THE CONTROL OF WEEDS.

New Zealand Journal of Agriculture, Volume XXXIII, Issue 1, 20 July 1926, Page 23

THE CONTROL OF WEEDS.

( Continued.)

3. METHODS OF DISTRIBUTION.

A. H. Cockayne,

Director of the Fields Division

There are many methods whereby weeds may become distributed from place to place, whether , by means of seeds, spores, or individual pieces of the growing plants themselves. A clear conception of the various ways in which each individual weed is likely to be disseminated is of importance, inasmuch as this knowledge may often lead to management practices being adopted that will lessen the danger of spread. So far as our introduced weeds are concerned, they have been brought info this country, either accidentally or intentionally, in the ordinary commercial intercourse with other countries. ■ ' ■ •

PRINCIPAL MEANS OF WEED-INTRODUCTION. The following are the principal means by which weeds have been and are being introduced in overseas transport :■ — As Seeds or 'Plants intentionally introduced for Cultivation on Farm or in Garden. Just as the introduction of animals from other countries for sport, pleasure, and professedly economic purposes has led to the development of animal pests, so intentional plant-introduction has been the cause of the establishment of plant pests in - the shape of weeds. The number of garden or farm-crop plants introduced that have become naturalized is not large, but among them are numbered some of our most noxious and dangerous weeds. Familiar examples are blackberry, originally introduced as a hedge and orchard plant; sweetbrier, introduced as nursery stock ; and gorse and broom, introduced for hedge-plant purposes. The majority of . the intentionally imported plants that have become serious weeds are of a ■ shrubby nature, and the desire for live hedges has been responsible for the introduction of many plants that have become either generally or locally distributed as weeds. . To those already mentioned, hakea, barberry, and elderberry can be added. When one considers the vast number of different species that have been introduced and the small number that have become naturalized to any extent it is clear that the danger of importing plants that may become dangerous is not really very great so far as numbers are concerned ; but at any time it is likely that a plant may be introduced that may become an unmitigated pest. For this reason it would appear to be a useful safeguard if there were some legislative regulations whereby promiscuous- importation could be, if not checked, at least supervised to a certain extent. At the present time any plant, no matter how harmful it may be in its original home, can be freely imported, and certainly there should be some power whereby plants distinctly likely to be troublesome here could be refused admittance. ' ■

As Seed Impurities in Agricultural Seeds generally.

The majority of our introduced weeds of agricultural and pastoral land can be traced to this source—for example,' thistle, dock, ragwort, spurrey, and a host of others. As both our annual crop seeds and grassland seeds were originally almost entirely introduced from Europe, it is easily seen most of our weeds are of European origin. The importation of weed-seeds in imported grasses and clovers goes on year after year, and tons of such a weed as sorrel are annually imported in clover-seed. This source of introduction in imported seed does not now result in the production of many “new” weeds, but simply means the bringing in of seeds of weeds already established here. Here again, however, it is likely that new weeds not present in the country' may from time to time gain a footing. No administrative action has been taken in New Zealand to limit the importation of weeds in ordinary commercial agricultural seeds ; but many countries have stringent laws against the importation of any seed containing more than a certaih percentage of impurities, or prohibiting the importation of any seed containing seeds of certain specified plants—generally, ' strange to say, the specified plants being established ones in those countries where the laws are in operation. In general it can be said that legislation against the introduction of any specified weed-seeds is quite ineffective, and only results in harassment of the seed trade without affording any real and sure protection. - ■ • ■ There are, however, two methods of dealing with imported seed that appear to be worth adoption. One is prohibiting the importation of any agricultural seed containing more than a certain percentage of impurities. This in no way would stop the introduction of weed-seeds,, but it would eliminate badly cleaned seed, would raise the general standard of all imported lines, and be decidedly beneficial. The other method that' would appear to be sound and practicable is the examination of all imported seed, and the rejection of that containing seeds . of 'plants not already established .in New Zealand and which are - clearly objectionable in those countries where they are growing. Without doubt, the matter is largely in the hands of the farmer himself by buying only seed guaranteed by the merchant to be free from any objectionable impurities.

As Seed in Ships’ Ballast.

• At one time this was a quite fruitful source of weed-introduction, but with the present almost universal use of water ballast is no longer serious. American wild rice-—-a bad grass-weed liable to block drainage - channels in the Northern Wairoa district Bathurst burr, and quite a number of South American weeds owe their presence in New Zealand to ships’ ballast.

As Seeds in Packing-material.

