PROPAGATION

Although many plants are easily raised from seed , some can be reproduced much more quickly and conveniently by vegetative propagation, that is, by cultivating suitable portions of the root, shoot, or even the leaf. Cuttings, layering, division, budding and grafting are typical methods, and with man-made varieties of plants such as delphiniums, lupins, Michaelmas daisies, phlox etc. which do not come true from seed, owing to their complex ancestry, vegetative propagation is essential if an exact replica of the parent is desired. Large roots with several crowns, like Michaelmas daisies, can be split into parts of suitable size by hand, or by inserting two hand-forks back to back and levering apart. The fresh young outside portions are then re-planted — preferably by grouping several small pieces than scattering single large ones. Division in this way is also the easiest method of increasing all plants that make separate shoots from the roots like campanula, saxifrage, achillea, thyme, sedum and many rock plants. Some plants with a single crown and woody trunk, such as heli-anthemum and lithospermum, can be lifted at any time between March and October except when the plants are in flower, when their rooting shoots can be pulled apart and re-planted.

Layering is the best way of propagating border carnations; climbers, such as wisteria, and clematis; plants with trailing stems, like strawberry, blackberries and loganberries, and hybrid berries like the boysen-berry. The method for carnations consists in selecting half-ripened shoots from well grown plants in midsummer, stripping off a few of the lowermost leaves, and with a sharp knife making a slanting cut halfway through the stem and passing through the lowest joint. The cut portion of the stem is buried with the incision open, pegged down firmly and kept moist. When the layers have ‘taken’ the new plants are severed from the parent and transplanted. With clematis, stems up to three years old layered in June will give strong plants for rooting next autumn. For details of ‘aerial layering’ see POLYTHENE.

Cuttings. The best and easiest way of propagating many garden plants is by taking cuttings, that is, suitable portions of the plant are cut off by means of a sharp knife, trimmed, and planted in pots or prepared beds until they ‘strike’ or form roots. This method is speedier than seeding. As already emphasised, most man-made varieties of plants, especially border perennials like delphiniums and lupins, do not come true from seed and cuttings are usually the only means of ensuring that the character of the parent plant is retained. It is particularly valuable for plants that do not produce seed. Success depends on taking shoots of the right kind and age at the correct time, and in providing suitable conditions for rooting.

Cuttings are mostly taken from the stem, though sometimes from the leaf or root. Stem cuttings are known as ‘soft wood’ or ‘hard wood’ cuttings according as they are derived from the young, soft growth of plants like viola, lobelia and catmint, or from the firm wood of more mature shoots of roses, box, privet, other shrubs and trees. Soft wood cuttings usually root well out of doors, and very easily if grown in a frame with a slight bottom heat. A temperature of 55 degrees F. is about right. They are best taken during the growing season from June to August. Well developed shoots should be selected and cut off just below the leaf. Lengths will vary according to species but 2— 2 ½ in- is a fair average. Very thin or extra thick cuttings should be avoided. The thickness of a pencil is often taken to be a good average. The lower leaves are then removed and the trimmed cuttings planted out of doors, or, if it is late in the season, in boxes, closed frames, or cloches.

With plants such as catmint, suitable growths can be selected from the new shoots thrown out by bushes cut back after flowering. Antirrhinum, pansy and viola cuttings should be taken from the sucker-like growth in the centres of clumps — hollow shoots are useless. Rock plants like aubrietia, hclianthemum, dianthus and saxifrage, strike very readily if new side shoots that have not born flowers are selected from the base of the plant. With some plants such as fuchsia, geranium and hydrangea, it is often best to use half-ripe growths for cuttings. Hard wood cuttings are usually taken after the leaves have dropped from the parent plant, from September to November. This time is also best for most evergreens and woody greenhouse plants, also soft fruits like currants and gooseberries. Heeled cuttings were always considered to be best, that is, side shoots stripped from the main stem with a butt or ‘heel’ of older wood at the end which provides a good surface for rooting, but it is very doubtful if a ‘heel’ cutting is any quicker rooting. They may be inserted in sheltered beds out of doors, or covered with cloches or hand lights if necessary. For certain shrubs, some heat is desirable and the cuttings should be placed 5 or 6 to a 3 in. pot in the propagating frame or warm greenhouse. Generally speaking, hard wood cuttings are 9 — 12 in. long.

Rose cuttings form what are called ‘own-root’ roses, and reproduce the parent plant exactly. Flowered side shoots may be used from ramblers, removing the dead blossoms just above a bud. Note that with other groups of roses, e.g. hybrid teas, propagation by cuttings is less reliable than budding on to a wild rose or understock. See ROSE — Propagation. A large number of plants can be easily propagated by means of root cuttings, in which portions of the root of varying thickness and size according to species are planted out in a pot or pan containing a suitable rooting medium. If the plants have large roots, pieces 2 — 3 in. long may be buried upright in the soil with the end that was nearest the crown uppermost; leaves will then develop at the crown end, and roots at the base. The method is specially useful for fleshy-rooted plants, such as anchusa, but root cuttings may also be taken from certain fibrous-rooted plants such as phlox, gaillardia and Oriental poppies. Primula denticulata and Pulsatilla vulgaris (Anemone Pulsatilla) can be increased in this way.

