Vegetative propagations or asexual reproduction are frequently used in horticulture or agriculture to propagate economically valuable plants and cash crops, especially the perennial species that produce sterile or no seeds. Asexual reproduction does not involve the union of gametes, production of seeds or spores, and changes in the number of chromosomes, which often used single cells (algae), tissues, organs, or specialized organs that undergo structural modifications. Regeneration through somatic cells, tissues, or organs is also classified as vegetative propagation. For example, micropropagation in the plant tissue culture, which is the biotechnology that is applied to mass propagate the clones asexually.
There are several reasons the plants “choose” vegetative propagation. First, some plants do not produce seeds, or the number of seeds is few. There are homologous hybrid seeds during the meiosis of gamete formation, which results in abnormal seeds or hybrid seeds with odd ploidy that are not viable. Also, propagation through seeds is time-consuming which involves a long dormancy period, whereas vegetative propagation offers a faster and genetically stable way to produce the propagules. Moreover, plants produced through vegetative propagation have no prop root which makes them more suitable for pot plants, indoor growing, or bonsai. Also, fruit plants propagated via vegetative methods set the fruit timing earlier and more controllable than those propagated from seeds, which is an advantage in the fruit industry. Grafted plants are more resistant to harmful pests and diseases, more robust that offer better quality and quantity in yield as compared to seedlings because they allowed identical plants to be produced. In dioecious plants, grafted plants can reduce losses by selectively performing grafting on the plant at specific “gender”.
Splitting is the easiest way to propagate the plants vegetatively as the new growths have a well-developed rooting system. It is sometimes referred to as cutting as there are some parts of the new offsets that remain in contact with the mother plant. Plants with modified, underground stem-like rhizomes often produce new buds available for splitting to produce the offspring—for example, ginger. Alternatively, above-ground stem modification of the weedy species produced runner, stolon, or offset, which is a modified root that develops into a new plant (strawberry). The underground splitting can be observed on tubers like potato, bulbil such as onion, and corm that can be water chestnut. Mechanical separation of the new growth and the mother plants using hands or knives is used to propagate the specialized stems and roots. Ornamental plants like aroids, ferns, orchids, and palms are commonly propagated by using splitting. The banana that produced offspring through the sucker is separated via splitting. Plants with new growth usually have an abscission region that allowed mechanical separation to be done with minimal damage.
Apart from that, cutting, layering, air-layering, and grafting are commonly used to propagate the plants, especially woody plants. To promote root production for higher survival rate, plant growth regulator such as auxins like indole-butyric acid is used to induce rooting. Stem cutting can be performed onto a branch, twig, or main stem at different degrees of hardness such as hard stem (rose), intermediate hardness (carnation), and soft stem (Magnolia sp.). Subsequently, leaf-cutting can be used for certain plants with a high regenerative rate like Begonia and Sansevieria. The leaves are cut and buried vertically into the medium, then kept in the shade and high humidity. Root cutting is less preferred as it is limited to some species like Casuarina. Cutting is favored in succulent propagation.
Layering and air-layering are better options than cuttings. Because cutting with the application of rooting powder has an inconsistent rate of root induction. Layering is done by bending down the stem of the plant towards the ground and followed by covering it with soil or substrate. The stem produces the roots and new shoots gradually. There are several ways to make layering: tip layering, simple layering, trench layering, serpentine layering, and mount layering. Tip layering covers the tip of the plant with soil, and then the new growth is developed. It can be used onto the fern of Adiantum sp. Crops like apple trees used simple layering. Simple layering and trench layering are somehow similar whereby the branch is covered with soil and new shoots are produced. For trench layering, the regenerated shoots are found more than that of simple layering and commonly applied on grapes. Serpentine layering can be sued on rose and grape by inserting several nodes into the soil while the internodal segments are exposed to the air. Then, mount layering is done by adding more substrate or soil at the base of the stem until it covers the new shoots. Therefore, the shoots produce the roots gradually.
Air-layering is performed by covering the shoots not onto the ground but “in the air”. It is commonly done by removing the peel of the chosen stem circularly. After making sure that the bark and its remnant are completely peeled, it is wetted, and rooting powder is applied. It is then covered with soil or a rooting medium such as coconut husk. Finally, the medium is wrapped or enclosed with plastic film and then tie with gardening wires. Aluminum foil is used as an additional cover to shade the medium and reflect the heat energy from the sun. Air-layering is widely used on woody crops like lemon and apple trees as well as bonsai.
