Blackberries (Rubus fruticosus) are fruit-bearing brambles that are hardy in U.S. Department of Agriculture plant hardiness zones 3 through 9 and most often propagated asexually. They can be grown from seeds, but the seeds are relatively difficult to germinate, and the plants grown from seeds do not always have the desirable traits of their parent plants.
Vegetative propagation techniques using cuttings or new growth from an existing blackberry plant, however, produce clones of that parent plant, and the clones usually duplicate the traits of the parent, including its fruit traits.
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To start blackberry plants from stem cuttings, cut 4- to 6-inch-long pieces of stem from the end of a blackberry plant's young, flexible stems. Ensure each piece of cut stem, or stem cutting, contains several leaf buds, but remove all dead leaves and offshoot stems from the stem cutting. Encourage root development by dipping the cut end of each stem cutting in rooting hormone.
Plant each stem cutting -- with its cut end pointing downward -- 2 inches deep in potting soil that contains perlite and is in a pot that has bottom drainage holes. Use one pot per stem cutting, and place the pots in a sunny location. Keep the potting soil moist but not saturated, and mist the cuttings on a regular basis with water from a spray bottle for the first two to four weeks after planting. Keep the new plants potted for up to one year, and then transplant them into an outdoor garden in fall or early winter.
To propagate from root cuttings, dig up roots from around blackberry plants; digging a furrow about 1 foot from an established row of plants is a simple way to access the roots. Cut 4- to 6-inch-long pieces of roots that are 1/4 to 1/2 inch in diameter.
The root cuttings can be replanted at a depth of 2 to 4 inches directly in another area of the garden. Alternatively, they can be planted in a pot filled with potting soil containing perlite to establish new plants for later transplanting; place them 2 to 4 inches deep in the potting soil. Root cuttings also can be stored in a plastic bag in a refrigerator for later planting, but prevent the cuttings from drying out before you replant them.
Established blackberry plants produce suckers, vigorous new shoots that grow from the bases of existing plants. You can produce new plants from this growth by severing the suckers' root connections to the parent plants, using a garden trowel or shovel. Dig up each sucker and its root system, and transplant it to a new location, setting the transplant at the same soil depth at which it grew before you dug it up.
Blackberries also reproduce by taking root where the tips of their established stems touch the ground. You can take advantage of this habit to produce new plants. To do so, simply pull the tips of flexible stems down to soil level in summer, and cover the tips with 3 inches of soil. A new shoot will emerge and develop roots from each point where a stem tip is buried. In fall, sever each new shoot from its parent plant, careful to keep each shoot with its roots. Transplant the removed shoots to a different spot, planting them at the same soil depth at which they previously grew.