Plant species are naturally endowed with the ability to regenerate themselves through self- or cross-pollination of their flowers to produce seeds. When physiologically mature, seeds germinate under optimum environments and generate new individual plants to perpetuate the parent plant.

Similarly, plant species whose seeds are not adequately viable to produce new plants and/or sterile (e.g. plantains) and depend primarily on the induction of vegetative sections (leaf, stem, root, flower stalks) containing viable buds are particularly more vigorous in asexual propagation of these plant species.

Seed propagation of crops is more ancient than asexual propagation and evolved with the origin of agricultural crop production in pre-historic times. Historically, human use of seeds marks the transition from nomadic food gathering to sedentary civilizations based on agriculture, in different parts of the world.

In recent times, technological advances have led to the development of micro-propagation, which involves the culturing of individual cells or groups of cells (tissues) under highly aseptic conditions to produce whole new disease- and insect-free plants.

Types of Propagation Methods

1. Sexual Propagation

This is a method of plant propagation involving the fusion of distinctly different sex cells (male, female) to produce a plant.

The fusion of sex cells allows the exchange of genetic materials leading to heterogeneity and the formation of hybrids, which vary widely in their appearance, physiological status, etc., and conform to the emerging plants’ diverse adaptation to varying environmental growth conditions.

2. Asexual (Vegetative) Propagation

This method involves the induction of a vegetative section or part of a living plant to form roots and subsequently, develop it into a whole new plant.

Plant multiplication does not involve the seed cycle (exchange of genetic materials) and therefore, it is the best way to maintain some species as clones; individuals identical to the parent.

Types of Propagating Materials (Propagules)

Methods of Crops Propagation and Factors Affecting Crop Production

1. Seed

‘Seed’ is the generative part of the plant used for propagation. A seed is a small immature plant (embryo) protected by a seed coat or testa, which is formed from the outer layers of the ovule after fertilization.

The seed is the basic unit of propagating many tropical crops, including yam and fruits (mango, pawpaw, passion fruit). Even in crop species whose primary mode of propagation is by vegetative means (e.g. mango, avocado pear), seed sowing constitutes an important method of regenerating new plants, and obviating the limitations (e.g. poor adaptation) of vegetative propagation.

Seeds are sown in three different ways, namely by broadcasting, drilling/row-seeding, and pocket drilling/ sowing in holes. Advantages of sexual propagation are ease of transportation of propagating materials, less cost, skill and work to raise seedlings, ease of vegetative propagation in mature plants, hybrid vigor, and associated benefits of the development of new varieties and wider adaptation to varying environments.

The disadvantages include slow seedling growth, non-prototype offsprings, the problem of ensuring uniform product quality since most seeds originate from cross-pollination (by wind, insects), weaker seedlings, and a longer period to plant maturity than vegetatively-propagated crop species.

2. Budding/Bud Grafting

A process consisting of the engrafting of the bud (scion) of a plant into the stem (stock) of another plant of the same genus. Generally, it is very suitable for propagating deciduous fruit (Citrus spp.) and shade trees. Budding of improved materials on regenerated coupons is one of the new methods of rehabilitating cacao.

In the most common T- budding pattern, the desired scion from a young, actively-growing shoot of a chosen crop variety is immediately slid into a T-shaped slit on the rootstock. The joined bud and rootstock are held by a winding of rubber- band/special tape/wrap which holds it until sealed, which prevents drying or contamination of grafted materials.

Chip budding is used for budding species whose barks do not “slip” (when cut, the bark easily lifts in one uniform layer from the underlying wood) easily without tearing. Bud grafting is faster, easier, and less messy than other forms of grafting discussed below. Bud grafting allows the production of plants identical to a parent plant.

Also, it may give increase the productivity of crops through the hardness, superior rooting capacity, drought tolerance, and insect or disease resistance of the rootstock.

However, the method is labor-intensive, requires great skill in nursery operations (and therefore, expensive), and can only be efficient when performed at very specific times when weather conditions and crop physiological growth status are optimum.

Also, the vascular cambium of both the bud scion and rootstock must be aligned to stimulate tissue growth on the basal ends before rooting.

3. Grafting

This is similar to budding in theory, but different in the sense that grafting involves the joining of the upper part (scion wood, 0.63-1.27 cm diameter and only with leaf buds) of one plant to the understock (rootstock) of another plant of the same species (clones, varieties).

Grafting is an old art and science of plant propagation in pears, citrus, mangoes, grapes, and other fruit trees, traceable back 4000 years to ancient China and Mesopotamia. Some plants graft naturally, where two branches are in close contact over several years (e.g. ivy).

Grafting allows gardeners to produce plants identical to a parent plant, allows growers to control the size and shape of a tree or shrub (e.g. apples), and gives more vigorous and earlier-fruiting plants.

