N.Z. alpine plants natural hybrids

Press, 13 March 1987, Page 14

N.Z. alpine plants natural hybrids

Oardener’s W DIARY

Derrick Rooney

If frequent natural hybridisation is, as some ecologists have suggested, an indication of vigorous and actively evolving plant communities, New Zealand’s mountains have some of the healthiest plant populations in the world. Natural hybrids abound in several important genera in our flora: the hebes, the clearias, the celmisias, the raoulias, some ferns, the southern heaths, and others.

Hybrids between different species, within the same genus, are not uncommon in many countries, especially in areas where the natural distribution of species has been affected by roading, farming, mining, or the removal of forests. An unusual aspect of natural hybridism in New Zealand is that we also have numerous hybrids between species belonging in different genera. The vegetable sheep

(species of Raoulia) cross with the South Island eidelweiss (Leucogenes grandiceps). The eidelweiss also crosses with the native “everlasting” (Helichrysum bellidioides) and with the matforming species of raoulia. The whipcord helichrysums, which some people believe should be placed in a different genus, cross with H. bellidioides. Back crosses also occur.

All this would be of merely academic interest were it not for the fact that some of these hybrids, orphans doomed to extinction in the wild, appear to have the potential to become first-class rockgarden plants. Only a handful are in cultivation and probably only one is to be found outside the shadehouses and alpine houses of specialist growers. This is a tiny, spreading shrublet with needle-like, silvery leaves whicfi makes a delightful

cushion in a sunny place in the rock garden. It has been determined to be a hybrid between Raoulia glabra, a loose, green mat plant of rocky places, and Helichrysum depressum, a whipcord shrub. “Roulia loganii” is sometimes offered by alpine nurseries. This plant, with woolly stems and close-set silvery leaves, grows into a cushion of packed, furry rosettes. It is a natural cross between Raoulia grandiflora and the North Island eidelweiss, Leucogenes leontopodium. While not always easy to establish in the rock garden, it makes a delightful pot for

the alpine house, and grows away well in a trough.

Like most of the natural hybrids within this distinguished group of alpine daisy plants, “Raoulia loganii” roots readily from cuttings. But putting roots on them is one thing; growing them on afterwards is not necessarily quite so easy. These plants come from stony or rocky places high in the mountains, and they seem to like a loose, gritty soil which is not allowed to dry out completely, and which contains some vegetable humus in the form of leafmould or peat. A mulch of coarse gravel at the collar seems to be beneficial.

So long as the texture is right, I don’t think the choice of ingredients in the potting mix matters too much. Some growers use the traditional loambased John Innes mix, which grows a very wide

range of plants very well. Some use peat-based composts, some bark mixes. I have been an advocate of peat-based mixes, but the rising cost of peat has forced compromise, and I have recently been experimenting with a mix based on conifer leafmould, with no peat in it. So far it seems satisfactory.

The important thing is not to fuss over the actual selection or proportions of ingredients, but to concentrate on getting the texture right, so that the mix stays evenly moist, but any surplus water drains through it immediately. Adding extra grit will not necessarily achieve this. In fact, I have been using less and less grit in my mix and the grit now accounts for only about one-fifth or less of the bulk. The leafmould has a naturally open texture, and I add rotted sawdust also helps to keep

the mixture open. The fertiliser requirements of alpine plants are minimal but plants in pots need some feeding, so I add slow-release fertiliser, some dolomite lime, and some bone flour. The mulch of coarse grit on top of the mix helps to avoid collar rot, which can be troublesome. “Coarse grit” is sand with particles, larger than 6mm and smaller than 3mm, screened out. My “normal” mulch is up to 2cm thick, but some of the scree plants, such as Cotula atrata, need only have the pot half-filled with compost, on which their rhizomes should rest. The pot should then be filled in the normal way with coarse grit.