PROPAGATION BY CUTTINGS

Cuttings are pieces of roots, leaves, or stems that are removed from the parent plant and placed in an environment that promotes their devel­opment into total plants. In most cases, the new plant will be genetically identical to the parent plant since propagation by cuttings is an asexual method of reproduction. As measured by sheer numbers of plants pro­duced, propagation by cuttings is the most widely used method of reproducing ornamental plants asexually. The reasons are that the tech­nique is quick, easy, and inexpensive. Greenhouse growers and nursery growers both propagate important commercial crops from cuttings.

Propagation by cuttings is possible because of the ability of plant cells to revert to an undifferentiated, actively growing (meristematic) condition, from which they can once again initiate the root, stem, and/ or leaf tissue necessary to form a complete plant. Years of research have been directed toward understanding why and how cuttings develop into complete plants and why certain species are easier to propagate than others. Although not everything is known or understood, these points are commonly accepted:

• Adventitious roots are initiated in herbaceous plants from points just outside or between the vascular bundles. In woody plants, they originate next to and out from the center of the vascular core. The adventitious root initials (growing points) may develop after cuttings are taken, or they may be preformed but dormant in the vegetative tissue before the cutting is taken. Individual species vary.

• Adventitious roots and adventitious shoots do not develop at the same rate within an individual plant or among different species.

• The rate of root and shoot initiation and formation in cuttings is controlled in part by growth regulators, principally auxins, gibberellins, and cytokinins. Some growth regulators occur naturally in the tissue. Others can be applied by the propagator. Those that occur naturally may promote or inhibit growth depending on the growth regulator and plant species involved and the concentration of the growth regulator.

• Auxin is necessary for the initiation of adventitious roots on stem cuttings. IAA (indole-3-acetic acid) is the naturally occurring auxin. NAA (naphthaleneacetic acid) and IBA (indolebutyric acid) are similar materials, but synthetic. The synthetic auxins, applied by plant propagators, are generally more effective than the naturally occurring IAA at initiating the formation of new roots on stem cuttings.

• The formation of adventitious roots is very slow in certain plants. Depending on the species, rooting may be inhibited by: (1) naturally occurring rooting inhibitors in the plant tissue; (2) the lack of one

or more rooting cofactors, found by several researchers to work synergistically with auxin in root initiation; or (3) a continuous

sclerenchyma ring between the phloem and the cortex that creates a physical barrier to developing roots as they attempt to emerge from the center of the vascular core.

• Cuttings possess polarity. They form new shoots at the end nearest the tip of the plant (the distal end) and new roots at the end nearest the crown (the proximal end). In commercial propagation operations, care must be taken to orient the cuttings properly in the propagation bench to prevent delayed and unsatisfactory rooting.

• The best cuttings result from healthy stock plants that contain adequate nitrogen for good shoot formation and high carbohydrate levels for easier rooting. Excessive nitrogen in the cuttings is likely to inhibit root formation. Lateral shoots make better cuttings than terminal shoots in certain species because the former contain more carbohydrates whereas the latter are very high in nitrogen.

• Adventitious roots form more quickly on stem cuttings in the dark than in the light. Stock plants of certain species can have their stems which are to be used as cuttings, wrapped in black plastic or tape where the roots are to form. Later, when the cuttings are made, new roots will form more quickly in the darkened portion. This is one reason why air-layering works as a propagative technique and why cuttings root best at their proximal end, within the darkened rooting medium.

• Stem or root cuttings taken from young plants root more quickly than cuttings taken from older plants. The juvenility factor is commonly acknowledged but only partially understood. Many commercial propagators maintain their stock plants in hedge form to sustain the juvenile growth phase rather than permitting the plants to develop into their normal adult tree form.

• Cuttings from lateral shoots and others from vertical shoots may develop into plants with different growth habits. Generally, strongly vertical plant forms will result only if the cuttings are from vertical shoots. The more easily a species roots, the less important is the position of the cutting on the stock shoot.

• Cuttings can usually be taken from the stock plant regardless of whether it is in a flowering or vegetative stage. When a species is difficult to root, better results are usually obtained when vegetative growth is selected for the cuttings. Research suggests that a reduction in auxin levels during flowering is a probable reason.

