THE GRASSLANDS OF NEW ZEALAND.

New Zealand Journal of Agriculture, Volume XXVII, Issue 5, 20 November 1923, Page 281

THE GRASSLANDS OF NEW ZEALAND.

SERIES II. THE TARANAKI BACK-COUNTRY.

E. BRUCE LEVY,

Biological Laboratory, Wellington.

2. GROWTH-FORM AND HABITS OF SECONDARY GROWTH IN RELATION TO CONTROL.

IN the control of secondary growth, or, in fact, of any weed, it is useful for the farmer to know the habits and growth-form of that weed and its behaviour under, or its response to, differential

farm-practice. By a . study of the life-history of the plant we often can pick out weaknesses manifest at some stage in its yearly cycle of growth, which can be turned to our advantage in the control of that weed, in much the same way as in the control of insect and fungoid pests. Indeed, it may be put down as an axiom in weed-control that any weed should be attacked at that point or period of the year where and when it is most vulnerable. This article will deal more or less with the growth-form and methods of propagation and spread of the most troublesome scrub plants of the Taranaki back-country, and will endeavour to indicate the vital weak spots in each class of growth.

The secondary-scrub growths causing most of ■ the trouble in the Taranaki back-country are hard fern, manuka, bracken-fem, waterfern, and piripiri. Of these hard fern and manuka are the most difficult to deal with.

HARD FERN.*

■ This fern (Fig; 20) forms a carpet-like mass 12 in. to 18 in. tall, fairly dense to e the bottom, which is often filled up with dry dead fronds of previous years’ growth. It spreads from out of the shelter of logs or. stumps, or from points of establishment on dry open knolls. Its spread is by means of branching, wiry, dark-coloured rhizomes, which creep over the surface of the soil. In this surface-growing rhizome lies the greatest weakness of the plant. Stock injure it readily by their treading, and it is particularly liable to destruction by fire in a season when the foliage is sufficiently dry to carry a hot fire (Fig. 21). The time of burning hard fern is important.- A hot fire is essential,

and burning should not be carried out until the soil-surface is well dried out. Early spring burning is often practised owing to the fact that during a severe winter the previous year’s fronds are all dried up, being killed by frost. These carry a surface fire, but the soil-surface is usually wet in the spring, so that the surface rhizomes remain undamaged and new fronds come away soon after the fire (Fig. 22).

Every endeavour should be made to get grass established on the hard-fern burns, so that stock may be enticed there and thus damage any small pieces of fern that may have been missed by the fire. The spring-time does not afford opportunities for the establishment of grass

on secondary-scrub burns,- and this is particularly true in the case of hard fern, for the great mass of surface rhizomes and thread-like roots form a hard compact surface most readily dried out in the hot weather of summer. Autumn burning in a dry season seems to afford the only hope of combating this weed; and while it is recognized in the Whangamomona district that one cannot always rely on getting a dry season, yet it would seem wiser to miss a season or two than to attempt the burn at a time when the soil-surface is not dry. The autumn affords the greatest chance or success, not only in the burning but also in the establishment of grass on the burn.

The growing-point of ,the surface rhizomes of hard fern “ feels ” somewhat for the light, and it will not penetrate a densely shaded cover. As the rhizome grows forward in contact with the soil it sends down thin thread-like roots, and at the same time somewhat slender fronds into the air. Therefore, as in the case of a great many other weeds, in order to spread satisfactorily, the surface rhizome must contact the soil, else the new roots cannot develop. By constant grazing of the turf around the clumps of hard fern the grass is kept short, and hence light is allowed to penetrate freely to the young outspreading rhizome, which consequently keeps close to the ground and spreads, rooting as it goes (Fig. 20). There is also nothing above to hinder the upward growth of the young frond. If a fairly dense shade and competition with other plants is provided (as by a good growth of grass induced by spelling the pasture), then the rhizome tends to rise slightly off the ground so as to avoid penetrating the denser growth. .When the rhizome thus loses contact with the soilsurface no roots are formed and its growth is inhibited, and what growth is formed is very liable to be destroyed by stock when these are once more turned on to the area.

