WEEDS AND THEIR IDENTIFICATION.

New Zealand Journal of Agriculture, Volume XX, Issue 3, 20 March 1920, Page 168

WEEDS AND THEIR IDENTIFICATION.

( Continued.)

ESMOND ATKINSON,

Biological Laboratory.

PERENNIAL SOW-THISTLE (SONCHUS ARVENSIS).

There are many plants among those naturalized in New Zealand which in their native countries are of little importance, but- which have become aggressive here; while, on the other hand, several plants known elsewhere as most objectionable weeds, though they have been observed in the Dominion for a number of years, have not hitherto proved particularly dangerous in this country... Among these may be mentioned the subject of this article, perennial sow-thistle (Sonchus drvensis), which is also known by the names • corn sow-thistle and creeping sow-thistle. f • '

Perennial sow-thistle is a bad weed in England and in many parts of Europe (its native habitat), and in parts of Canada it is looked upon as the worst of all weed pests. While it does not follow from this that perennial sow-thistle may ever reach a position of first importance among weeds in New Zealand, it is already extremely troublesome under certain conditions.

As regards distribution in the Dominion, perennial sow-thistle was first recorded - from Auckland many years ago, and has since been noted in several parts of the Manawatu district, and in Canterbury and Southland. Beyond these broad facts not much is known of its distribution, and further details on this point would be welcome.

DESCRIPTION

Perennial sow-thistle is a tall plant, reaching the height of 3 ft. or 4 ft. ■ It is, as a rule; much more slender than either of the two common sow-thistles (Sonchus oleraceus and S. asp er), - and the stem is only slightly branched; but it is in the root-system that the greatest difference is to be seen, for in the species now under consideration there is a thick wide-spreading horizontal rootstock which throws up leafy shoots at frequent intervals. In general appearance this rootsystem is not unlike that of Californian thistle, but it differs in being shallower,, the horizontal rootstock often being not more than 4 in. or so below the surface of the soil. The whole plant is full of a bitter milky juice.

The leaves are crowded round the lower part of the stem, but become fewer towards its top, so that the part below the flower-heads is practically leafless. The root-leaves are the largest, being 6 in. or more in length ; they are frequently less deeply lobed than the lower stem-leaves, but otherwise resemble them closely. The lobes of all the leaves are irregularly placed, often much twisted, and as a rule their tips tend to point towards the base of the leaf. The lower leaves,

in addition to the large indentations, have their margins toothed with small soft spines, but this character is absent from the upper leaves.

The flower-heads are borne in a loose spreading cluster at the top of the stem in large specimens, or in those in which the top has been cut off, long stalks bearing smaller clusters of heads often arise from the axils of the upper leaves. The heads themselves are quite different in appearance from those of the common sow-thistles ; they are far larger (up to 2 in. in diameter) and brighter yellow in colour, and have much the look of heads of dandelion or catsear.

The involucre (the ring of green bract's from which the yellow rays spring) varies in shape according to the age of the head ; in the bud it is rounded with a flattened top, in the mature flower-head more or less cylindrical, and in the young fruiting-stage conical. As the fruit ripens and the feathery pappus spreads out the involucral bracts become reflexed and lie back against the stalk. All these stages are shown in the illustration. In the common New Zealand form of the plant the involucres and the stalks below them are covered with light-coloured glandular hairs—that is, hairs provided at their tips with minute knobs which secrete 9. sticky susbtance. There is, however, another form of the plant in which no hairs whatever are found. 1

The achenes, or “ seeds,” average about J- f in. in length by /gin. in breadth, and are of a dull dark-reddish-brown colour. In shape they are roughly oblong and slightly flattened, and at one end bear a light-coloured crown, which marks the point of attachment of the pappus. The achenes are longitudinally marked with strong ridges, one on each edge and five on each side, the middle ridge and those at the sides being the most prominent, while in addition the whole surface is covered with deep transverse wrinkles.

SIGNIFICANCE AS A WEED, AND CONTROL METHODS.

In New Zealand perennial sow-thistle is harmful only as a weed of cultivated land, where its strongly developed root-system and its large production of seed render it very aggressive. The ordinary processes of cultivation, unless they are merely preliminaries to further control work, will only have the .effect of breaking up the rootstocks into pieces and, since a small piece is capable of producing a new plant, of further distributing the weed.

1 am indebted to Mr. J. Beverley, Plant-breeder, Central Development Farm, Weraroa, for an account of some experiments carried out by him with a smothering-crop of Weraroa vetchling, a species of Lathyrus, closely allied to the Tangier pea (Z. tingitanus). Part of an area occupied by the sow-thistle was sown down on 7th May, 1919, with the vetchling, which came away well and made a fine growth, and when on 24th January last it was harvested for seed there was no sign of the weed on the surface. One small plant has since appeared, but there is no doubt that in the main the use of the smothering-crop has been effective. Another part of the same thistle-infested area was laid down in grass in 1917, and, except for a few scattered plants none of which are more than 1 ft. from the edge, all trace of the weed has disappeared. Sheep will eat perennial sowthistle readily, and this fact, together with the results of the smothering-

crop and grassing experiments referred to, which indicate that a wellaerated soil is essential to the plant, shows that as a weed of pastures it is negligible, . and that there is no danger of it spreading from cultivated land to land where there is a good sole of grass.

Perennial sow-thistle is thus an excellent example of a plant'which under a certain set of agricultural conditions may be a most serious pest, while under different conditions, in places perhaps only a few yards away, it may disappear entirely from the ranks of weeds. An intelligent study of its behaviour under these •. different conditions should go far to disabuse those people who consider that the significance of any particular plant as a weed is one of its intrinsic characters instead of one which ■ is supplied by environment. This is the fundamental fact on which all efforts at weed-control should be based (though this is far from being the case in practice), and if it'Were thoroughly realized no weed-control legislation would really be required.

The control methods against perennial sow-thistle where. cultivated land has become badly infested may be summed up as follows, two courses being open (i) The 'land should be. laid down in grass; or, (2) where this is ■ impracticable, some smothering-crop, such as that mentioned, or tares and oats, should be used, if. necessary for two years in succession. The planting of potatoes, which are so often looked upon as a good cleaning-crop, is not to be recommended, as the aeration which they give the soil allows the underground stems of -the thistle to last through the season. <

The achenes of perennial sow-thistle are occasionally found as impurities in commercial lines of seeds, particularly in imported alsike and timothy, but they are of much, rarer occurrence than those of the two annual sow-thistles.

. Qualities of a Herd Boar. — The qualities and characteristics of a herd boar are matters of the greatest importance. . Every pig-breeder should know and have fixed in his mind just what qualifications to require in the animal that is to head his herd. A sow directly affects only the pigs she farrows, while the boar affects every litter in the herd. ■ The most important qualities required in a boar are : Good, big bone ; well grown ; a long, wide, deep body ; a level back, with short neck and head ; a wide, deep, roomy chest,. low hams, short legs, and standing well on his feet ; fairly large ears, wide between eyes, good colour, fine hair, good action, gentle disposition, yet showing strong masculine qualities and vigour ; from a good family which is prepotent, prolific, and has quick feeding-qualities. If the sows are weak in certain qualities it is well that the boar be strong in those qualities. The breeder should always keep in mind an ideal of the best type of conformation, and try to improve his herd by mating such .animals as will come nearest to bringing about that ideal. Remember that it is the best cuts and the highest quality of dressed pork or bacon that is the ultimate object, and not points of the show-ring. — K. W. Gorringe, Instructor in Swine Husbandry.