Some species, notably quite a few perennials, are very readily propagated by root cuttings. Though this may seem drastic, it is easy and well tolerated by many plants. Healthy stock should be used, lifted prior to the growing season, and washed free of soil. Select young healthy roots the size of a pencil for woody plants, smaller for non-woody ones, and cut straight across, just below the crown. Long roots may be cut into multiple sections.
As with stem cuttings, I use a transverse cut to mark the upper end of the cutting, the one closest to the crown, and an angled one pointing downward into the soil to identify the lower part of the cutting. Cuttings MUST be oriented correctly.
How cuttings are grown determines how long they should be: if rooted in a warm propagator, cuttings 2 to 3" long should be adequate; if rooted in a cold frame, make them 3 to 4" long, and for those left to overwinter in the cold, unprotected ground, 4" or longer is required.
Plant in loose, barely moist, well-drained soil in a cold frame, or simply pot up and bring indoors. Poke a hole into moist, well-draining medium with a pencil and gently lower the cutting until its top is even with the soil surface, one or two per pot. Cover with a thin layer of grit or fine gravel. Keep in a humid environment, and water very sparingly.
Note that stem and leaves often develop before the new root system, but that the new roots must appear before increasing with watering. Also, DO NOT use Rooting Hormone, as this actually has a detrimental effect.
Rhizome cuttings are treated in a similar manner, though each section should have at least one shoot; sections are planted horizontally, with the shoots pointing up. This technique works well for many diverse plants, from rhizomatous-rooted bamboo to hops, as well as numerous others.
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