Propagation timing is everything

Press, 23 September 1988, Page 10

Propagation timing is everything

Gardener’s W DIARY

Derrick Rooney

When do you propagate perennial plants? When they are ready for it, of course. If you can’t do it when the plants are ready, do it when you are ready. Most herbaceous perennials are quite tough, and will survive being chopped up at almost any time of the year. There are some exceptions, of course: bamboos and summer-green kniphofias, for example. Bamboos (and other ornamental grasses, and pretty well all monocotyledons, for that matter) usually transplant successfully only if the move is made when they are entering a phase of active root replacement.

Unlike trees and shrubs, which have a permanent framework of woody roots, these plants renew their root systems annually, and in some cases twice a year. If you chop them into pieces when the old roots are retiring and the new ones have not emerged their chances of survival, are much reduced.

Kniphofias are best split up in late January or early February; bamboos in September, irises two or three weeks after they finish flowering. The rule of thumb with nonbearded irises is that if they have new white roots about 2cm to 3cm long, they may be moved. Si-

berian and Spuria irises, however, are best transplanted in March. Division is one method of propagation; cuttings provide another. It is not generally realised among gardeners that a wide range of perennials may be propagated by cuttings, of either the stems or the roots, even though these methods have been used for many years to propagate some important plants, such as chrysanthemums (now renamed Dendramthema!) and delphiniums. No elaborate equipment is necessary — a cold frame that can be kept closed and lightly shaded is perfectly adequate. Cuttings may be treated with root-promoting hormone powders or dips before planting, but these are not necessary in most cases. I now use hormone only for the slow-rooting species, such as the lovely Convulvulus cantabricus. Shoots scm to Bcm long are ideal for cuttings and should be cleanly severed from the crown with a sharp blade. They seem to root better when a very thin sliver of crown is retained as a "heel." A point worth re-

membering is that cuttings from young plants root much more readily, and grow away more strongly, than those from old plants. Some commercial growers keep special “stock plants” and propagate them continuously until they cease to produce vigorous shoots. The plants are then discarded. Home gardeners, who need only a few plants, need not go to so much trouble. One batch of cuttings is usually enough. If you are propagating from a garden-grown plant, however, don’t take more than two-thirds of the shoots as cuttings. The method of propagation least familiar to amateurs is by root cuttings. This method is used commercially to multiply such plants as oriental poppies, some Asiatic primulas, verbascum hybrids, and other plants not readily increased by more conventional means.

Root cuttings are essentially similar to stem cuttings and should be treated in almost the same way — the only difference really is that root cuttings are completely buried. Two good plants with which the beginner may experiment are the dawn poppy (Eomecon chionanthum) and the hop. Both grow easily and rapidly. The technique is simple. Roots of about pencil thickness are best, but any size that can be

handled will do. Using a sharp blade, chop them into pieces about scm long. Make a cut straight across at the top and a sloping cut at the bottom, so that you don’t accidentally plant the cuttings upside down (don’t worry if you do; they’ll probably grow anyway). Fill a tray (polystyrene mushroom trays are excellent for the purpose) with sandy potting mix, but not pure sand, and water it. Use a pencil or a pencil-thick stick to make the rows of holes for the root cuttings, plant them, and sprinkle a little potting mix over them. Put the tray in your cutting frame or shadehouse.

That’s all you have to do, apart from keeping the trays of root cuttings watered. Root cuttings may be made at any time of the year, but early spring is probably best. The resulting plants may be pricked out and potted or planted in early autumn, or left until the following spring, by which time, if they haven’t been overcrowded, they should be nice plants. Perlite may be used instead of sand in the potting mix. This material, a kind of natural volanic glass, looks like finely ground vermiculite and seems to have mechanical properties that promote strong root growth. I now use it in all my cutting mixes. It is expensive, but a little bit goes quite a long way.