Plant propagation



Plant propagation is the process of creating new plants from a variety of sources: seeds, cuttings, bulbs and other plant parts. Plant propagation can also refer to the artificial or natural dispersal of plants. Plant propagation is often done in a nursery before being planted out where required.

Sexual propagation (seed)
Seeds and spores can be used for reproduction (through e.g. sowing). Seeds are typically produced from sexual reproduction within a species, because genetic recombination has occurred. A plant grown from seeds may have different characteristics from its parents. Some species produce seeds that require special conditions to germinate, such as cold treatment. The seeds of many Australian plants and plants from southern Africa and the American west require smoke or fire to germinate. Some plant species, including many trees do not produce seeds until they reach maturity, which may take many years. Seeds can be difficult to acquire and some plants do not produce seed at all.

Asexual propagation
Plants have a number of mechanisms for asexual or vegetative reproduction. Some of these have been taken advantage of by horticulturists and gardeners to multiply or clone plants rapidly. People also use methods that plants do not use, such as tissue culture and grafting. Plants are produced using material from a single parent and as such there is no exchange of genetic material, therefore vegetative propagation methods almost always produce plants that are identical to the parent. Vegetative reproduction uses plants parts such as roots, stems and leaves. In some plants seeds can be produced without fertilization and the seeds contain only the genetic material of the parent plant. Therefore, propagation via asexual seeds or apomixis is asexual reproduction but not vegetative propagation.



Techniques for vegetative propagation include:
 * Air or ground layering
 * Division
 * Grafting and bud grafting, widely used in fruit tree propagation
 * Micropropagation
 * Stolons or runners
 * Storage organs such as bulbs, corms, tubers and rhizomes
 * Cloning, striking, or cuttings
 * Twin-scaling

Propagation Environments
Many environmental factors influence growth and production of plants. The primary processes affected by environmental factors are photosynthesis and transpiration. To maximize the photosynthesis necessary for plant growth, growers must reduce the factors that limit photosynthesis or increase the factors that promote photosynthesis.

Photosynthesis, and therefore growth, can be limited by factors associated with the growing medium, other organisms, or the atmosphere. Limiting factors related to growing media include mineral nutrients and water (generally supplied by irrigation). Photosynthesis can be limited by the absence of beneficial organisms or by the presence of harmful organisms. Atmospheric factors that affect photosynthesis include light, temperature, relative humidity, and carbon dioxide levels. Propagation structures, or environments, are any area that is modified to encourage the growth of plants by controlling these atmospheric factors during all phases of growth, usually in a nursery.

Nursery facilities vary considerably in their complexity. Propagation environments can be as simple as a garden plot where water and fertilizer are applied, or as complex as high-tech greenhouses that also modify all atmospheric factors.

Minimally controlled propagation environments
A minimally controlled environment is the simplest and least expensive of all types of propagation environments. The most common type is an open growing compound. It consists of an area where plants are exposed to full sunlight and is usually nothing more than an irrigation system and a surrounding fence. Nurseries use open compounds for plant propagation and also to expose crops previously grown inside structures to ambient conditions during hardening. Plants can be grown on elevated platforms, benches, or pallets to improve air pruning of the roots, or directly on a layer of gravel (to provide drainage) that is covered with landscape fabric (to control weeds). Irrigation is provided by sprinklers for smaller containers or driplines for larger ones; plants obtain nutrients from controlled-release fertilizers that are incorporated into the growing media. The compound needs to be fenced to minimize animal damage, and, in windy areas, a shelterbelt of trees around the compound can protect from desiccation and improve irrigation coverage.

Semicontrolled propagation environments
In semicontrolled propagation environments, only a few of the limiting factors in the ambient environment are modified. They comprise a wide variety of growing structures ranging from simple cold frames to shadehouses.

Cold Frames
Cold frames are low-to-the-ground structures consisting of a wood or metal frame with a transparent covering. As their name suggests, they have no heating source except for the sun. Cold frames are the most inexpensive propagation structure and are easy to build and maintain. Because conditions inside can stay relatively warm and moist, cold frames can be used for seed germination or rooting cuttings. They can also be used to protect seedlings and cuttings from heavy rains and wind.

The ideal location for a cold frame is an area with a slight slope to ensure good drainage. A sheltered spot against the wall of a building or greenhouse provides additional protection. Some nurseries sink the floor of the cold frame 15 to 30 cm into the ground to use the earth for insulation. Other nurseries make their cold frames lightweight enough to be portable so they can move them from one section of the nursery to another.

It is relatively easy to build a cold frame. Frames may be made using a variety of materials, including wood (preferably decay resistant); recycled plastic lumber also works well. Never use creosote-treated wood or wood treated with pentachlorophenol because these substances are toxic to plants. The cold frame needs to be built so that it is weather-tight and so the top can be opened partially or fully to allow for various levels of ventilation, watering, and the easy removal of plants. The cover must be able, however, to be attached securely to the frame to resist wind gusts. Heavy plastic film is an inexpensive covering but usually lasts only a single season. Hard plastic or polycarbonate panels are more durable and will last for several years. Cold frame kits may also be purchased and are easily assembled; some kits even contain automatic ventilation equipment. Cold frames can be labor-intensive because they need to be opened and closed daily to manage temperature and humidity levels. A thermometer that can be conveniently read without opening the cover is mandatory. In a cold frame, plants grow best at 65 to 85 °F (18 to 29 °C). If air temperature goes above 85 °F (29 °C), the top must be opened to allow ventilation. In the tropics, cold frames usually need to have shadecloth suspended above them to help moderate temperatures.

Heated propagator
A heated propagator is a horticultural device to maintain a warm and damp environment for seeds and cuttings to grow in.

This can be in the form of a clear enclosed bin sitting over a hotpad, or even a portable heater pointed at the bin. The key is to keep the moisture in the clear bin, while keeping lighting over the top of it, usually.

Seed propagation mat
An electric seed-propagation mat is a heated rubber mat covered by a metal cage which is used in gardening. The mats are made so that planters containing seedlings can be placed on top of the metal cage without the risk of starting a fire. In extreme cold, gardeners place a loose plastic cover over the planters/mats which creates a sort of miniature greenhouse. The constant and predictable heat allows people to garden in the winter months when the weather is generally too cold for seedlings to survive naturally. When combined with a lighting system, many plants can be grown indoors using these mats.