From the foregoing, then, it would appear that control of hard fem centred about three practices : (1) Burning of the hard fern in autumn in a season sufficiently dry for a hot fire to be secured ; (2) the sowing of these bums with suitable grass-seed ; and (3) the spelling of the pasture to induce a good grass-growth about the hard-fem clumps. The spelling also enables a heavy stocking at any one period • of the year when the fern is most injured by treading.,

BRACKEN-FERN.

Bracken-fern has an element of very great weakness in that its frond is eaten by stock. The rhizome of bracken-fern is well below the soilsurface, usually 6 in. to 8 in. deep, but at times 1 ft. or even more on. certain light sandy or pumice country. The rhizome is thick, and in it is stored an immense amount of reserve plant-food. In the early spring from this underground rhizome new fronds are formed, and these draw during the early period of their growth —that is, until they uncurl — on the food-reserve in the underground rhizome. The new fronds appear above ground as tender, brittle, curled structures, most susceptible to injury by grazing animals (Fig. 23). This curl stage is, without. doubt, the weak point in the annual growth of the bracken-fern. Every frond broken off in the curl stage means a reduction in the vitality of the plant, and just as long as the process can be repeated each time new fronds appear the draw upon the

reserve food in the'rhizome becomes so great that ultimately the supply becomes depleted and no more fronds appear above ground. . If the frond is allowed to . get out of the curl stage it becomes tough and stringy and less palatable to stock. Again, just as soon as it uncurls it begins manufacturing plant-food, which goes to maintain the foodsupply,. and hence the vitality of the underground rhizome (Fig. 24).

In order to deal satisfactorily with the bracken-fern in the curl stage some food other than the fem must be provided, else the stock rapidly go back in condition. The fairly long dormant period of bracken-fern during winter usually makes it possible to get grass well established on the fern-burn before the young fronds begin to appear, which is usually in October or late September. ’ Bracken-fern control therefore appears to resolve itself into an attack on the plant along two lines : (1) Crushing by stock in the early spring and summer, while the fern is in the curl stage, and (2) the production of pasture plants on the bracken-fem area during its dormant period —March to October —in order that this stocking may be satisfactorily carried out.

WATER-FERN.

Water-fern requires a good deal of moisture and shade in order to thrive. It is troublesome mostly around stumps and logs, or on altitude farms where the rainfall is heavy. It does best in somewhat loose soil containing plenty of humus in the form of leafmould or rotten logs. The rhizome is well below ground, being on the average 2 in. to 3 in. deep. It is stout, but not so well supplied with, reserve food as is bracken-fern. The young fronds are fairly numerous, large and fleshy, and grow for the most part in winter and spring. Frost, however, injures the growth in the winter. If stock can get in among water-fem they can be very ’ damaging with their feet, owing to the big, fleshy, curled fronds being readily broken off. Moreover, from the fact that the rhizome does not contain the same reserve food as in bracken-fern, the plant is much more readily crushed out than is bracken. Stock also eat the herbage to a small extent even when the frond is expanded. On water-fem areas it is difficult to get a bum, so that pasture plants are introduced with greater difficulty. Water-fern scarcely ever comes out into the open, and usually as soon as the logs are removed and when the land has become more consolidated .by stock water-fern ceases to trouble. On certain farms in the country under consideration, however, water-fern is still spreading considerably, forming large clumps.

MANUKA.

Manuka, once it becomes well established, cannot be controlled by stock alone and the slash-hook must be employed to fell this scrubgrowth. This is a costly business, but there is apparently no alternative. To bum standing manuka that is bearing seed is worse than useless. ■ Manuka once cut will not grow out afresh from the cut stumps (though any small branches left low down on the stump will grow), and were it not for the fact that this plant seeds so profusely its control would be comparatively easy. The seed habit may be termed almost a mania with manuka : one plant in a single seasen

will produce millions of seeds, all of which apparently are fertile and capable and eager to produce a young plant. This apparent readiness

of germination is, as far as the writer can see, the weak point in manuka, provided the problem is attacked in the right way. Either the plant must be cut and burnt before the seed is shed, or else cut and left on the ground until-such time as the seeds have germinated

and have established as young plants among the cut dead manuka (Fig. 25). The seeds germinate readily in spring (end of September), and provided opportunity offers it would appear that all seeds germinate the same year that they mature and fall (this point needs more definite investigation, however), so that if a fire can be run over the whole cut surface every manuka seedling will be killed outright. Provided there

has been a good fire one may search in vain on an old manuka burn for young manuka seedlings.