The Environment for Rooting

Once separated from the stock plant, cuttings are vulnerable to injury from assorted environmental factors. To ensure the rooting of the greatest number of cuttings, several precautions must be taken by the propagator.

Moisture Moisture is critical to the successful rooting of the cutting. The irony of the situation is that the leaves need to produce food to sup­port root formation, but lack of roots prevents the cutting from taking up water the leaves need for photosynthesis to produce food. The more slowly a species forms roots, the more critical the situation becomes. To reduce water loss from transpiration, a mist line is widely used by

propagators. Seldom applied continuously, intermittent mist under the control of a time clock maintains high humidity around the leaves of the cuttings. The film of water that settles onto the leaves also keeps the cuttings cooler than the drier air.

Temperature Temperature controls the rate of root and shoot devel­opment in cuttings. If the air temperature in the propagation bench is too high, shoots will form faster than roots, placing even greater mois­ture stress on the cutting. Instead, the rooting medium should be warm­er than the air to promote root growth in advance of shoot development. To accomplish this, an electric heating cable and a thermostat may be installed in the propagation bench. With controlled daytime air tem­peratures between 70° and 80° F, night temperatures of 60° F, and the rooting medium slightly warmer at all times, the cuttings should root satisfactorily.

Nutrition Nutrition supplied during rooting may influence the quality of the cuttings produced. Since the rooting medium is not high in nutri­ents and the misting leaches minerals from the plant tissue, it is often beneficial to add a liquid fertilizer to the mist water.

Acidity/Alkalinity The pH of the rooting medium can affect the num­ber and quality of adventitious roots produced. Media having a pH near neutral have generally been found to produce the best root systems. There are selected exceptions, however. For example, azaleas root better in an acidic medium.

Light quality and intensity As noted previously, high carbohydrate levels in cuttings promote good root formation. Therefore, high light intensity from sunlight or an artificial source high in red and/or blue wavelengths (most usable by plants) is necessary for good root develop­ment. As the cuttings develop, the day length may begin to affect the shoots either positively or negatively. Research has shown the effect of photoperiod to vary depending on the species of plant being propa­gated, the season of the year, and the type of cutting.

Oxygen content As long as the propagating medium is porous and the bench well drained, sufficient oxygen will usually enter the medium to satisfy the plants’ needs. If the cuttings are found to be producing adventitious roots only near the surface of the medium, it is an indica­tion that the medium or bench is not draining well.

Types of Cuttings and Methods of Propagating Them

In Table 14-3, various types of cuttings are compared. Many species of plants can be propagated by more than one technique. Which method is most practical is a decision to be made by the commercial propagator (Figure 14-4).

Hardening-Off the Cuttings

Hardening-off means the gradual adaptation of plants to environmen­tal conditions more stressful than their present ones. Cuttings rooted under conditions of high light, high humidity, and warm temperatures must be hardened-off soon after the roots form if normal growth is to

TABLE 14-3. A Guide to Propagating Cuttings

Type of Cutting

l

Typical plants Used

When to Take Cuttings

Part of Plant to Select

Location of the Cut

Length of Cutting

Hardwood

stem

(deciduous)

Some deciduous trees and most deciduous shrubs

Late autumn to early spring

Central and basal portions of the stem

Basal cut: just below a node Distal cut: 1/2 inch above a node

4 to 30 inches

Hardwood stem (narrow – leaf evergreen)

Young seedling stock plants of most narrowleaf evergreen trees and shrubs

Late autumn to late winter

Mature terminal shoots of previous year’s growth

4 to 8 inches below the terminal shoot

4 to 8 inches

Semihardwood

stem

Woody, broadleaf evergreens

Summer

New shoots, only partially matured

Basal cut: just below a node

4 to 6 inches

Herbaceous

stem

Succulent herbaceous shrubs and florist crops

Usually successful at any season

Strong stem sections, only partially matured

Basal cut: just below a node

3 to 5 inches

Leaf

Tropical foliage plants

Successful at any season

Leaf blade Or Leaf blade and petiole Or Pieces of the leaf blade

Leaf-Bud

Tropical shrubs, broadleaf evergreens, and greenhouse crops

When the foliage is actively growing

Leaf bud, petiole, and a piece of stem with bud attached

At a node

Root

Deciduous shrubs

Late winter or early spring

Root sections

1 to 6 inches (larger roots make larger cuttings)