Also, two varieties can be grown on the same tree to facilitate pollination (e.g. in apples). However, budding grafting is labor-intensive, expensive, inefficient in poor weather and plant growth conditions, and where cambiums of both scion wood and rootstock are not precisely aligned.

Other disadvantages are graft incompatibility, sucker production in grafted plants, and death of rootstock due to rooting of the scion arising from planting the graft union below the ground. There is a need to protect the grafted area from dislodging the scion out of alignment, especially by bracing. Also, there is a great risk of the top growth being very brittle thus, failing to harden off before cold weather.

9 Reasons for Budding and Grafting

The reasons for budding and grafting include the following;

1. Opportunity to change varieties or cultivars for crop improvement;

2. Optimising cross-pollination, especially in fruit trees that are not self-pollinating;

3. Advantage of particular (desired) rootstocks, especially in respect of superior growth habits, disease and insect resistance, and drought tolerance;

4. Benefit from interstocks, valuable in a situation of graft incompatibility;

5. To perpetuate clones by grafting onto seedling rootstocks when clones of plant species (e.g. conifers) cannot be economically reproduced from vegetative cuttings due to the low rooting percentage of cuttings

6. To produce certain plant forms e.g. weeping or cascading forms as in weeping hemlock (Tsuga Canadensis Carr var. pendula);

7. To repair damaged plants, arising from maintenance equipment, disease, rodents, or winter storms, through arching, approach grafting, or bridge grafting;

8. To increase the growth rate of seedlings, especially in the seedling progeny of many trees requiring 8-12 years to fruit with natural development; and

9. To facilitate virus indexing, through the confirmation of the presence or absence of the virus by grafting scions from the plant onto another plant that is highly susceptible and would quickly show symptoms of infection.

4. Layering

This involves bending a branch/part of the stem of a growing plant and anchoring (with a rock or peg) and burying a portion of it, to establish a new root system at the point of contact between the bent part and the earth (i.e. on the shoots that are still attached to the parent plant).

A light soil increases rooting success as will wounding or girdling of the buried portion. Treatment with a rooting hormone (e.g. Rootone, Hormondin^R, Hormonex^R) is most desirable, particularly one containing a fungicide. Plants with flexible branches are particularly suited to this method.

As soon as the new plant is established, the connection with the parent plant is severed and the new plant becomes independent. Layering is a good propagation choice when only a few plants are needed.

A heavy soil will reduce rooting success while covering the tip of the parent plant (bent shoot) kills it. Other types of layering are serpentine/compound layering, continuous/trench layering, and mound/stool layering.

5. Cuttings

A cutting is a vegetative plant part that is severed from the parent plant to regenerate the parent plant (by regaining lost tissues), thereby forming a new plant. Both woody and herbaceous plants are asexually propagated by cuttings of stems, leaves, and roots.

Cuttings regenerate new plants through the change of mature cells into meristematic cells that are found at rapid growth sites like buds. As in layering, the use of rooting hormone as a dip, preferably one containing a fungicide, helps to hasten to root, increase the number of roots, or give uniform rooting, except on soft fleshy stems.

The rooting medium may be coarse sand, vermiculite, soil, water, or a mixture of peat and perlite. A major advantage of cuttings is the practical regeneration of whole plants from actively-growing plant parts/organs.

The cutting technique also, typically as with other asexual methods of plant propagation, produces several whole new plants, which are genetically identical clones of the parent plant.

Important disadvantages are that cuttings should be made as soon as possible after the collection of plant material; not all species of plants can be propagated from cuttings (e.g. Acacia spp.); cuttings must be shielded from direct sunlight, especially if they are under glass or plastic; stock plants (plants used for asexual propagation) should be healthy and well-branched as should the tools and conditions for preparing cuttings to ensure healthy new clones; choice of correct rooting medium to achieve optimum rooting within the shortest possible time.

There are many types of cuttings. Based on the vegetative part of the plant providing the cutting material. There are:

6. Stem Cuttings

This technique is the most commonly used method of propagating many woody ornamental plants and houseplants; sweet potato, sugar cane,, and cassava are food crops propagated from stem cuttings.

Stem cuttings of many favorite shrubs are quite easy to root whereas those of a tree species are more difficult to root. A glasshouse is not necessary for successful propagation by stem cuttings but it is critical to maintaining high humidity around the cutting.

Facilities for rooting cuttings include flower pots, trays, a small hoop frame, and/or an intermittent mist system. Materials for making stem cuttings should be vigorous, have new growth with no flower buds, and be free of diseases and insects.

Cuttings should be 5.08-10.16 cm long, cut from older stems, and have 2-3 leaves (2-3 nodes) attached. Dipping the base of the stem, including the node area, into a rooting powder stimulates rooting.

