The division of plant mass is not very different from taking cuttings. Both methods of propagation are asexual, or vegetative rather than sexual, as is the case when propagating from seed. Two main benefits of asexual or vegetative propagation are: New plants are already mature, or are capable of reaching maturity much faster than when propagated from seed. Some plants such as hybrids will not reproduce true to parent stock when propagated from seed, in fact, some of these kinds of plants will not even produce viable seed. Asexual propagation is such cases is the only way to get new plant starts.
Propagating from cuttings involves the removal of a piece of the parent plant such as a branch, shoot, leaf or piece of root. Propagation be division, as described here, involves the separation of plants by cutting apart the root mass, clump, tuber, rhizome, bulb, or corm.
The most obvious method of propagating by dividing is to simply cut the root that links that links a parent plant from its offspring and start the new plant in another location where it develops on its own. Any kind of plant that sends out roots, runners or offshoots and produces new plantlets, can be divided in this way. It is important to allow the new plant, plantlet, or sucker to begin growing sufficient root mass of its own before severing the connection to the parent plant. Premature division is likely to kill the new plant if it cannot take in moisture and minerals form its own roots.
Many species of plants grow from bulbs or tubers. These plants multiply by growing additional bulbs or tubers to the side of the originals. In time the mass of plants will spread or grow thicker. In most cases, the thinning out of this mass is very beneficial and will allow for healthier continued growth of the original plants and the advantage to the gardener is the additional plant starts that can be obtained for free.
Many kinds of shrubs and trees grow from underground runners that send up new plant starts. They are divided by severing the connecting root from the parent stock at the appropriate time. Other kinds of plants are divided by separating their tubers or rhizomes: Banana, Elephant Ear, Canna, Caladium, Anemone, Tuberous Begonias, Bleeding Heart, Cyclamen, Dahlia, Gloxinia, Bird of Paradise, Calla Lily, Iris, Lily of the Valley, Mint, Rhubarb, Yams and Potatoes. Some of the plant types that are divided by separating the bulbs are: Amaryllis, Hyacinth and Grape Hyacinth, Iris, most kinds of lilies, Liriope, Crocus and Tulips. Corms are much like bulbs except that the corms can be divided with each section being capable of producing new plant starts. Among the kinds of plants that can be divided by separating the corms are: Crocus, Calchicum, Freesia, Garlic, and Gladiola. Other species are simply separated at their root mass, such as Hostas and Creeping Phlox. Cut through the root clump with a sharp knife and the division is complete. Further separation may be possible if individual plants are preferred over a new clump of plants.
The best season to divide nearly all plants, unless they are in bloom, is early to late spring. Summer is risky because of the intense heat. Division is possible during early to mid-autumn. This is riskier in the North. Cold weather will occur much later in the South which gives the new roots time to grow before the threat of frost. In any case, the division of plants should always be undertaken in the early morning or late afternoon hours, when the heat and sunlight are less intense. If a large quantity of plants is to be divided, it is important to have large containers of water present in which to keep the newly divided plants cool and moist before replanting in their new location or in containers. These plants should not be exposed to full, direct sunlight for several weeks or until the roots are well established. Some species such as most Hostas should not be exposed to full sunlight at all. Banana, Elephant Ear and Canna will thrive if full sun after only a week or two.
The depth for planting the new plant starts will vary from species to species. Generally, when the depth of the original plants is know, and those plants were thriving, the same depth should be adhered to when replanting. A generous watering will be appreciated by plants that have been recently divided. This should always be done during morning hours. Do not allow these plants to stand in water however, because that will deprive the plants of much needed oxygen. Water abundantly, then let water drain away for several days before watering again. If plants show signs of wilting and the soil is dry, water immediately. Wilting will also occur because of lack of oxygen. In this case the soil is likely to bet soaked or packed with clay which holds water and allows little oxygen to circulate. In this case do not water but get water to drain and loosen up the soil and add sand and mulch. The best types of soil and fertilizers will vary from species to species. Fertilizing or feeding should be resumed after roots are re-established, usually within a few weeks.
Adequate precautions should be taken for the protection of the roots during winter months. Depending on the species, a heavy mulch may be required. If new plants are to reside in containers for the winter, plants will require mulch or other protection from frost. Houseplants and all tropicals will need to be wintered indoors in northern climates.