The Cultivation of New Zealand Plants
Chapter III. — Methods of Cultivation
Methods of Cultivation.
In selecting plants for cultivation it is essential to recognize the limitations of your garden. Under this head comes first that point determining its climate, namely its situation in New Zealand, which at once gives a general idea of what can and what cannot be cultivated. It is on the degree of cold to be experienced in winter that a general horticultural classification of plants is based. In this book they are divided into three categories:—very hardy, (vh.), able to tolerate rather more than 16 degrees of frost; hardy page 17(h.), barely able to tolerate 16 degrees; and half-hardy (hh.), only able to tolerate a few degrees, or it may be none; between these classes there are, of course, intermediates. Speaking generally, frosts become less intense, fewer, and of shorter duration in proceeding from south to north. The Canterbury Plains (see map*) are a notable exception, for, in relation to altitude, they experience the hardest frosts of any part of the Dominion. Proceeding inland from the coast, or with considerable increase in altitude, frosts are more severe and of longer duration. But, no matter where, low-lying ground is considerably colder in winter than the adjacent hillsides.
Next the rainfall, and the average number of rainy days, must be considered. Thus the capabilities of arid Central Otago, on the one hand, and of Westland, on the other, stand as extremes. Other factors of moment are the average and maximum summer temperatures, the amount of sunshine, and the prevalence of high winds.
The soil of the garden is a most important feature. There are great differences in its fertility, its drainage-requirements, and the ease with which it can be worked.
The nature of the garden, its size, its physical features, and its aspect must determine finally of what its plant-inhabitants are to consist.
Regarding the cultural treatment of any plant it is important to know in what kind of situation it grows in nature, in what part of New Zealand, and at what altitude. Occasionally, such information may mislead. For instance, Griselinia lucida grows either perched high on the trunks of trees, or on rocks, yet it succeeds well as a shrub in ordinary garden soil; many species confined to bogs, or wet ground, thrive page 18best in gardens in fairly dry soil. Indeed, plants in general have much wider horticultural capabilities than a knowledge of their habitats would suggest, and it is almost axiomatic, that most plants, no matter under what circumstances they grow naturally, will do best in good, deep soil.
On the other hand, a knowledge of habitats tells a good deal about the sun and shade requirements, and the frost tolerating capacity. Thus, plants of the forest interior, those of cloudy high mountains, and those of the Subantaretic Islands usually require an aspect where there is shade during the hottest part of the day; and, paradoxical though it appears, certain alpine plants will not tolerate severe frost, though this applies rather to exotic alpines than to any of ours.
Photo. W. B. Oliver
Fig. 3. Veronica gigantea (the tree-koromiko) growing as a true forest-tree, Chatham Island.
Photo. W. D. Reid
Fig. 4. A low hedge of Veronica parviflora in the Wellington Botanic Garden; in the background is the black tree-fern, Cyathea medullaris.
