Clematis. From the pruning standpoint there are two kinds of woody Clematis—those which bloom on shoots made during the current season and those which bloom on one-year-old wood. The first group includes practically all the large flowered hybrids, the lanuginosa type, the jackmani type and the viticella type.
These bloom on summer shoots of new wood and therefore can be pruned when dormant, as severely as necessary, even so far as cutting them down to the ground. Among the most valued kinds which can be cut back severely are C. texensis (Scarlet Clematis), C. paniculata ( Japanese Clematis), and C. virginiana (Virgin's-bower).
The Arida and patens types, including C. montana (Mountain Clematis), blossom on one-year-old wood. These are pruned with some sacrifice of bloom by thinning out the crowded growths before they start in the spring and by cutting off the immature tips of the shoots. An alternative is to wait until they are through blooming before pruning. These are not so rugged as those in the first group and are not reliably hardy in regions more severe than those experienced on Long Island, New York.
If you are not sure of the group to which your plants belong, your best plan would be to assume that they are the kinds that bloom on the shoots of the preceding year. Then you will not be in the position of preventing them from blooming by cutting off all the growth of the preceding year.
Cytisus praecox (Warminster Broom), C. purgans, and C. scoparius (Scotch Broom) have a tendency to grow leggy. Consequently, they should be pruned by clipping them as soon as the flowers have faded. They do not respond to hard pruning—usually it is fatal to cut them back to the brown wood. Although they are deciduous most of the functions of the leaves are taken over by the young green shoots hence for all garden purposes they are evergreen.
Deutzia, except for D. gracilis, which is compact, low-growing (about 4 feet), and seldom needs pruning, should be treated the same as Philadelphus.
Hydrangea. This genus contains both those which bloom on old wood and those which bloom on shoots developed during the growing season. The progenitor of the so-called "French" or Florists' Hydrangea is H. macrophylla. It is safer, with all of the many varieties of this species, to avoid cutting off any of the branches during the period of its dormancy, because, with the exception of one or two varieties, the buds which produce the blooms are either terminal or those near the tips of the canes.
They should be pruned as soon as the flowers have faded by cutting out some of the weaker shoots, especially those which have blossomed. This is a good seashore shrub which is dubiously hardy inland at the latitude of Philadelphia. It can, however, be protected by covering the entire plant with earth, or by tying the shoots together and wrapping them with burlap, straw, or salt-meadow hay, which should enable them to withstand temperatures down to zero.
H. quercifolia (Oak-leafed Hydrangea) is trimmed in the same way as the preceding. The treatment accorded it is determined by the preferences of the owner, whether the flowers or the leaves are more important. If he wants flowers, he should try to keep the terminal buds from freezing, or if it is the leaves that are wanted, the shoots of the preceding year may be cut back in the spring, removing about one fourth of their tips.
H. petiolaris is the Climbing Hydrangea. This needs little or no pruning. All that is necessary is to cut back the flowering branches if their growth is so lush that there is danger of injury to the supporting tree or when it becomes so large that it may break away.
Schizophragma hydrangeoides (Hydrangea Vine).
This is very similar to the preceding, differing in the sterile flowers,
which have only one sepal, while those of Hydrangea petiolaris have three
or four. Prune as indicated above.
These in general are pruned by annually cutting out a few of the older shoots and by cutting the flowering branches freely for decorating indoors. There is one species, P. pubescens, which is almost unbelievably gawky if left unpruned. This requires pinching out the tips of young shoots in order to make a well-branched and symmetrical shrub. As in the case of Forsythia, it is the two- and three-year-old branches which produce the best display of flowers. Consequently, some of them should be removed immediately after flowering to make room for the one- and two-year-olds.
Rhododendron calendulaceum (Flame Azalea) is one of the deciduous Rhododendrons popularly known as Azalea. This Azalea is believed by some to be the most beautiful of all native shrubs. It may be pruned while it is dormant with some loss of bloom. However, if it is vigorous, pruning can be left until the flowers are faded.
Syringa. Syringa vulgaris (Lilac) is commonly budded or grafted on Ligustrum ovalifolium (California Privet) or L. vulgare (Common Privet). When the graft has taken, the young plants are planted out in the field .and deeply set with only the tip of the graft showing. When these plants are dug up for their first transplanting, the privet understock, if it is not already dead, can be cut away leaving only the lilac roots. Saddle grafting is the method used because the base of the scion is placed in such a way that it readily calluses and ultimately produces its own roots.
Wisteria (Wisteria). There is a widespread but erroneous belief that Wisteria has to be grafted to enable it to bloom. Failure to bloom may depend in part on the inherent nature of the individual, but chiefly, I think, on the environment and how it is pruned and treated.
Seedlings and cuttings should bloom in two or three years if they are in full sun and in soil not overly rich in nitrogen and pruned during July by shortening the shoots of the current season. These shoots should be cut to within about 6 inches of the branch from which they originate. Leave only those needed to extend the growth of the plant. It seems as though they are more likely to bloom when only a limited space is available on the trellis on which they can climb. Thus those which are trained to make standard plants or to bush form are likely to bloom earlier because of the severe pruning during the growing season.