The stem should, however, be dry when dipped. Four main types of stem cuttings are identifiable based on the growth stage of the stock plant, which is very critical in the rooting of cuttings, namely:

1. Herbaceous cuttings- cuttings taken from non-woody plants, such as perennials and houseplants e.g. Chrysanthemums, rose. Cuttings are 5.08-15.24 cm stem pieces, with a terminal bud.

2. Softwood cuttings- cuttings are taken from soft, succulent, new growth from non-woody stock plants before the new growth starts to harden (mature). The cuttings are used to propagate flowering shrubs.

They must be taken after rain or water is required to keep them cool in the morning. The larger diagonal cut gives more area to develop roots. Cuttings should be kept in water before rooting.

3. Semi-hardwood cuttings- cuttings prepared from partially mature (firm) wood of the current season’s growth, just after a flush of growth. The method is used for propagating many broadleaf evergreen shrubs, some conifers, holly, rose and cacao (using the tips of shoots).

4. Hardwood cuttings- cuttings taken from tissue that has become woody (firm) and the plant is mature and dormant with no signs of active growth. Several cuttings can be made from the same branch of some shrubs.

Basal cuts should be just below a node, while the uppercut should be slightly above a bud. Cuttings should be kept moist until rooting. The system is most often used for deciduous shrubs and many evergreen species e.g. grape, fig, and rose. The three types of hardwood cuttings are straight, mallet, and heel cuttings.

7. Leaf Cuttings

Leaf cuttings are used almost exclusively for propagating a few indoor plants. Leaf cuttings do not include an axillary bud, and thus, can only be used for propagating plants that are capable of forming adventitious buds.

The method involves the use of a healthy leaf blade or leaf without petiole in propagating new plants, following the same procedures as for stem cuttings, particularly treating leaf cuttings with growth hormones to stimulate rooting and quick bud development.

There are several types of leaf cuttings, and for all of them, the old leaf is not part of the new plant and is thus, usually discarded. In most cases, the old leaf provides the energy food source for nurturing the newly-generated plant e.g. Bryophyllum pinnatum.

Whole leaf with petiole-This involves a whole leaf with about 3.81 cm of the petiole. The lower end of the petiole is dipped into a rooting medium after which one or more new plants form at the base of the petiole. The old petiole may be reused after the new plants have formed their own roots. African violets and peperomia are propagated in this way.

Whole leaf without petiole- This method is used for propagated plants with sessile (petiole-less) thick, fleshy leaves. The leaf is inserted vertically into the medium after which one or more new plants will form from the auxiliary bud. The leaf may be removed after the plant forms its own roots.

Split-vein- The veins on the lower surface of a leaf from the stock plant (e.g. Begonia and snake plant) are slit before the leaf-cutting is laid on the medium. The rooting medium is used to hold down the margins of a curling leaf.

A variation of this method involves inserting leaf wedges cut with at least one main vein into the medium with the main vein partially covered. In both cases, new plants form from the base of the split vein and leaf wedge.

Leaf-bud cuttings- These are used for many trailing vines and when space or cutting material is limited. Each node on a stem can be treated as a cutting. A leaf-bud cutting consists of a leaf blade, a petiole, and a short piece of stem with an attached axillary bud.

The cuttings are placed in the rooting medium with the bud covered 1.27-2.54 cm and the leaf exposed. Rubber plant, Camellia, Rhododendron, and blackberry are propagated using this method.

Flower stalks- This follows the same technique as leaf cuttings. It involves plant propagation from a flower stalk, usually with large leaf ears devoid of flower buds. This enhances chimera production in African violet.

8. Root Cuttings

This involves the propagation of plants from a section of a root. In some species, the root cuttings produce new shoots which subsequently form their root system whereas, in others, root cuttings develop a root system before producing new shoots. Plants propagated from root cuttings include blackberry and rose.

In most cases, root cuttings of woody plants are usually taken during the dormant (inactive growing period) season when roots have large carbohydrate levels. Root cuttings can also be taken from actively-growing plants i.e. throughout the growing season.

In plants with large roots that are normally propagated outdoors in a hotbed, the root cuttings should be 5.08-15.24 cm in length, with a straight cut at the proximal end and a slanted cut at the distal end of the rooted cutting.

In plants with small roots, the root cuttings are 2.54-5.08 cm in length and are laid horizontally about 1.27 cm below the soil or sand in a flat. The flat is then placed under shade, which is removed after new shoots appear.

9. Divisions

Divisions are segments produced by cutting or breaking a crown or clump of suckers. Suckers are aerial stems formed from adventitious buds.

Each segment consists of a bud and some roots, which when replanted grows into a new plant vertical to the parent plant. In cocoyam and taro, plants are propagated from young shoots. Pears and raspberries are propagated by suckers.

10. Bulbs and Corms

Bulbs are specialized underground stems whose leaves are used as food-storage organs. The fleshy stem part is usually very short (compressed) and attached to a basal plate while the fleshy leaves (bud scales) protect the terminal bud, which eventually grows into a new plant under an appropriate favorable) environment.

