WEED – CONTROL.

New Zealand Journal of Agriculture, Volume VII, Issue 1, 15 July 1913, Page 34

WEED - CONTROL.

THE SCIENTIFIC ASPECT.

A. H. Cockayne.

Under natural conditions no plant, of any kind can be looked upon as a weed. In such circumstances the mastery of one plant over another, and of one type of vegetation over another, is only a natural sequence occurring through the aggressive plant or aggressive type being more suitable for the existing conditions of the locality than were the previously dominant types of the vegetation. This replacement that is continually occurring in nature must be caused through some modification in the action of the factors that influence plant-growth. These modifications may be caused either externally through an increase or decrease in the intensity of one or more of the factors uninfluenced in any way by J he vegetation, or internally by a change brought about in their action through the direct influence of the vegetation during the course of its development. . Any modifications in the factors influencing plant-growth will, under natural conditions, only take place slowly, and especially will this be true with external changes. Internal changes may, however, operate fairly rapidly, and in such cases the vegetation must be looked upon as a temporary one, liable, to rapid replacement. Associations of plants that are liable to be replaced through the growth of the individuals altering the existing conditions for plantgrowth may be looked upon as instable associations, and those types that do . not alter the conditions and are not liable to be replaced can be termed stable formations. Thus invasion, replacement, and migration of plants can, under natural conditions, be looked upon as the occupying of the land by those plants which individually and collectively are best suited to the conditions, and there is no sense of desirability for any special plants so far as the land itself is concerned. As soon as the land is occupied by man and utilized for any specific purpose there arises the desire to grow certain plants to the exclusion of others, and those that are not desirable from the personal aspect are termed weeds. Whenever undesired plants grow freely in any locality it is evident that their requirements are more perfectly attuned to the conditions than are those of the desired plants. The whole

object of soil-utilization is to grow successfully only those crops that are required to the exclusion of all other plants. When the conditions are unsuitable for the development of certain plants their culture has either to be abandoned or else the conditions must be artifically modified. to adjust them to the requirements. Where the requirements are equally suitable for the development of both the required crop and for weeds, and they cannot be modified so as to render them unsuitable for the development of the. latter without injuring the prospects of the required crop, then special methods for the mechanical removal of the weeds become necessary. This necessity, of course, -becomes greater in proportion to the increase in the conditions favouring the development of the weeds over those favouring the plants that it is desired to grow. The principles governing natural replacement through internal modifications are well illustrated by crop rotation, where the - ' growing of one crop should in all cases improve- the conditions for the succeeding one; and when this method of farming is properly carried out the suitable conditions for the development of undesired plants are largely eliminated, and the weed question naturally becomes automatically controlled. Under natural conditions pure associations consisting of one species are not unusual in certain cases, but they generally modify the conditions for plant-growth, and in consequence are less stable than are those consisting of a large number of species. Here the modifications caused by one species are often counterbalanced by those changes exerted by other species, and the struggle is largely one between the different components, leading either to an equilibrium or else to a comparatively pure association when the conditions favour the development of one species over any of. the others. When this stage has been reached the stability of the association becomes considerably lessened, and quite a new type of vegetation, is likely to become established. In pure associations that are not properly closed— i.e., the limit of the individuals that the ground can support has not been reached —new plants which are as suitable for development as that species which is already dominant may enter into competition. In ordinary farming a great deal of the land is given over to growing pure associations which are naturally unstable, and in very many instances they cannot be looked upon as closed. In consequence of this, invasion by undesirable plants is the universal rule on land devoted to agriculture. The intensity of the invasion is in all cases directly correlated with ’ the degree of suitability of those conditions favouring their development.

Wherever artificial conditions are introduced by man they should in all cases be such as favour the development of the plants that it

is desired to grow, as otherwise they must increase the difficulty of reducing the growth of the undesired elements to a minimum, and thus intensify the weed problem. The effects of man’s operations, the utilization of land, must only be looked upon as altering existing conditions vastly more rapidly than can unaided nature and in those special directions that are necessary for the successful production of crops. . The primal principles that govern natural invasion and replacement must be the same as those which operate in what may be termed artificial invasion and . replacement. The study, therefore, of the behaviour of the plants under natural conditions should assist very largely in determining the factors which have to be controlled at will to avoid and control the invasion of farming-land by undesirable plants. It is thus seen that in the control of weeds the determining of how to alter the factors that' influence their spread is really of much greater influence than the mere adoption of mechanical methods for their partial or total. removal, although these may be the only practical ones to adopt in many individual cases where the weeds concerned show a great range of adaptability for varying conditions.