Wisteria floribunda ( Japanese Wisteria) may in some varieties, such as W. f macrobotrys, have racemes of flowers 4 or 5 feet long, but W. sinensis (Chinese Wisteria) with racemes 6 to 12 inches in length is more popular. Perhaps the reason for this is because the Chinese Wisteria opens all its flowers at the same time, whereas the Japanese forms open gradually from base to apex.
Old shrubs which are making poor growth may be given new lease on life if they are cut back almost to the ground. It should be recognized, however, that such pruning is to be avoided as much as possible. Ideally, the treatment for shrubs which tend to become decrepit with age is to maintain constant juvenility by the removal, annually if necessary, of old worn-out wood. If this is done it greatly lessens the danger of upsetting the balance between roots and tops.
Thus it may be desirable to spread corrective pruning over several years, or cut about half of the wood marked for removal during the winter and then check the over vigorous growth by removing the remainder during the following summer. Done in this way, it obviates the loss of blossoms for one year, possibly two or three. Among the shrubs that are amenable to this treatment are Philadelphus (Mock Orange), Deutzia, Syringa (Lilac), Weigela, and Forsythia.
Shrubs which produce their blossoms on wood one year old or older cannot be extensively pruned when they are dormant without reducing the number of flowers produced in the spring. This does not necessarily mean that pruning this class in winter when they are dormant is absolutely prohibited.
Lilacs can be pruned without much loss of bloom the following season if the parts removed are restricted to suckers and weak, twiggy, and spindling branches. Occasionally branches bearing flower buds may be deliberately sacrificed so that those remaining will have a better chance to develop. Forsythia (Golden Bells), Jasminum nudiflorum (Yellow Jasmine), and Chaenomeles ( Japanese Flowering Quince) can be pruned late in winter and the prunings brought indoors to be placed in water, where they will open their flowers in advance of those left on the bush.
As a general practice, however, it is well to prune these early-blooming shrubs which flower on old wood, immediately after they have finished blossoming. Pruning should consist of the removal of worn-out wood and correcting irregularities of growth.
Comparatively late bloomers from old wood such as Kolkwitzia (Beauty Bush) and Buddleia alternifolia (Garland Butterfly Bush) can be pruned by cutting off many of the branches which have produced flowers. Cutting the flowering branches for indoor use serves, in part, the same purpose as regular pruning and in some cases, for example Lilac, it is an important means of preventing the plants from increasing too rapidly in height.
Cut-back and die-back shrubs
There are some shrubs the tops of which are not entirely winter-hardy in locations where the temperatures may fall to 10 degrees or lower. Examples are Abelia grandiflora, Buddleia davidi (Butterfly Bush) and its varieties, Vitex (Chaste Tree), Callicarpa japonica (Beauty Berry), Caryopteris incana (Bluebeard), of which Blue Mist and Heavenly Blue are outstanding varieties, and some varieties of Rosa (Rose). They are pruned in the spring, cutting them back to the living wood. In severe climates it may be desirable to mulch these shrubs early in the winter with straw or something similar to protect the roots.
There are some which can be treated as cut-back shrubs even though the tops may not suffer as a result of the winter. Among them are Hydrangea arborescens grandiflora (Hills of Snow) and Spiraea bumalda Anthony Waterer. These may be pruned by cutting them almost to the ground line in the spring or the flowers may be cut off as soon as they begin to fade.
Ailanthus altissima (Tree of Heaven) and Paulownia tomentosa (Empress Tree) are sometimes grown to a single stem which each year is cut to the ground before growth starts in the spring.
Treated in this way, they make strong shoots with leaves which are extraordinarily large. These are used to obtain an exotic effect where subtropical bedding is practiced.
Hibiscus syriacus (Rose of Sharon or Shrub Althea) and Tamarix pentandra (Summer Tamarisk), which blossom on shoots of the current season, can be pruned before growth starts in the spring. Shrubs which may be similarly pruned are Hydrangea paniculata grandora (Peegee Hydrangea), Hybrid Perpetual, Hybrid Tea, and other Roses.
Hydrangea paniculata grandiflora has been widely condemned by garden writers, including this one, but it has its good points, especially if it is pruned correctly. This means cutting back, about one third, the shoots of the preceding year. Many gardeners and landscape architects object to this Hydrangea, saying that the foliage is coarse and the flower heads are too flamboyant. Perhaps this is true when they are pruned back in the spring so that each stem can produce only one or two growth shoots, but when they are pruned moderately the plant looks entirely different. When it is left without pruning, more especially in the variety praecox, the plant is really quite handsome.
H. arborescens grandiflora (Hills of Snow). This is commonly pruned by cutting it down almost to the ground line in the spring. It probably is the best way to treat it.
Those shrubs which are grown for the color of their twigs in winter, such as Red-osier Dogwood, should be pruned before growth starts to promote the production of an ample supply of one-year-old twigs. This can be done by cutting them severely in the spring when they are dormant, followed up by pinching out the tips of the shoots when they are about 10 or 15 inches long. The Pussy Willow belongs in this same group so far as technique of pruning is concerned.
If the desideratum is to have "pussies" produced on long whiplike growths, pruning should be severe just before leaf growth starts. Those who prefer the pussies displayed on short, twiggy growth should refrain from excessive pruning by limiting it to shortening the shoots produced the preceding year about one third.