The time of cutting and . burning, therefore, is very important in manuka-control. In the ordinary course of events the manuka-seed crop is ripe about the end of July, and most of the capsules have opened and have shed their seed on to the ground by the end of August. There are, however, a small proportion of capsules that do not open with the main crop. These may 'remain closed for a whole year or even longer, so that no matter what time of the year the manuka is cut there is always some seed on the. plant. These latter capsules, so soon as the plant is killed by cutting or by fire while standing, mature, and, opening up, shed their seed on to the ground. If the

manuka is cut before spring they will have opened along with the main crop, and they will germinate along with the rest. If the manuka is left until summer before it is cut there will always be a certain number of seeds shed on to the ground that have not germinated at' the. time of firing, and it must be remembered that a dormant seed is much more difficult to kill by fire than is a young tender seedling. Any seeds that are not killed will, of course, readily germinate and become established soon after the fire.

Late spring bums will kill virtually all the young seedlings, but this time of the year does not offer opportunity for getting grass-seed established on the burned surface. The fact must always be borne in mind that manuka (and, for that, most weed-seeds) will not germinate and establish within a close grass sward. It is only when the grass sward opens up that such become established. Autumn burning of .cut manuka not only destroys manuka seedlings, but also affords ■opportunity for the sowing of grass-seed with a good chance of its successful establishment. Manuka which has been cut and which has lain for long loses the greater portion of its leaves, and thus it may not carry a fire so well as when fired soon after being cut. As the :winter is the most opportune time for the farmer to get out to cut his manuka, spring burning is often practised. Where there is already a certain amount of grass such as danthonia, New Zealand rice-grass, &c., among the manuka, spring burning is probably more beneficial to the grass than autumn burning would be, provided the bum is done before the new season's growth has commenced. On areas, however, where sowings of grass-seed are being made the bum should be in the autumn.

Standing manuka that is old enough to bear seed should never be fired. This is the opinion of the best farmers of the Wairarapa and Hawke’s Bay districts, where they have had considerable experience with manuka. The heat of the fire is seldom sufficient to consume the seed-capsules on the living plant, and the heat really acts as a ripening process to the capsule. The latter consequently opens soon after the fire has been through, and the seed is shed on to the ash left by the fire. Here there is no competition, and the conditions are ideal for the establishment of the seedling (Fig. 26). Numerous instances have come under the writer’s notice where thousands of young manuka-plants to the square foot have come up on standing manuka burns (Fig. 27).

In the Whangamomona district also patches of hard fem are usually associated with the manuka until the latter becomes dense. If such areas are burnt standing the fire sweeps over the top of the hard fern, leaving its rhizomes comparatively undamaged (Fig. 28). Felling the manuka on these areas means that a much more intensely hot fire is secured close to the ground surface, and thus the hard fern may be entirely killed out by the firing. The manuka-plant itself is readily killed by fire, and this fact .can be made use of in' the burning of manuka areas while the plants are still small—but only provided no seed-capsules have matured. To' do this there must be some growth such as danthonia, brown-top, or paspalum present that will carry a fire.

The burning. of standing manuka on country ■ where the brackenfern has 'not been killed out before the incoming of the manuka almost invariably leads to an immediate pure bracken-fern association. On all that country where bracken-fem forms a phase in the forest succession this return of bracken is likely to happen so soon as the growth that replaced the bracken in the succession is destroyed by fire (Fig. 29). It would appear that the rhizome of bracken-fem is capable of remaining in a dormant state in the soil for many years, and once the shade of the top growth, is removed the bracken once more springs into being. On the second-class soils where bracken is not thriving luxuriantly manuka will often come into the standing fern, and in the course of eight or more years will so overtop the bracken that its growth is subdued, and the association to. all intents and purposes becomes a pure manuka association. Now, should this manuka be burnt standing, the growth that follows immediately is not manuka but bracken-fern, which holds sway until such time as the manuka seedlings, established after the fire, again grow through the fem and once more begin to overtop it.