Bulbs can be propagated by removing small bulblets (young bulblets) or offsets (mature bulblets/large buds as in lilies) that form at the base of the parent bulb. The small bulbs mature into plants that produce flowers in 2-3 years.

In other crops such as grape hyacinth (Muscari spp.), scooping (complete removal of the basal plate) and scoring (making three knife cuts across the base of the bulb enough to destroy the main shoot) removes apical dominance and encourage bulblet formation, more than from offset propagation.

Also, in daffodil (Narcissus spp.) bulblets form from bulb clipping and twin scaling, where respectively, bulblets develop from the basal plate between scale leaves and at the edge of the basal plate.

A corm is the swollen base of a stem enclosed by dry scale-like leaves. It is a solid stem structure with nodes and internodes. The natural increase of new corms and cormels (miniature corms) in the field enhances corm propagation. The same procedure of propagating bulbs applies to corms, corm bits, and cormels. Both methods are used in gladiolus, lilies, onion, plantain, and bananas.

In bananas, corm bits are better sources of planting material because corm bits uniquely assure clean planting material free of nematodes and weevils, easily prevent the transfer of banana weevils and nematodes from infested bananas to new plantations, provide enough planting materials from the few available scarce suckers, are easier to treat with Furadan (nematicide/insecticide mixture) than suckers, and are easier to transport for planting than suckers.

11. Runners/stolons

These are the lateral stems or vines of crops such as grass species and sweet potatoes, which run and grow horizontally on the soil surface to produce nodal adventitious roots and subsequently plantlets.

The organs are cut into smaller sections, each with one or more buds, and partially buried in the ground to produce new plants. Examples of plants propagated through these organs are strawberries and yarrow.

12. Tubers

The “tuber” is a specialized kind of swollen, modified, and compressed stem structure that functions as an underground food storage organ developed from either the base of the stem (stem tuber) or the root (root tuber) of a plant.

The tubers of root crops such as yam and Irish potato are sliced into setts from which axillary buds (“eyes”) sprout into new plants or by planting whole tubers.

In Nigeria for instance, setts are treated with pesticide dip containing 100 g Mancoceb + 70 ml Basudin in 10 litres of water to ensure healthy seed yams at harvest.

Sweet potato is propagated from the tuberous roots (swollen secondary roots modified for food storage) which are capable of producing shoots at the proximal end and new roots at the distal end. The tuberous root propagation in dahlia is achieved by crown division.

13. Suckers

A sucker is a branch of the parent plant that will occasionally appear in a leaf axil of the plant. Propagation can be achieved by cutting the suckers from the parent plant and rooting in a rooting medium, e.g. African violet. Some trees and roots have shallow roots which produce separate plants called suckers.

Cutting through the roots around the suckers helps to separate the suckers, more easily with smaller suckers about 60 cm. The advantages of suckers compared to corm bits are ease of preparing suckers for planting and ease of identifying sucker qualities (bad, good) through observations of the parent plant.

Suckers are best planted at the onset of the rainy season to allow them access to sufficient and prolonged water supply. In bananas, young plants or offsprings (side shoots/suckers) are produced by a mature plant, namely water sucker (a weak side shoot with wide leaves, a surface runner), sword sucker (a side shoot with narrow pale sword-shaped leaves), maiden sucker (a large sucker with wide leaves, that has not yet flowered) and peepers (a young shoot with scale leaves). Sword suckers are the only suckers suitable for propagating bananas.

14. Rhizomes

Rhizomes are horizontal stems running at or just below the ground surface, specially modified for food storage organs. Unlike roots, rhizomes have nodes and internodes, with the nodes containing growing points (“eyes”).

When cut into smaller sections or segments containing one or more viable buds and scale leaves, the buds sprout into new plants. Ginger, iris, couch grass, and strawberry are propagated from rhizomes.

15. Micro propagation or Tissue Culture

This modern technique of plant propagation is based on the principle that each plant cell has the potential to grow into a new plant exactly like the parent plant. In this method, an individual or small group of plant cells (tiny pieces of bud, leaf, and stem) are manipulated in a way to enable them to produce a new plant.

Mass propagation of sweet potato tubers is achieved by in vitro culture of nodal segments in MS medium containing 9% sucrose under continuous darkness using the Jar Fermentor Technique. Begonia and roses are also propagated by tissue culture using the meristem-tip. The advantages of this method are the speed and efficiency of plant propagation and the production of disease-free (aseptic) plants.

Disadvantages include spontaneous natural mutations and very exacting conditions for growing tissue culture materials, such as absolute sterile conditions, strict control of temperature, light, humidity, and atmosphere with costly electronic sensors and computer equipment.

Methods of Crops Propagation and Factors Affecting Crop Production