Straw, and other material used in the packing of commercial, goods can frequently contain weed-seeds. For instance, the horned poppy, abundant on sandy soils around the Wellington coast, was introduced in machinery imported for the Patent Slip at Wellington, . and Californian-thistle seeds have been found in the straw wrappers around bottles.

As Fodder used for Stock during Overseas Transit.

This method of introduction was very common in the early days of settlement, but does not exist at present, as under Stock Act Regulations no . fodder used for live-stock on board ship is allowed to be landed. Fodder, and particularly chaff, which' is. frequently ; imported into New Zealand,. often contains considerable- quantities of weed-seeds ; but, generally speaking, they are of plants that are already well, established here. Whenever importations of produce are being made on an extensive scale the danger of weed-importation is stressed by the local farming community; but certainly the clear method whereby this danger can be eliminated is for New Zealand to grow the whole of her stock-fodder requirements.

On Imported Stock.

Certain weed-seeds, especially those liable to cling to wool or hair, are quite likely to be imported on live-stock.. Thus many of the Merino rams imported during the past few years contained quantities of burr clover in their fleeces, and Bathurst burr has been imported from time to time in the same way.

MEANS OF WEED-DISSEMINATION WITHIN NEW ZEALAND.

There are three main agencies of dissemination of" weeds throughout the country—natural, human, and animal.

Natural Agencies.

Wind. -Wind is the most potent natural agency in the distribution of weeds, and many plants have developed seeds of a type that render them capable of being distributed long distances even by winds of moderate intensity. Well-known seeds of this character are particularly common in that family known as the Compositae —the . thistle and daisy family. They are mostly provided with feathery structures that enable the seeds to be suspended -in the air for long periods. At the present time there is hardly any forest country in the North Island that when cut down and burnt will not almost immediately produce an abundant crop of thistles or similar weeds, due to the immense, number of wind-borne seeds that are in or on the ground. The clouds of thistledown that one so "frequently sees moving along in the wind are a striking testimony to the effect of wind on seed-dispersal. The fact that forested country may annually have virtually tons of weed-seeds falling on the ground, and yet there is no establishment until the forest has been opened up or removed, can be viewed as giving the key to rational weed-avoidance. It shows how difficult if not impossible it is for plants to gain a foothold if suitable conditions for establishment are absent. So far as weed-control is concerned — on grassland, at any rate — is the adoption of methods making establishment difficult that is biologically sound. Running Water and Floods. Streams and rivers, particularly when they are in flood, carrying as they do immense quantities of soil, are an important source of weed-dissemination. Not only seeds, but portions of plants themselves may by this means be transported great distances. The distribution of goat’s-rue over all the land periodically flooded by the Manawatu River is a striking example of the waterdispersal of a weed, and there are many such examples in New Zealand.

Human Agencies.

General Transportation 'of Goods. — -The main distribution of weeds by human agency comes about in the general distribution of agricultural, materials in the course of their ordinary transportation over the country. Oats, chaff, . hay, and other transported fodders, such as feed-meals-that have been insufficiently ground and which are contaminated with weed-seeds, all play an important part in weed-distribution. With regard to certain weeds, notably Californian thistle, contaminated, fodder has been more responsible for rapid and general distribution than has been the use of impure seed for sowing purposes. In the case of Californian thistle a peculiar idea has arisen in districts where thistle-infested oats and chaff are not uncommon —namely, that the seeds of this thistle will not germinate unless they have passed through, an animal. It has been repeatedly demonstrated that such is riot the case ; and one of the plots at the recent Dunedin Exhibition, with an abundant crop of Californian thistle derived from seed taken from an infested line of grass-seed, ' clearly showed the fallacy of such an idea.

The avoidance of weed-distribution in contaminated fodder appears to be almost impossible. The Australian system of prohibiting the sale of such material has in practice been found to be more or less unworkable, and cannot in any way be carried out so as to fulfil its objective. It'certainly must be said, however, that the indiscriminate distribution, of weed-contaminated fodder in New Zealand has and is intensifying our weed problems ; therefore it would appear to be sound to reduce this source of distribution so far as practicable. In the first place, the farmer on clean land should exercise great care in his purchases of fodder, and always bear in mind that the buying of weed-infested material will tend towards reduced production and increased expenditure on his farm. Again, the farmer likely to produce such material should, carry out all practical methods to minimize the damage he is liable to cause the country. In this respect the clear recognition that he is damaging the country at large, by selling badly weed-infested material without doubt would do much good .in influencing him in not growing sale crops on the notoriously dirty portions of his farm until they were more or less adequately cleaned. It should also influence him to take advantage of certain practices with regard to early and late sowing and early and late harvesting, whereby the ripened weed-seed crop would not coincide with that of the crop he grows and intends to sell.

