These never should be cut back into wood that is leafless. The best way to handle Pines is to shorten the "candles" when they have attained their full length before the needles are produced and/or cut the long terminal shoots back to the first or second whorl of branches. Ordinarily it is, or should be, unnecessary to prune a Pine except when a formal hedge is desired or to remove insect pests such as the pine-shoot weevil or in the case of foundation planting when they are getting so tall they obscure the view from the windows. These remarks also apply to Abies and Picea, substituting "young growth" for "candles."
In this connection I would like to remind you that it is important to start curbing those plants which are likely to exceed their bounds, before they reach the stage which is your limit for them. The reason for this is that a small portion of wood of the current season must be retained to maintain the plants in health. This brings a further reminder that only small- or slow-growing species should be used in the areas where low-growing plants are necessary, or that you should plant those that can take close shearing, such as Japanese and English Yew, Hemlock, and Arborvitae.
The best time to prune coniferous evergreens in general is just before new shoots start in the spring, followed by shearing new growth when it has attained its full length or just before this, usually in June or early in July. Treated in this way, there will be additional new growth to mitigate the shorn look throughout the winter which occurs when pruning is done in September. Three prunings each year are permissible only when the plants are approaching the limit in size. In the case of Pines, after the main terminal shoots have been cut back, the "candles" of those remaining may be shortened by removing one half to three fourths of each candle.
To sum up, coniferous evergreens ordinarily are no problem except for the occasional cutting back of a shoot outstripping its neighbour; in the case of foundation planting, to prolong the time lapse before it becomes necessary to remove them; when a specimen of formal shape is required as an accent in a formal garden; when needed to form a hedge; or for topiary.
Broad-leaf evergreen trees and shrubs
Broad-leaf evergreens seldom need extensive pruning except when they are used to provide formal accents and when they are used for hedges or edges, or when it is necessary to remove broken branches and those which have been injured as a result of a severe winter.
The number of broad-leaf evergreen trees that can endure sub-zero temperatures is definitely limited. The hardiest ones are the Ilex opaca (American Holly) and, if you can find a source for it, I. pedunculosa.
Although there is, or was, a tree of Magnolia grandiflora (Bull Bay) growing in Brooklyn, New York, usually it is not reliably hardy in climates that are more severe than that of Washington, D.C. It is not too happy even on the northern part of the Pacific coast, where winter conditions are no problem but the lack of heat in summer might be a limiting factor. This can be overcome, to some extent, by growing it against a wall as is done in England, to take advantage of the extra heat radiated from the wall.
Magnolia virginiana (Sweet Bay) is only semi-evergreen and takes a long time to grow to a sizable tree. Pruning these evergreen trees is usually restricted to the removal of unwanted branches or shortening "wild" shoots which grow out far beyond their neighbours.
Ilex aquifolium (English Holly), opaca (American Holly), I. cornuta (Chinese Holly), and I. crenata ( Japanese Holly) are the most important of the evergreen hollies. The berries of all these are red except the last-named, which bears black fruits. South of Washington, D.C., I. vomitoria (Yaupon) and I. cassine (Dahoon) are used for hedges and topiary. Many examples may be seen in restored gardens at Williamsburg, Virginia.
I. aquifolium and I. opaca are grown under orchard conditions to provide Christmas greens and berries, the first-named on the Pacific coast and I. opaca in Florida. They are also used for hedge plants in regions where they are thoroughly winter-hardy. One clipping a year in summer just before the young shoots attain their full length is usually all that is needed. The annual cutting of the crop in December normally is all the pruning needed by those which are grown for Christmas greens.
I. crenata is very similar to Boxwood in its foliage characters. It can be distinguished easily, however, by its alternate leaves as compared with the opposite leaves of Boxwood. The straight species, I. crenata, may grow 20 feet tall in regions with mild winters. Some of the varieties, such as helleri, convexa, and globosa, are dwarf forms. The only pruning that any of these require is done in the spring, when the shoots injured by winter are cut off.
The Dahoon and the Yaupon need little pruning except when a formal effect is desired. They can be sheared as often as necessary during the growing season. The flowers of English Holly are produced on the growth of the preceding season; consequently pruning, if any, should consist of shortening the shoots of the current season before they attain their full length. The purpose of this is to avoid having the berries obscured by long vegetative shoots.
I. glabra (Inkberry), a semi-evergreen, tends to grow lax and gawky. It should be pruned by cutting back the shoots of the preceding year in the spring and the tips of the young growth again in June or July.
