Plants can be propagated sexuality by seed or asexuality by techniques that include cutting, grafting, and layering, and by the use of plant structures such as runners, suckers, crowns, bulbs, corms, rhizomes, and so on. Micropropagation involves the use of tissue culture.
Propagation of varieties and cultivars by seed requires careful control of the seed source. The genetic complement of the seed must remain the same as that of previous generations of seed. Most agronomic (field) crops have seed certification programs. In the case of horticultural crops (vegetables and ornamentals), seed genetic quality is maintained by the seed companies producing the seed and embryo. After harvesting, the seeds are stored in the appropriate environment (usually dry and cold) until planted. Before planting, the stored seeds are sampled and tested for viability (the ability to germinate) by a test such as cut, float, X-ray, germination, tetrazolium, or excited embryo.
In many species at the time of seed maturation and harvest the embryo is dormant and requires some change in environmental factor to break dormancy. Stratification (exposure to cold temperatures) will be break dormancy of some seeds. Other seeds require soaking or leaching to remove a chemical inhibitor present in the seed.
Seed germination is a series of events that include imbibition of water, activation of hormones and enzymes, then embryo growth and development. Environmental factors that affect seed germination include water, temperature, aeration, light (for some seeds), pathogens, and salts.
Asexual propagation involves reproducing an entire plant from a part of another plant. Except in some cases such as mutation or chimera, the new plant has exactly the same genetic makeup as the first plant. The technique is very useful when genetic integrity cannot be maintained by seed production or when seed production is inefficient.
Cutting production involves removing a part of a stem or root of a plant and regeneering the missing organs and tissues by providing an environment that favors the regeneration process.
Grafting is the attaching of a twig called the scion from a plant with one genetic complement. The scion and rootstock must be closely enough related for there to be grafting compatibility. There are several different kinds of grafts, including whip and cleft. Budding is similar to grafting except the scion is a bud.
Layering is similar to cutting except the part to be propagated is not removed from the parent plant until the missing stem or roots have generated. Layering can be simple, mound, or air.
Other plant structures that can be used for propagation are planters that can be separated from the parent plant or crowns, bulbs, corms, and tubers that can be divided. Plantlets can come from runners, which extend from the main portion of the plant or form naturally in some species along leaf margins and at leaf bases.