The policy of crop quarantines in different localities; and the prohibition of the sale of weed-infested crops, should, in my opinion, be; regarded as a last resort. Education, rather than compulsion and. restriction, is more likely to have permanent benefit, and the use of clean seed and the production of clean crops should be looked upon by every farmer not only as a necessity but as a duty. Farm Implements.-— These implements are. a frequent source 'of weed-dispersal. This is especially true of threshing-machines and drills. It is hard to see how the position can be improved, but the real recognition that they are sources of contamination would at least do some good, and tend to their better cleaning, particularly in the case of threshing-machines. Stable Manure. — Stable or farmyard manure is often a source .of weed-distribution. Care should be taken to store manure in such a

way that the fermentation processes really destroy the weed-seeds present. The proper rotting of stable manure should be looked upon as essential. Unless such rotting is carried out the . disadvantages of stable manure as a means of infesting land with weeds may outweigh its advantages in increasing the fertility of the land to ...which it is applied.

Animal Agencies.

There are two main types of animals responsible for the dissemination of weedsnamely, ordinary farm live-stock and birds. Ordinary farm live-stock, such as horses, sheep, and cattle, distribute weedseeds either by .carrying them on the outside of their bodies or by eating and then voiding them in their excreta in a condition capable of germination. The influence of live-stock in both these directions is very great—-fortunately, indeed, in many cases to the advantage of the grassland farmer, for useful as well as noxious seeds can be so distributed over wide areas at no cost to the farmer. For example, the spread of white and many other clovers on to land where they have never been sown can be almost wholly attributed to the action of live-stock. The swamp-reclaimed portions of the Ruakura Farm of Instruction offer remarkable examples of the value of stock in spreading useful seeds. Cattle and horses are greater factors in the spread of weeds by means of manure droppings than are sheep, but the .latter carry large quantities of seeds in their fleeces, particularly during certain times of the year.

Just as many seeds are specially constructed to enable their easy conveyance by the agency of wind, so many seeds are constructed by nature in a manner that carriage on the wool or hair of animals is facilitated. The following are some examples of these adaptations :

Minute size of seed enabling it to remain for a considerable time in the fleece of a sheep : Foxglove and mullein are good representatives of this class. The presence of foxglove on a farm, often attributed to the use. of impure seed —a supposition generally wrong —can often be traced to the bringing of stock from foxglove-infested country.

Fruits or seeds with hooks or barbs: Piripiri, burr clover, and Bathurst burr are familiar examples of weeds that are capable of being widely distributed by live-stock.

Fruits or seeds having hairs : Tauhinu and many of the smallerseeded composites are provided with a hairy pappus, and large quantities of such seeds may be carried in fleeces. - Danthonia, with its hairy seed, is regularly distributed over wide areas.

Fruits or seeds that tend to pierce the skin : Barley-grass, hairgrass, rice-grass are representatives of this class. The distribution of the last-named is valuable.

Buying-in of Stock. — ln the purchase of stock considerable attention has often to be paid both to the weeds present on the land they have been grazing on and the time of year they, are bought, in order either to avoid or at times to encourage the distribution of seeds on to any particular farm. Thus, for instance, in coastal districts great care has often to be taken to avoid the purchase of stock from tauhinu country during the period when the seed is likely to be in the fleece, which is in late summer and early autumn. Again, the purchase of stock from piripiri-infested country is quite dangerous for new burn country that

is beginning to lose the fertility derived from the ashes. Foxglove is another weed that is likely to be introduced by bought-in stock. . As has been mentioned, the seeds clinging to the wool of stock are quite likely to prove advantageous in certain cases. . This is particularly true of country which it is desired should come into danthonia. The purchase of sheep from country where danthonia has seeded freely is regularly effected with the express intention of bringing in considerable amounts, of the seed in the fleeces. Birds are. a frequent cause of weed-distribution. Among weeds that are spread far and wide by this means may be mentioned blackberry, ink-weed, and black nightshade. .Waterfowl, again, frequently convey seeds over long distances. The general distribution of the floating buttercup within a few years of its introduction is a case in point.

DISTRIBUTION ON THE FARM. So far as the distribution of weeds on individual farms is concerned, all the methods above enumerated come into play with regard to their actual conveyance, but the actual increase and infestation of any farm, or any portion of a farm, with weeds is largely a matter of what type of management is carried out, and whether it tends to assist or restrict their establishment and spread. The principles here involved will be dealt with later when methods of control are being discussed.

(To be continued.)