Perhaps a reminder is in order that, except for I. burfordi, a variety of the Chinese Holly, most hollies arc dioecious—the staminate (male) and pistillate (female) flowers are produced on separate plants. Therefore it is essential to have at least one male plant for every 20-50 females to insure a good crop of berries. Other ways of solving this problem are to bud-graft the male plant on the female, or, if you know of a male plant from which you can cut branches when the flowers are open, place their butts in a bucket of water and put them in the vicinity of the tree that is to be pollinated. Honeybees and other insects will do the actual work.
B. julianae (Wintergreen Barberry) is reputed to be the hardiest of the evergreen species. This and B. triacanthophora and B. verruculosa (Warty Barberry) need practically no pruning except to shorten shoots which may spoil their symmetry.
Buxus (Boxwood or Box)
Buxus sempervirens (Tree Boxwood) is capable of growing into a small tree up to 25 feet. B. sempervirens suffruticosa (Edging Boxwood) is the variety that is most commonly grown in America. This is the one which is a feature at Washington's home at Mount Vernon. In course of time it makes a handsome billowing mass up to about 40 inches high or more, but usually it is clipped to a formal hedge or edging.
For edging it should be sheared in the spring and again in late August or early in September. Even with this regular pruning the plants will, in course of time, grow too large to serve as an edging. In such cases it is customary to dig up entire plants, if they are not more than 1 foot high, divide them, and reset them deep enough to provide the desired height, or shoots 6 inches or so long can be put in as cuttings late in August and set out the following spring. The remainder of the overgrown edge can be dug up and planted where it has a chance to develop.
Calluna vulgaris (Heather)
There is only one species of the genus Calluna,
but it is variable and there are numerous named varieties. They should
be pruned, usually with hedge shears, by cutting off the faded spikes
of bloom, in the spring before new growth starts.
Unfortunately Camellia is not reliably hardy north of the Mason-Dixon Line. However, there are plenty of adventurous gardeners who don't mind taking a chance of growing them outdoors even in the Hudson River Valley. The two best-known kinds, apart from Camellia thea (Tea Plant), are C. japonica and C. sasanqua. The last-named blooms in late fall and early winter, while japonica blooms in late winter and early spring. Ordinarily there will be no need for pruning. If it does become necessary, it is comforting to know that it can be done without permanent injury even though it may involve cutting into the old wood. If necessary, they should be pruned immediately after blooming when the new growth is about to begin.
Daphne odora, D. cneorum, and D. burkwoodi are the most important species of evergreen Daphne for outdoor gardeners. D. burkwoodi and D. cneorum are the hardiest. D. odora marginata is hardier than odora, but this is not reliably winter-hardy in regions north of Philadelphia. Little pruning is needed on any of these evergreen Daphnes except perhaps that the flower heads of D. cneorum may be sheared off as soon as the flowers are faded.
Unlike its close relative Calluna, this genus contains more than 500 species. The winter-hardy kinds include Erica carnea, E. ciliaris, E. cinerea, and E. vagans, which can be pruned by shearing off their flowers when they fade. The tender kinds, which need the protection of a greenhouse in our climate, include E. hyemalis, E. persoluta, and E. melanthera. They can be pruned with hand pruning shears by cutting off the shoots that have borne flowers as soon as the flowers have faded. These Heaths are frequently obtainable from florist shops at Christmas and Easter time, where they are sometimes featured as "genuine Scots heather," which is naughty of the vendors because these species of Erica are really not Scotch at all. They are native in South Africa.
Although the Heaths and Heathers are included in the broadleaf evergreens, actually their leaves are narrower than those of some of the conifers which are called narrow-leaf evergreens.
Euonymus (Spindle Tree)
Euonymus kiautschovica (patens) is semi-evergreen except in mild climates. The only pruning needed is the occasional removal of a branch which gets out of bounds.
E. fortunei is at least as hardy as the English Ivy, especially if it is grown on a north wall or where it is not exposed to winter sunshine.
E. f carrierei is a shrubby kind which may require pruning to - make it retain its shrubby form. The variety vegetus also can be trained as a shrub. It is more easily done if there is no support of any kind nearby to encourage it to climb. There is now available a series of shrublike cultivars produced by Corliss Brothers, Inc., of Gloucester, Massachusetts. They should be immensely valuable for foundation planting and for hedges. Among them are Emerald Cushion, a low spreading kind, and Emerald Pride, which in outline is like an old-fashioned beehive.
E. japonicus ( Japanese Euonymus) is much less hardy than the fortunei group. It is a favorite in England as a hedge plant because it stands shearing quite well.