In sowing, the soil in the pot should be moist throughout. No seed should be covered with soil to more than its own depth, and small seeds sown in boxes, or pots, intended to be kept under cover, should have hardly any or no soil put over them. Even the comparatively large seeds of Veronica require no covering, so long as the surface-soil is not allowed to remain dry for any length of time. Dry sphagnum moss crushed up very small makes an excellent covering for the soil, or it alone may be placed over the smaller seeds. Seeds of trees, and shrubs, can be sown in lines in the open, but the soil should be very free and not liable to cake, nor should it be allowed to become too dry. Seeds of herbs and semi-woody plants, and of high-mountain species in general, should be sown in pots, pans or boxes. Kerosene-tins, cut to a suitable size, so as not to be too deep, serve well for raising seeds, and their shape economises space. If possible, these pots, etc., should be placed in a propagating house, cold frame, or some contrivance where they can be shaded. Many seeds, including even those of high alpine plants, may be successfully and rapidly raised in gentle heat. The pots and pans should be deeply plunged in sand. The surface soil should never be allowed to get really dry, neither should it be very wet; it must be watered gently as required. A piece of glass placed over a pot helps greatly towards conserving moisture. In a few weeks, seedlings will commence to appear. These, when established, will need less watering.page 20
The soil for raising seeds should be such as to allow water to readily pass through it. Leaf-mould (hardly procurable in New Zealand), well-rotted stable manure (no longer a common commodity), soil made from rotted turf, peat, well-decayed garden refuse, surface-soil from the bush—any thoroughly rotted vegetable-matter in fact, mixed with plenty of sand, and according to its consistency, a little loam, may be used for seed-raising. The special enemies of the seedlings are a too moist atmosphere, weeds, and slugs. The first may be easily dealt with; the weeds can be pulled up as they appear; and the slugs destroyed at night. Later, if the seeds are very slow in germinating, the soil will get covered with moss, or a fiat liverwort. These should be carefully removed when they first appear. When the seedlings are large enough to handle, they may be pricked off into boxes, or if very strong, may be planted at once into the open. In certain cases, the seedlings may be left until the pots are full of roots, when, the whole potful, soil and all, may be planted without disturbing the seedlings. Difficult plants may frequently be established by this rough and ready method.
Many trees and shrubs may be raised from cuttings, and prostrate plants by small rooted pieces of the creeping stems. Cuttings of many plants may be taken in autumn, or early spring, from short side branches of terminal shoots. All the uppermost leaves must be left on, and the lower half, the leaves having been removed, be firmly fixed in the soil. Many cuttings root readily in loamy soil in a shady part of the garden. Those of Veronica may be struck at almost all seasons. The species best grown from cuttings, or seed, will be seen in the chapters dealing with the different species suitable for cultivation. Persons possessing a glass-house, where bottom-heat can be page 21secured, can grow various species from cuttings which fail to strike in the open.
Those with small gardens, who have not much time to spare for gardening, should make a liberal use of young slow-growing trees and the smaller shrubs. Nearly all those of New Zealand are evergreen, and there are great differencs in their forms and the colour and shape of their leaves. So they look well all the year round, and many are showy when in bloom. In any case, they are a most welcome substitute for the privets, escallonias, Euonymus and small conifers now planted ad nauseam. In fact, they more than hold their own with the select evergreens of temperate gardens.
Amongst the most rapid-growing are species of the following genera:—Alseuosmia (hh.), Aristotelia (vh.), Brachyglottis (hh.), Carpodetus (vh.), Cassinia (vh. to hh. according to species), Chordospartium (vh.), Coprosma, the larger-leaved species (vh. to hh.), Edwardsia (vh.), Gaya (vh.), Hoheria (h.) Leptospermum (vh.), Melicope (h. to. hh.), Myoporum (h.), Myrtus (vh. to h.), Nothofagus (vh.), Nothopanax (vh. to h.), Notospartium (vh.), Olearia (mostly vh.), Pennantia (h.), Phebalium (hh.), Pittosporum (h. to vh.), Plagianthus (h.), Quintinia (h. to vh.), Senecio (mostly vh.), and Veronica (h.to usually vh.).
For a small garden, in order to lend variety, one plant of each tree or shrub should be sufficient, or, if more, they can be draped with various climbing plants, e.g., Clematis indivisa (vh.), C. hexasepala (h.), Parsonsia (vh.), Rubus australis (vh.), R. cissoides var. pauperatus (vh.), most graceful, and of uncommon appearance, with its innumerable leaves reduced to midribs, covered with yellow prickles, not-page 22withstanding the disparaging remarks by Farrer in "The English Rock-Garden," vol. II., p. 226.
Sloping banks, even if extremely dry and steep, may be made an effective feature when decorated with veronicas (see figs. 17, 18), especially those of the ball-like form (V. buxifolia var. odora, V. glaucophylla, V. Traversii), while in front may come the small, spreading species, some with glaucous leaves, margined with red. Rubus Barkeri, with its leaves differing in hue at every season, but always beautiful, is ideal for draping a low wall supporting a steep bank (see fig. 1), as well as for the bank itself.
