a big bed of rugosa roses infested with white scale, and in winter I cut off every plant down into the ground; the next spring, from the suckers in the earth, up there came a dense mass of young shoots, all perfectly clean.
Avoid heavy cutting, hacking or pruning of shrubs at one time by timely and judicious pruning every year. In most cases, a very little pruning will be sufficient.
Among shrubs that need scarcely any pruning are azaleas, Deutzia gracilis, sweet fern (comptonia), wax myrtle, mezereon, ceanothus, tree peonies, shrub yellowroot, and Thunberg's spirea. On the other hand, shrubs that are benefited by being cut down to the ground every winter are callicarpa, Desmodium penduliflorum and Japonicum, the "blue spirea" (caryopteris), and the shrub-like perennial wild senna (Cassia Marylandica). Among the larger shrubs that severe annual pruning benefits are the great panicled hydrangea and the tamarixes. Cut the hydrangea back to its first or second joint and the heads of flowers will be much larger than they would be if more wood were left. The African tamarix blooms in May; cut it hard back as soon as it has done blooming, but never at any other time. The Chinese tamarix blossoms in August and September; cut it hard back in winter only. Some advise severe annual pruning for the althea, or Rose of Sharon; but I do not, for I do not admire a stumpy shrub. Keep it low-branched, but let the shoots get up and spread out.
The pure white, single-flowered one is the prettiest of all, and it needs very little pruning.
A vista of hardy herbs, shrubs, and trees.
Among our commonest garden shrubs are spireas, deutzias, mock-orange, weigelas, snowballs, lilacs, forsythias, magnolias, kerria, and sweet shrubs; and a word about these may suggest how to treat the others. Take Van Houtte's spirea: all it needs is occasional thinning out of the old wood; do not shorten the arching sprays. The crenata deutzias and mock-orange (generally known as syringa) shrubs are likely to grow very tall and full-branched from the bottom. Thin them well out from the base, and cut some of the tallest stems back half way, but do not shorten the side branches or well-ripened arching sprays. Lilacs, either the named varieties, Persian, Villosa, or the late-blooming tree species, as Pekinensis or Japonica, seldom need any pruning, except a watchfulness for suckers from the stocks on which they have been grafted: remove these as soon as seen. Weigelas need only thinning, and if they show a tendency to overluxuriance and sparsity of blossoms, cut in their roots in a deep circle three feet away from their stem.
The Japanese snowballs need no pruning, but the common one gives much bigger blossoms from stout, vigorous young shoots than from twiggy old wood; therefore, keep cutting out considerable of the old wood and encouraging young.
The woods as a background for informal borders of shrubbery and flowers. Cut-leaved staghorn sumach, Hydrangea hortensis, and Lilium testaceum.
Philadelphus, known also as syringa and mock-orange. This Is a good single specimen, standing in relation with a background. (The name syringa properly belongs, as a botanical term, to the lilacs).
Magnolias need no pruning aside from the timely shortening of a too far-reaching branch; but if they are likely to get bare at the bottom, tie down some of the lower branches close to the ground to fill up the space. Kerria and rhodotypus may get too thick; thin them a little, and from the bottom. Sweet-shrub needs only regulating, and, in the case of old plants, merely a shortening of the heavier branches.
Forsythias should be pruned just after their flowers are past. Cut them in rather hard.
Shrubs of doubtful hardiness, as some of the privets, Japanese red-bud, and styrax, should not be primed until early spring; then all injury from winter can be cut out. Evergreen azaleas (amœna), leucothoe, kalmias and rhododendrons need no general priming; but in the case of the rhododendrons that have been hurt by the winter, their injured branches should be cut hard back into sound wood, when a fresh growth may start from adventitious buds.
Roses may be pruned with comparative safety at any time in winter; but I never like to touch them until the winter is about over, because in some seasons, from tenderness of variety or injury to unripe wood, some kinds are likely to get hurt down to the snow-line. The H. P. or June roses we cut pretty low down; this gives us strong shoots and big flowers. Such as Madame Plantier we let grow into big bushes. The Crimson Rambler is let alone, and it repays us with immense wreaths of vivid blossoms. The prairie and all other running roses are simply thinned out, and not shortened back.
In the case of some of the finer Japanese shrubs, or small trees - for instance, the dwarf, vari-coloured maples, magnolias of the Watsoni and parviflora type, and pterostyrax - as they advance in years and get large, a branch, a big limb, or maybe half the plant, may die off in summer with a good deal of the appearance of fire-blight in pear trees. As soon as this is noticed, cut out these diseased limbs well below the affected parts.