Hedera helix (English Ivy) and its many varieties need annual pruning when they are grown as edging plants, as a hedge on a support of some kind, and where they are climbing on a wall of a house. The time to do this, in every case, is in the spring when new growth is beginning. Usually leaves as well as the shoots which developed the preceding year are cut off. This makes an unsightly mess for a few weeks but new growth quickly covers it. Paradoxically, hedge shears are used for trimming except in hedges, where hand shears are used to shorten projecting shoots.
The arborescent forms, which retain their character if the shoots are rooted from cuttings, need no pruning except for removal of the flower heads and wayward shoots which extend beyond the general outline, or when a reversion to the juvenile climbing form develops.
Hypericum moserianum (Goldflower) is a hybrid of H. calycinum and H. patulum. These are suitable ground covers in mild climates and can be used in the shade if it is not too dense. Pruning consists of cutting them back almost to the ground every spring.
Kalmia (Mountain Laurel)
Kalmia latifolia (Mountain Laurel) requires about the same kind of treatment given to Rhododendron. K. angustifolia (Sheep Laurel), sometimes called Lambkill, usually has a thin, open habit of growth. Pruning by shortening new shoots in the spring might overcome this defect.
Leucothoe (Fetter Bush)
-Leucothoe catesbaei (Fetter Bush or Dog Hobble) is the best known and the hardiest of the evergreen species of Leucothoe. Usually no pruning is required, but if for any reason it is necessary, it is easy and consists merely of cutting off shoots which have borne the blossoms. This can be done any time after May when the flowers have faded.
There are two evergreen species of Pieris (Andromeda) commonly grown—P. japonica and P. floribunda. Like Leucothoe, they produce their flower buds in the fall, ready to open in April or May. It is seldom necessary to prune them. Any pruning should be restricted to the removal of wayward branches that may spoil the symmetry of the bushes.
Pyracantha coccinea and its variety lalandi are the hardiest of the Firethorns. Pruning of those grown to bush form consists of shortening "wild" shoots during the summer. When they are trained to grow against a wall (espaliered), they should be pruned after flowering by shortening all shoots except those which have borne flowers and those which are needed to extend the growth.
One form of pruning which can be practiced on Rhododendrons annually to their advantage is the removal of flower clusters as soon as they have faded. This prevents the formation of seeds, which is a drain on the plant's energy, especially in the case of newly set plants. No tools are necessary, as the cluster is easily removed by giving it a sideways downward pull with thumb and fingers. Obviously this is impractical when the bushes have attained a height and spread of more than 6 feet.
Pruning Rhododendrons in the sense of cutting back branches is to be avoided whenever possible. If, however, the branches need thinning to make the bush more shapely, or if the bush has grown too tall for its location, pruning is permissible because, as a rule, new shoots are freely produced even by old wood when the plant is cut back.
Pruning should be done immediately after flowering, just before new growth starts in the spring. In the case of those species, for example R. maximum, which make new shoots before the flowers open, the flower display must be sacrificed to avoid having to cut off branches which already have started growth. Any pruning which demands severe cutting back should, if possible, be spread over a period of two or three years—cutting half or one third one year and the remainder after that.
The preceding remarks apply also to that group of evergreen Rhododendrons commonly known as Azalea. In addition, if a compact bush is desired, any new shoots which show a disposition to outstrip their neighbors may have their tips pinched off early in the summer.
Skimmia japonica and its variety reevesiana are hardier than is generally believed. The last-named has survived about ten winters outdoors, planted among Pachysandra on the north side of our home about eighty miles north of New York City. Usually it does not need pruning, but if for any reason it is necessary, it should be done in the spring when the new growth is about to start.
Other tender or barely hardy broad-leaf evergreens include
Aucuba (Japan Laurel)
Aucuba japonica and its variety variegata (Gold-dust Tree) can be cut back severely in the spring if necessary to reduce its size or to improve its shape.
Laurus (Bay Tree)
Laurus nobilis, the true Laurel, better known perhaps as Bay Tree, is the tree which provided the material for the laurel wreaths bestowed on the winners of athletic contests by the ancient Greeks and Romans. Because of its amenability, it is commonly grown in tubs or large pots, sheared either to a hemispherical, pyramidal, or ball-like head on a clear trunk.
These formal-shape laurels were formerly imported from Europe and were in great favour about fifty or sixty years ago, but with the coming of the Quarantine No. 37, which virtually prohibited the entry of them and similar plants, their popularity declined and now they are seldom seen. Difficulty in finding a place to store them for the winter and of obtaining help to bring them under cover in the fall and put them out in the following spring doubtless are factors in their loss of favour.