This alternation of manuka and bracken-fern associations is not very common in the Whangamomona County, for usually the brackenfern is - killed out before the incoming of the manuka, and here, in a state of nature, manuka does not form a phase in the succession; the soil being sufficiently good to carry either a wineberry-lacebark-fuchsia successional association, , or else one of tutu-veronica-fivefinger-karamu-kohuhu, &c. Manuka there comes only after the soil has become considerably depleted.

Manuka-control, it will be seen, is largely an attack on the plant in the seedling stage; and the regulation of practice, both in the felling and in the burning, should be towards effecting the most damage at this stage. Spelling the pasture while the manuka is young, so as to get sufficient grass-growth to carry a fire, attacks the plant in another vitally weak spot.

PIRIPIRI

This weed demands for its spread and prosperity a turf closely grazed by an animal that will not eat its foliage. Sheep-grazing alone fulfills these conditions almost perfectly. The plant in its habit of growth comes into the group of pasture - weeds that spread by means of an overground runner which clings close to the ground, rooting as it goes. Hard fern, it will be noted, also belongs to this group. The surface runner of piripiri is drawn up to the light by shade-forming plants much more markedly than is hard fem, and in this character lies the greatest weakness of piripiri as a plant. Cattle beasts also will eat it. The conditions of the dry knolls, which are usually the point of establishment of piripiri in a new burn, favour the spread of this plant because the grass-growth there is usually too poor to completely cover the ground. Consequently the runner of piripiri is able to keep contact with the ground, and thus the plant spreads to the edge of the knoll. The better grass-growth surrounding' the knoll usually inhibits further spread until such time - as this grass becomes closely grazed down.

Piripiri itself is almost entirely neglected by sheep, and they will not browse among it unless forced to do so by lack of feed elsewhere. The sheep, however, graze close up to the edge of these piripiri patches, keeping the grass here extremely short (Fig. 30). Thus light is allowed into the overground runners, and the outward spread of the plant is assured. If these edges of the piripiri clumps are shaded (such as by spelling the pasture as in the case of hard fern) the runners may readily be made to leave the ground, the new growth turning upwards in order to reach the light. When this happens the runners no longer send out roots into the soil, but travel along the surface of the grass as loose trailing stems most susceptible to injury, by the grazing and treading of stock (Fig. 31). Cattle eat the piripiri to some extent, and they have not the aversion to going among it that sheep seem to have.

The facts that cattle will eat the plant and that the growth-form may be altered considerably to its disadvantage by stimulating shadeforming plants among and around it suggest the following methods of procedure in its control : (1) Spelling the pasture to shade the overground runners of the plant so that these lose contact with the soil, being drawn up to the light ; and (2) the grazing of these spelled areas by cattle. .

GENERAL.

From the foregoing it will be seen that the habits and growthform of a plant are big. factors in deciding what methods should be employed to successfully control that growth. The importance of the grass sward and . of the stock capable of being maintained by means of that sward are manifest throughout. These really are the two great weapons available to farmers for the control of most weedgrowth, and any- method for the control of weeds that does not take these two important factors into consideration must, in the writer’s opinion, be economically unsound. It is realized that the farmer in the Taranaki back-country is up against, the great difficulty of securing this grass sward and in getting it to hold, and this is one of the first phases of study and experiment We are undertaking in our work in Whangamomona County.

(To be continued.)

Brown-top Seed. Special investigation work into the possibilities of extension of our export seed trade to America in brown-top (Agrostis tenuis) seed was carried out at the Department’s Biological Laboratory during the past year, and possibilities of extension in this direction have opened up. Collections of the various Agrostis species have been made throughout each district in New Zealand, and these have been studied and the dominant forms determined as to species. The dominant form of Agrostis in", New Zealand has now been determined by Dr. C. V. Piper, Agrostologist, United States Department of Agriculture, as being brown-top (Agrostis tenuis), for the seed of which he reports there is, in his opiriion, an extensive market in America. The outcome of this research will have quite an important bearing on the economy of grassland farming on our first-class short-rotational grassland soils.

* For botanical description and illustrations of hard fern, common bracken-fern, and water-fern see The Bracken-ferns of New Zealand,” by Esmond Atkinson, this Journal, January, 1923.