Asexual or Vegetative Propagation(Meaning and Methods)
Asexual, or Vegetative Propagation, involves the use of vegetable parts of crops as planting materials. Plant parts that can be used are stems, roots, and leaves.
Asexual propagation is suitable for the cultivation of crops that do not produce seeds.
Examples of crops that can be propagated vegetatively are cassava, yam, potato, banana, breadfruit, cocoyam, onion, pineapple, etc.
Methods of Vegetative Propagation:
There are two main types of vegetative propagation namely:
1. Natural method of vegetative propagation.
2. Artificial method of vegetative propagation.
Natural Vegetative Propagation:
Many plants produce propagation parts naturally. These parts can be used to produce new plants.
The methods vary and include the following:
a. Suckers: These are short underground horizontal branches ending in terminal buds. They are modified stems that produce natural branches. Examples of crops, that can be propagated by suckers, are pineapple, banana, and plantain.
b. Bulbs: These crops store food in their thick fleshy inner leaves, and have short, underground stems from which adventitious roots grow out.
The leaves are covered by auxiliary buds, and at the centre, the central bud.
The major function of bulbs is food storage and vegetative propagation.
Common examples of bulbs are onion and garlic.
c. Rhizomes: These crops grow horizontally, below the soil surface. They are underground stems with nodes along the stem, from which lateral buds and roots grow. New plants develop from such buds.
Examples of crops, that can be propagated by rhizomes, are ginger and some grasses like spear grass.
d. Corm: These crops store their food in the stem which becomes very swollen. The corm develops into a plant by the terminal bud. An example of a good corm is cocoyam.
e. Runners: These crops grow horizontally, above the soil surface. They are too weak to stand upright or erect.
The new plants arise from the nodes and adventitious roots found in runners. Good examples of a runner are sweet potato and carpet grass.
f. Stem Tubers: These crops are swollen underground stems, with buds and tiny scale leaves. They store food in form of starch. In stem tubers, a new plant can be produced when a portion of the tuber, containing the “eye” is planted. A whole tuber can also be planted, or cut into setts, examples include yam and Irish potato.
g. Root Tubers: These crops are swollen fibrous roots. They store food in the form of starch. Good examples of root tubers are cassava, sweet potato, etc.
Artificial Vegetative Propagation:
This method involves the use of a cut portion, from the vegetative body, of the older parent plant. The aim of this method is to multiply crops, where new plants are grown from cut portions of a plant. The cut portions of the parent plant may be a stem or a union of a bud with a rootstock. Artificial vegetative propagation can be carried out by:
a. Cutting: This is a process when a part of a plant is cut into portions, and planted to produce new plants.
Cutting may be from stems, leaves, or roots.
Cuttings are green plants of stems or wood, which develop into new plants. The lower cut end of the stem is usually treated with rooting hormone, to promote root growth, the cutting is then planted in moist soil. In some plants, such as Bryophylum, cuttings can be from the leaves.
Sweet potato, cassava, and cocoa are propagated by stem cutting, while breadfruit cuttings are from the roots.
b. Layering: In this method, roots are made to grow on one of the branch stems of a growing plant. When the roots are established, the branch is then cut and planted in moistened soil, to continue its own life.
Layering is used to produce new plants of cocoa and coffee.
There are two types of layering;
1. Mound Layering: Here, a branch of stem growing near the ground is bent over, so that one or two of the nodes could touch the ground, and the node is then covered with soil.
Some good examples of crops that are propagated by mound layering are tomatoes and cocoa.
2. Aerial Layering or Marcotting: Air layering is a method of propagating new trees and shrubs, from stems still attached to the parent plant. The stem is wrapped with damp moss to encourage roots to form.
In this method, the bark of a healthy branch is removed in a ring form, up to the cambium region.
This method is used to propagate garden shrubs and fruit trees like mango and lemon.
c. Budding: This is the union of a bud with a stock. It is the act of uniting a bud from one plant with the stock of another plant, of the same or different species. The bud is the scion, while a plant providing a well-established root system of another plant is called the stock.
A piece of stem bearing a bud is cut from one plant, using the inverted T method. A T-shaped cut is then made at the bark of the stock plant, and the bud is fitted, tied with raffia, and sealed with wax, to prevent attack by pathogenic organisms.
The plant that receives the bud is called the stock. The bud and stock are thus united.
The crops propagated by budding are; cocoa, citrus, and rubber.
d. Grafting: This process involves the joining of a scion of one plant, to the stock of another plant, from the same species.
The plant whose root still remains in the soil is called the stock, while the top shoot is called the scion.
The shoot of a rootstock is cut off with a clean slanting or v-shaped cut. The scion is prepared with a clean slanting cut. The two surfaces are laid together, and the graft is bound with wax or tape.
Budding and grafting are special methods, they are very important in agriculture. They are used to propagate many fruit trees.
In grafting, the scion contains more than one bud, while in budding only one bud is used as the scion.
The grafting operation is similar to that of budding.