It's time to propagate

Press, 22 January 1988, Page 19

It's time to propagate

January may be, as I observed last week, a low point in the gardening year, but there is still plenty of work to do — sprinklers to be moved frequently, hedges to be trimmed, flowered stems to be cut down from plants which might produce unwanted seed, shed bark to be picked up, bulbs to be cleaned and repotted, cuttings to be taken. Many kinds of shrubs, large and small, evergreen and deciduous, will grow from cuttings taken now, dipped in hormone powder, and kept in a warm, humid atmosphere. Select young, shortjointed shoots which are just beginning to firm up; a shoot is ready to be planted as a cutting if it almost, but not quite, snaps when bent sharply

between the hands. For later cuttings, in autumn, firmer shoots are required. This sounds like dreadfully imprecise advice, but it’s the best I can do. Experience will soon show you what kind of shoots are best for cuttings, and when, anyway. Some will put down roots within a few weeks or occasionally a few days at this time of year — I have had hydrangea cuttings potted up within seven days when everything was just right. Most cuttings will take longer, however; a month, six weeks, sometimes more. I might have a bit to say on this subject next week. Meantime, I’d like to make some points about plants which, if they must be divided, are best split up and replanted between midsummer and autumn, however crazy a time this may seem to disturb them.

Many beardless irises fall in this group. It’s really too late, now, to divide bearded irises; you can still do it, but at the risk of losing next year’s flowering. These should have been done before the end of December; three to four weeks after flowering seems to be the ideal time, in my garden at least. With the exception of a few species, which are long lived and slow increasing, bearded irises need to be divided

Gardener’s') W DIARY I

Derrick Rooney

and replanted in freshened soil every two or three years. Older clumps lose vigour and produce fewer, smaller flowers. A light

‘Too late now to divide bearded irises’

dressing of superphosphate around bearded irises in February seems to improve their flowering, but I usually forget. Louisiana, Spuria, and Evansia irises can all be divided and transplanted now, provided water is available forr them until they are re-established. The small Evansias — Iris cristata, I. lacustris, I. gracilipes — and their white forms are best reestablished in pots in a shadehouse before being planted out in autumn, when weather and soil are cooler. This treatment seems to result in more vigorous clumps. The “roof iris,” I. tectorum, seems tolerant of disturbance at almost any

time of year, but is, I think, best propagated from seed. Divisions do not seem to thrive as seedlings do and older clumps frequently suffer from virus. Some strains of this iris, especially of the lovely white form, seem very prone to virus infection and have unthrifty, yellowish foliage. I have a few seedlings, to be planted out next month, of a tetrapioid white form (in which the doubling of the normal number of chromosomes should result in increased vigour and flower size).' A few seedlings of the so-called Taiwan form, which has bigger flowers and healthier foliage than the ordinary blue form, are also coming on nicely, and I hope to get seed soon of the Burma form, which reputedly has very large clear lavender-blue flowers without the usual darker mottling on the petals. Small Spuria irises, such as I. graminea and I. sintenisii, are, I think, best divided in early spring, just as new shoots are emerging from the ground, and one of the loveliest little ’uns, I. kerneriana, dislikes division or disturbance at any time and is best raised from seed. The larger Spurias and the Louisianas take a brief rest in late summer; the old leaves look very tatty at this time of year,' especially if they weren’t

sprayed in spring against leaf spot. These are programmed to make a spurt of new rhizome growth in autumn, and can be safely replanted (after division) now in anticipation of this, but they must be kept moist until well established. Both kinds favour heavy soil but can be grown successfully in lighter soils which are well enriched with organic matter, as also can Siberian and Higo types. Tetrapioid forms of the latter, with their silken texture and full-petalled, dinner-plate-sized flowers, are among the most sumptuous of garden blooms.

All these irises prefer acid soil, although the Spurias aren’t as insistent on their dislike of lime as the others. When in doubt, use dolomite, not ordinary garden lime.

Bearded irises and Pacific Coast irises prefer more mineralised soil. The former are lime tolerant, rather than lime demanding, and the latter prefer a soil on the acid side of neutral. Pacific Coast irises — developed from a cocktail of western North American species — are best propagated from seed, like their predecessors, the Innominata and Douglasiana types. However, named varieties and superior seedlings must be divided if they are to be increased, and this may be done at just about any time of the year if they are showing new, white roots about 2cm long, but is probably best done in late summer or early autumn, as soon as there has been sufficient rain to initiate growth.

These irises are conditioned to taking a rest in midsummer, after flowering, as this is the dry season in their homeland. It’s to leave them

alone until they show signs of renewed growth. Siberian and Higo irises are not very closely related but have similar growth cycles, and you get better increase from them by leaving their division until late March or even early April. They are long-lived plants and there is no point in disturbing them at all unless you want to make new clumps. Pieces for replanting in the open ground should consist of no fewer than three fans. Smaller pieces may be re-established in gritty soil in pots or planter bags. Siberians may also be divided in early spring, but autumn divisions, if they are to be replanted directly, make, I think, better plants. Higo irises dislike spring disturbance and may take a couple of seasons to recover from it, if they recover at all. These irises are often grown in water but they do just as well, and usually better, in ordinary border conditions. The Japanese, who developed this type of iris, flood their iris fields at flowering time but they do it for aesthetic, not

‘These irises are conditioned to taking a rest in mid-summer’

horticultural, reasons. Higo irises in an extended range, of colours are now being developed by hybridisers in Australia and the United States and many of the new varieties are “rebloomers,” flowering twice in a season. There appears to be a bright future in gardens for this type of iris. Some other members of the greater iris family, such as the early-flower-ing Dietes and Moraea from South Africa and the Libertias from New Zealand and South America, may be divided now. Tradescantias are still putting out a few flowers; if cut back and split up now, and kept moist for a few weeks, they will have another good flush of flowers in autumn. The small, early-flower-ing Kniphofias may be divided more successfully now than at any other time of the year; summer and autumn-flowering types are best divided in spring. The larger types with distinct stems or trunks — such as K. caulescens, K. northiae, and K. tuckii —- may be propagated by cuttings, taken either now or in the spring. No Kniphofia likes to be disturbed in winter. Another plant that can be very successfully propagated from cuttings taken now is the large Evansia iris, I. wattii. This unusual plant has long, bamboo-like stems topped with fans of broad, light green leaves, from which, in the second year, branching heads of light blue butterfly flowers appear in spring if the winter has been a mild one. It requires protection from wind and severe frost. Stem cuttings, taken now and planted in gritty soil, will root in a few weeks and make strong plants within a