The borders for the shrubs and other plants, in the first place, should have been dug deeply (see James Young's "Rose Growing in New Zealand," in this series, pp. 18-22), but the clay subsoil, though to be loosened, should not be brought to the surface. By really deep digging alone even poor soil conditions can be greatly improved. It is surprising, however, how many New Zealand plants can be established where the cultivation has been most superficial.
Photo. J. Crosby-Smith.
Fig. 5. Celmisia coriacea var. stricta growing at 5,000 ft. altitude on the Takitimu Mountains, Southland.
Photo. W. D. Reid
Fig. 6. Gentiana patula growing at 4.000 ft. altitude on the Old Man Range, Otago.
Photo. G. L. Adkin
Fig. 7. Celmisia spectabilis growing in the herb-field on the Tararua Mountains, Wellington.
In the drier parts of the South Island (Central Otago, Canterbury Plains, E. Marlborough) the cultivation of high-mountain species is more difficult, but with care, knowledge, and an abundant supply of water, much can be achieved. In the North Island alpines grow fairly well near Wellington, and will certainly grow better still in more elevated localities. Proceeding further north, the conditions for such plants become worse, just as they become more favourable for the half-hardy plants.
The essential requisites for cultivating New Zealand alpine plants are perfect drainage, abundance of water, a cool soil, shelter from high winds, and, where required, a certain amount of shade. Though it is commonly believed that they should be grown on a rockery, quite a large proportion, even in the less favourable parts of New Zealand, succeed in the open page 24border, even if fairly sunny, side by side with exotic herbaceous plants. The following grow readily in such a position:—Most species of Carmichaelia, Celmisia spectabilis, Coriaria lurida, Helichrysum bellidioides, H. prostratum, H. microphyllum, H. Selago Linum monogynum, Muehlenbeckia axillaris the mat-forming raoulias, Rubus parvus, Senecio bellidioides, S. Haastii, and many small veronicas, including V. Lyallii and V catarractae. But in the districts favouring alpines, the greater part of such plants, confined to between the 1,000 ft. line and the 5,000 ft. line can be readily established without any special provision except a free soil and good drainage. Even on the lowest part of the Canterbury Plains many of the more difficult species have been cultivated successfully on flat ground where there was shade most of the day, but they were watered copiously during dry weather. So, too, in the vicinity of Wellington, there are growing on a bank, shaded during the hottest part of the day, the following not particularly "easy" plants:—Celmisia bellidioides, C. glandulosa, C. sessiliflora, C laricifolia, C. prorepens, C. viscosa, Gunnera albocarpa, Myosotis albida, M. explanata, Ourisia caespitosa, Plantago Brownii, P. Hamiltonii, Ranunculus novae-zelandiae, and Raoulia subulata.
Notwithstanding the above remarks, a well-designed rockery is a pleasing adjunct to a garden, if the garden be large enough. If it did no more, it is easier to see and admire the smaller alpines than when they are grown on the level. Also, stones laid on the surface of the ground help to conserve the moisture. The shelter and close proximity of a large stone, partly sunk into the soil will sometimes act like a charm for certain difficult plants. A heap of river-shingle a few feet high mixed with plenty of good loam, makes an excellent, though not ornamental, rockery, until it page 25is well-covered. Where the drainage is insufficient two or three feet of soil should be removed and a thick layer of stones put into the bottom, the hole to be filled up with stones, sand and loam, well mixed together. On the surface there may be placed a thin layer of gravel or broken stones, and good-sized pieces of rock, partly-buried, make the shelter certain difficult species require when planted on the shady side.
The above are hints rather than explicit directions. The main point to remember is that the plants demand ample soil for their roots and plenty of opportunity for these to descend as far as they require. Nor should any plant be so placed that water can accumulate round it. Stagnation spells death to alpine plants in cultivation.
* On this map are the lines of latitude on which the distribution of plants mentioned in this book is partly based.