*Notes: Deciduous hardwood stem cuttings are handled differently depending on when the cuttings are taken and when the local climate allows planting. Where winters are mild, cuttings can be made in the autumn and planted immediately. Some callus formation and rooting will often occur before winter. Cuttings taken in late fall or winter can be stored outdoors in mild areas or in cool, nonfreezing storage indoors. They should

Preparation and Requirements

Preferred Rooting Medium

Special Requirements

Special Methods of Handling

Treat cuttings with a root-promoting material; plant immediately or place in a box of peat until callus forms, then plant; insert into rooting medium so that 2 inches to one-fourth of the cutting is planted.

Loose sandy loam

Select cuttings from stock plants growing in full sun

*See notes

Remove all leaves from lower half of cutting; treat with IBA and a fungicide; place into rooting medium up to the leaves

Sand or Half perlite and half peat

High light intensity and high humidity are necessary but not heavy wetting. Bottom temperature of 75° to 80°F is recommended.

Cuttings may root more easily if a piece of old wood is left at the base. Also, wounding the basal end of the cutting may help promote rooting.

Remove all leaves except two or three at the distal end; if leaves are large, cut them in half to reduce transpiration; treat with a root-promoting material; place in medium up to the leaves

Half perlite and half vermiculite Or Half peat and half perlite

Use intermittent mist and bottom heat.

Take cuttings in early morning to ensure turgidity, or keep wood cool until ready to take cuttings.

Remove all leaves except two or three at the distal end; if leaves are large, cut them in half to reduce transpiration; place in medium near but not up to the leaves. Use of a root-promoting material is optional.

Half perlite and half vermiculite Or Half peat and half perlite

Use intermittent mist and bottom heat.

Take cuttings from stock plants kept cool to ensure turgidity. Keep cuttings cool and plant as soon as possible.

It is usually necessary to use a leaf or section of leaf with one or more wounded veins included. With some species, the basal ends are inserted into the medium. Other species are pinned flat onto the medium, vein side down. Treatment with a root-promoting material may help.

Loose sandy loam Or Sand

High humidity is needed. Also, polarity must be noted.

There is great variation in the techniques used. Some can be propagated best in greenhouse flats, others in closed petri dishes.

Treat cut surfaces with a root-promoting material. Insert into the rooting medium with bud buried 1/2 to 1 inch beneath the surface.

Half peat and half perlite

Select cuttings with well-developed axillary buds. Maintain high humidity and bottom heat.

Cut ends may be treated with a fungicide. Plant either with the crown end upward and at soil level, or horizontally 1 to 2 inches deep.

Sand Or Sandy soil

Correct polarity must be noted.

Cuttings are frequently obtained as nursery crops are being balled – and-burlapped.

be placed upside down or horizontally in moist sand, sawdust, or shavings. Callus tissue and root initials will usually form during the storage period. Some cutting stock can be collected during the winter and kept in cool, nonfreezing moist storage until early spring. Cuttings can then be made and planted immediately.

figure 14-4. Cuttings being placed in the propagation bench to promote root formation (Delmar/Cengage Learning)

occur. Allowing the cuttings to remain in the propagation environment longer than necessary for rooting can cause damage to both new roots and new shoots. However, the abrupt cessation of misting or other spe­cial propagating conditions can result in severe shock to the cutting.

Commercial propagators use various techniques to harden-off cut­tings. Among these techniques are the following:

• Gradually decrease the misting. Either reduce the misting interval or the time of system operation.

• Root cuttings in peat pots for direct transplanting or directly into their sales container. This eliminates any chance of root disturbance.

• Transplant during the dormant season. The reduced metabolic rate of the cuttings reduces transplant shock.

• Hold transplants temporarily under high humidity. This allows roots to adjust to the new medium with no disturbance to the foliage.

• Remove the propagating frame from around the cuttings. The cuttings can then send roots into soil beneath the rooting medium and continue their growth undisturbed.