PROPAGATION OF CROPS

Meaning of propagation

Propagation is the process of raising young plants from seeds or by vegetative parts.

Methods of crop propagation

There are two main methods of crop propagation. Viz:

1. Sexual propagation
2. Asexual propagation

Sexual or seed propagation

Sexual propagation is the development of crop plant through the sowing of seeds. Crops which can be propagated by this method are tomato, okro, onion, oil palm, coconut, maize, rice, cowpea, etc. the seeds of these crop may be planted directly in the field e.g. rice or firstly in a nursery before transplanting to the field e.g. oil palm, cocoa, tomato.

Advantages of sexual propagation

1. It is easier to increase crop population by using this method.
2. Sexual method is more convenient because seeds are easy to handle (that is, easy to carry, store, procure and sow).
3. Sexual propagation enhances crop improvement through selection and breeding to generate improved varieties of crops.

Disadvantages of sexual propagation

1. The crop plants that are seedless cannot be used for propagation.
2. The gestation period of some crops is very long. It therefore takes time for such crops to mature and fruit if their seeds are planted.
3. Pests normally eat up some seeds in the soil during the process of germination.

Qualities of seeds

1. The seeds should be matured.
2. The seeds should be well dried.
3. Large seeds should be used as they give good yields.
4. They also have high vigour.
5. The seeds should be free from holes as they will no longer be viable.
6. Seeds should not be stored for long time as they may not germinate.

Seed treatment

Before sowing, seeds should be treated. Methods used include:

1. Chemical dressing of seeds: Seeds are treated with appropriate chemical to prevent disease organisms and pests.
2. Pre-sack-the-seed: Here, hard seeds are soaked in water to facilitate germination.
3. Scarification: This is the process whereby the surfaces of seeds are loosened to enhance emergence of plumule and radicle.

Asexual or vegetative propagation

Asexual or vegetative propagation is the propagation of crop plants using the vegetative parts of the parent plant. For example, root, stem and leaves. Crops that can be sown in this way include cassava, rubber, pineapple, sugar cane and banana.

Natural methods of vegetative propagation

Some crops plants are naturally adapted to vegetative propagation and are easily grown by this method.

The following are the commonest portions of the plant used for the purpose of natural propagation.

1. Rhizomes: These are underground stems which can be cut into sections and replanted e.g. gingers and canna.
2. Suckers: Many plants produce offshoots, complete with roots, which can be severed from parent plant and replanted elsewhere e.g. banana and pineapple.
3. Bulbs and corns: These are swollen portions of stem (corns) and scale or foliage leaves (bulbs) which are produced by certain plants, e.g. Onion (bulb), cocoyam (corm).
4. Runners: These are produced by the parent plant and are modified stems which travel over the soil and produce rootsat intervals e.g. banana, straw berries.

Horticultural methods of vegetative propagation

Four commonly used horticultural methods of asexual propagation are:

1. Cuttings
2. Layering
3. Grafting
4. Budding

Cuttings: This is a method in which parts of the parent plant are detached and planted in the soil so as to grow. A gardener can make use of the root e.g. apply by using its suckers or stem as in cassava or the leaf as water leaf plant.

The length of each cutting varies from 5cm-30cm depending on the type of crop plant being propagated.

When to make cutting: Cuttings will root better or most easily if they are taken during the rains, but they also be taken during dry season provided they are well watered and shaded while they are rooting.

Layering: Layering is the propagation of plants from parts of the stem by which the stems are allowed to develop roots before cutting them off from the parent plant.

Procedure involved in layering:

1. Select a branch of ripened wood which can be easily bent down to reach the soil without breaking.
2. Cut the branch half way through with a sharp knife immediately below one of the leaf buds, passing the knife along the cut to split the branch for a distance of 2cm -5cm.
3. The slit portion is called the tongue and this may be kept open by inserting a small pebble into the incision.
4. Bend the branch down to ground level and remove the soil to a depth of 5cm -8cm at a point in line with the cut.
5. The prepared portion of stem is then lowered into the depression in the soil and secured in position by a forked peg
6. The stem portion is then covered with a mixture of sand and leaf mold.
7. The layered stem must be kept shaded and moist. in layering, a rooting hormone compound may be inserted in the cut section to induce root formation.

Grafting: Grafting is placing together two cut surfaces of the same or of different closely related plants in such a way as to cause them to unit and grow together. The plant onto which this graft is inserted is called the stock. The plant material inserted on to the stock is called the scion.

Selection of a suitable stock is very important for successful grafting.

Budding: This is a form of propagation which involves taking an “eye” or bud with a small portion of bark attached from a young branch, and inserting it in a corresponding cut made in the stem or branch of another plant. Budding is commonly use in citrus. During budding, inverted T or a T-shaped head cut is made at 45cm from the ground on the stem of the stock plant. The T-shaped cut is slightly raised so as to expose the cambium. The bud is then carefully inserted into the raised bark and pressed firmly in order to unite together the cambia of both bud and stock. This is immediately tied with a plastic material to keep the stock and scion in close contact until union is completed. It is advisable to prevent air and water from the cut until the bud as “heal” or “taken” together with the shock. This is evident when the bud becomes greenish. The bud later shoots in about two weeks and as it becomes pronounced, the cut of the stock above the bud is removed. It is recommended that the cut surface should be painted in order to arrest bacterial or fungi infection.

Advantages of budding and grafting

1. Stocks of valuable varieties of trees may be raised when other methods fail.
2. Crop plants with uniform qualities are produced.
3. Fruit trees can be multiplied and brought into bearing early.
4. Damaged parts of a crop plant can be replaced by grafting.

Marcotting: This is the propagation method in which the bark of a branch or crop plant is peeled off with a sharp knife, up to a considerable length of about 5cm long.

The peeled section is covered and tied with organic manure. This is watered until roots are generated. When the roots become fully established, the branch is cut of the plant and the rooted branch is sown to give rise to a new crop plant.

This method is suitable only for plants whose roots do not produce roots with ease. Examples include mango, lemon and shrubs.

Advantages of asexual propagation

1. Crop plants have uniform growth rate.
2. Production of new plants is quicker using this method e.g. flowers etc.
3. It is cheaper and more economical than seed propagation.
4. The crops have high vigour.
5. Crop plants can withstand adverse soil and weather.

Disadvantages of asexual propagation

1. Cuttings cannot be stored till the next planting season.
2. Spread of diseases from parent plants to offspring is possible.
3. Budding and grafting require special skill.
4. Most crop plants produced by asexual propagation do not enhance or support pollination.