Begonia rex, gloxinia, ramonda, Saintpaulia onantha (South African violet), streptocarpus, sachenalia (Cape cowslip) and certain other plants can be propagated by means of leaf cuttings. A well-grown leaf is removed, doubled over, and pressed with the fingers till the midrib cracks. The leaf is then laid flat on a pot or pan containing a suitable rooting medium, and the end of the stalk pressed in slightly. Small stones are placed on the leaf surface to keep the midrib in close contact with the soil. Roots eventually form from the midrib, and leaves on the upper side. Watering of leaf cuttings should be done by immersing the pan or pot in lukewarm water so that the soil becomes moist but the surface of the leaf remains dry.

A suitable bed for cuttings can be made by digging a trench in a sheltered part of the garden and filling it to within several inches of the top with a 9 — 12 in. layer of equal parts of sharp sand and loam. The trench should be of a depth that will just allow the tops of the inserted cuttings to be flush with the general soil level. Set in the cuttings to about one-third or half of their length, and make sure that their bases rest on die soil.

The above soil mixture may be used for frames, which should be kept closed and shaded from sunlight to prevent excessive loss of water from the plants. If the surface of the soil becomes caked, it should be lightly forked. Note that the mixture recommended by the John Innes Horticultural Institution for rooting cuttings is I part by loose bulk of medium loam to 2 parts of peat and I part of coarse sand.

Many plants which are difficult to ‘strike’ will root quite easily if treated with growth-regulating substances or hormone preparations as they are often called.

Cuttings can also be rooted in vermiculite, a natural substance found in the U. S. A. and South Africa and allied to mica. Its great advantage is a capacity to remain moist for not less than 14 days. Vermiculite is very light so that the rooted cuttings can be lifted with no risk of damaging the roots. Seeds can also be germinated in this material, which is often used in conjunction with peat or sand, as it is sterile, I.e. devoid of plant nutrients.

After Care of Cuttings. All types of cuttings must be kept fairly moist, some more so than others. For example, lupin cuttings should be moist at all times until they are rooted — usually about a month — whereas delphiniums are less particular notwithstanding their dislike of dryness at the roots during the summer. Once established, lupins will tolerate very dry soils, despite this insistence on ample moisture in the initial stages. Hard wood cuttings usually succeed with one watering directly after insertion as they are taken during a period when the sap is not running freely and there is relatively little transpiration. Always go over hard wood cuttings after hard frost to ascertain if this has lifted them out of the ground. If so re-firm as necessary.

An interesting development is the mist technique to keep soft cuttings really damp. The leaf surface is continuously covered with a film of moisture, the mist being intermittent. To achieve this an ‘electronic’ leaf usually consisting of a strip of plastic material with two carbon electrodes is placed among the natural leaves of the cuttings. So long as the surface of the plastic remains wet it conducts electricity. The connecting relay which controls the spray lines stays closed and the spray line is turned off. As the ‘leaf dries the relay opens and the spray line is turned on, forming a fine mist which wets the leaf once more and turns the supply off. By this means a fairly constant humidity is achieved. There are various forms of this device and experiments continue to improve the technique.

Budding and Grafting. Budding of vines and other plants was practised in pre-Christian times. It is simply a form of grafting and is defined below. Grafting is joining tv/o distinct parts of two plants to form a permanent union. One portion, known as the stock or understock, furnishes the root system, the other, called the scion, supplies the portion to be increased, and contains the eye or bud. In budding the scion comprises one bud only, with a small piece of bark attached . With other ways of grafting there are several buds. Grafting is generally undertaken in very early spring. There are various methods, e.g. whip and tongue grafting, saddle grafting, splice grafting. Camellias, clematis, wisterias, begonias and rhododendrons are frequently propagated by grafting, acers (maples), elms, horse chestnuts and flowering cherries by budding. Roses are increased by budding at the same time — June to August — as other plants propagated in this way. They can also be grafted under glass in winter. See ROSE — Propagation, for full instructions on both methods.

Apples and other tree fruits are increased by budding or grafting. Stocks can be obtained from certain nurserymen and planted about 15 in. apart, allowing 3 ft. between rows. At nurseries budding is the more usual method, grafting being undertaken the following March or April if the bud fails to ‘take’. Side shoots of the current year’s growth are best for budding, the lower half of each shoot being the most suitable. Make a T-shaped cut in the bark of the stock about 6 in. above soil level (with roses the cut is made as close to the roots as possible). Raise the flaps of the bark on each side of the downward cut, by means of a budding knife or the back of a pen-knife. Buds or eyes are inserted in the stock as with roses and the union is usually made within 3 to 4 weeks. The following February the budded stocks are cut down to just above the union. For grafting, dormant scions are collected in winter (often when pruning). Heel these in a shallow trench, preferably in a cool, shady corner. Each scion should have 3 to 4 buds, the soft unripe tips being thrown away. Cut down the stocks to within 3 in. of soil level. Make a slanting, upward cut about I y2 in. long on the stock, followed by a downward cut to form a tongue near the top of the upward one. A similar tongue is cut on the scion, this being inserted into the tongue of the stock. Tie with moist raffia and seal with warm grafting wax.

Cut off the top portion of the stock, so that it slopes inwards. Leave all shoots that grow on the scion until they are about 7 in. long and choose the strongest to furnish the main stem or trunk of the new tree. This must be staked with a bamboo cane or other support.

Suckers. Some plants can be increased by rooted suckers or horizontal underground growths (do not confuse with runners, e.g. strawberry runners, which are horizontal shoots running along the soil surface). Suckers are simply dug and planted in their new positions. Raspberries, Rhus typhina (stag’s horn sumach), Populus tremula , mint and Michaelmas daisies are increased in this way.

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