WEEDS AND THEIR IDENTIFICATION.

New Zealand Journal of Agriculture, Volume XXXVI, Issue 1, 20 January 1928, Page 44

WEEDS AND THEIR IDENTIFICATION.

( Continued.)

Esmond Atkinson,

Biological Laboratory, Wellington.

PIGWEED (AMARANTHUS RETROFLEXUS) AND LADY’S THUMB (POLYGONUM PERSIC A).

Certain plants have been illustrated and described from time to time in this series for the reason that they were newcomers, with their potentialities as weeds here little known. It is plainly desirable that in the case of a plant rapidly spreading through the country as many people as possible should be able to recognize it when they see it. Other articles have dealt with plants noted for their poisonous properties. Attention is now drawn to another class, consisting of that great army of common weeds of every type of country, among which experience shows that much, confusion still exists. Some plants not resembling each other particularly closely have similar or identical popular names ; others may be superficially alike but occupy quite different positions economically..

It is intended to use many of the drawings of this series hitherto published and all of those appearing from now onwards as contributions to a complete set illustrating the whole of the weeds found in New Zealand. It seems to the writer that a verbal description of a plant is as a rule either too technical to be of much use to any one but a botanist, or else too 4 vague to ■ convey at all a definite idea of its appearance. What is being aimed at, therefore, is to make the illustrations the final court of appeal, only such facts as colour, scent, variation in size, &c., being described in words.

The drawings here presented illustrate two common weeds about which inquiries are often received at this Laboratory.

Pigweed.

This plant, which is an annual of tropical American origin, is now widely distributed as a. weed of waste places and of . arable land in the warmer parts of New Zealand. It is commonest north of Auckland, and that is where it is most conspicuous as a weed, but it has been found in parts of the South Island, particularly in the Nelson District, and has been reported from Timaru northwards. It belongs to a small family which is more closely allied to the beets and fathens than to any other. ,

Fig. i shows the general appearance of the plantnatural size and at. (b) one of its most characteristic features is drawn in outline. This is the long, red, rather fleshy tap-root, which is the name given to roots descending straight down into the soil without branching much. It is this root that gives pigweed its other popular names of redroot and redshank. The stem is rather thick, green, I ft. to 3 ft. high, and branching to a certain extent, though small plants bearing ripe seeds may often be seen with only a single stem —upright and unbranched. The flowers are greenish, very small individually, but in a finger-shaped spike sometimes several inches long. There .is a

single seed in each flower, the seed being very small (c), more or less lens-shaped (see (d), which shows the seeds from the front and side enlarged), brilliantly shining, and black or brown in colour. As the flowers do not fade in the way that those with conspicuous petals do, little change is visible from the bud onwards, so that many plants apparently in their early stages will be found to be full of ripe seeds.

Generally speaking, proper cultivation will check the spread of pigweed, the thing to aim at being to get rid of it on the first appearance of the flower-spikes. This is the safest way to make sure that no seeds fall to the ground, where they have the power of remaining dormant for many years. They are often found as impurities in various commercial lines of seeds, especially clovers.

Lady’s Thumb,

Lady's thumb is a European annual now widely distributed, belonging to the family which includes the docks, sorrels, rhubarb, and buckwheat. There are a number of popular names besides the one used here-—eg., persicaria, smartweed, and knotweed while it shares with pigweed the name redshank.

The plant varies in appearance from a compact one a few inches high (when it is in the open by itself) to one with straggling stems several feet long (when it is growing among other plants). The drawing shows the shape of the stems with their swollen joints which give it the name “ knotweed,” while “ redshank ” describes their colour, which is red or purplish - red, varying in depth with the amount of light that falls on them. The leaves generally have a dark but dim irregular blotch (Fig. 2, a and &), and this distinguishes the weed from other common ones. The clusters of small flowers are pink, and the shining black seeds are of two distinct shapes— -flattened, ■or triangular in section like a dock-seed (c).

■■ Lady’s thumb, which is common throughout both Islands, is a weed of damp cultivated ground, and needs a fair amount of moisture for its full development. It is then often very troublesome, particularly in pastures and also in grain crops, where, owing to its growing faster than the cereal, it chokes out the young plants. Thorough ■cultivation time to prevent the formation of seedis necessary to hold this weed in check. The seeds occur as impurities in American samples of clover.

Arthritis in Lambs.—“ An investigation into the cause of this condition was undertaken during the past season,” states the Officer in Charge, Wallaceville Veterinary Laboratory, in his annual report for 1926-27, “ and specimens were received from four meat-works. An organism has been isolated from these with ■such frequency and uniformity as to strongly suggest that it is the cause of the arthritis, and this is further borne out by the fact that arthritis can be caused in lambs by injecting this organism into the circulation. It now remains to discover how the organism gains entrance to the body, before means can be taken to prevent its occurrence, and this will involve field observations rather than laboratory work.”