48 



DISCOVERY 



as the black nightshade (Solatium nigrum} are allowed, 

 or even if self-set potatoes are left to beconic " weeds," 

 the pest may support and propagate itself on them, so 

 that it remains in existence until potatoes are again 

 cropped in the field. Cereal pests are carried over from 

 one rotation to another by grasses which can usuallv 

 support them, while weeds like ciiarlock, wild mustard, 

 etc., can act as hosts for turnip and cabbage pests. 



Weeds may also be poisonous, and if such are allowed 

 to appear in pastures, they may poison stock and the 

 farmer may suffer much financial loss. Fortunately, 

 this is a comparatively rare occurrence in England, 

 but in Colorado the whorled milkweed has become a 

 very dangerous pest owing to its poisonous property. 

 In one county a lo-acre patch of this weed was respon- 

 sible for the loss in 1917 of sheep worth 20,000 dollars, 

 and in 1918 for 18,000 dollars worth. Another case 

 is quoted where a flock of sheep was driven over a sec- 

 tion of the country where this weed was growing, and 

 the sheep, being very hungry, ate with disastrous 

 results, for it is stated that the bodies of 400 sheep had 

 to be removed before a motor could get through. 



California suffers in a different way from a weed 

 which has been introduced from the borders of the 

 Sahara, probably in ballast. This is the Puncture 

 Vine (Tribulus terrestris), which produces a number 

 of prostrate stems, some eight feet long. At each 

 joint a number of burs are produced, and each bur 

 possesses two or more sharp spines about the size of 

 carpet tacks. These are so disposed that, whichever 

 •way the bur falls, one spine will always point upwards. 

 These will readily puncture a bicycle tyre, and even a 

 motor tyre if it is somewhat worn. 



One does not readily associate weeds with water, 

 although duckweed will soon be recalled to mind, as 

 ■well as deeper growing plants which spoil fishing, and 

 even navigation. The American pondweed (Elodea 

 canadensis) was introduced into England and became a 

 menace to navigation in some of our waterways, but 

 in Florida and Texas they have even a worse pest of 

 rivers, namely the water hyacinth. It is a beautiful 

 aquatic plant and is highly prized for use in aquaria in 

 some parts of America, but its spread in St. John's 

 River and the enormous sums spent in attempting its 

 suppression have earned it the name of the " million- 

 dollar weed." It seems to have obtained an entrance 

 into the river from an ornamental pond, and now not 

 only seriously impedes river traffic, but causes an annual 

 loss of one-fourth of the value of the logs rafted down the 

 river. It also interferes with fishermen setting their 

 nets, piles itself up under low bridges until the pressure 

 has overturned them, and it collects in masses and 

 becomes a menace to health when it decays. A curious 

 instance of loss caused indirectly by it has recently 

 occurred. A couple of aviators were circling over the 



central portion of Florida, and spied what they thought 

 was a beautiful green meadow. They came down on 

 it, only to find that they had alighted on a vast expanse 

 of water hyacinths, under which were several feet of 

 water, and it was only at great expense that the two 

 aeroplanes were salved. 



In conclusion, a word as to some of the means of 

 getting rid of weeds. There is no very easy way, 

 especially when they are in fields. The farmer has to 

 do what he can when there are no crops on the ground, 

 but this occurs rarely, except during winter, when the 

 weeds are dormant. He therefore tries periodically 

 and systematically to rid his fields of these pests, and 

 takes what he calls a cleaning crop. This is usually a 

 crop such as potatoes, turnips, or mangolds, for during 

 most of the summer when the weeds are growing he can 

 either turn his men in to hoe up the weeds, or else use 

 horse- or tractor-driven machines to do the same work 

 without damaging the crop. When grain is grown, 

 hoeing or cultivating can only go on for a short time 

 after the plants have appeared above ground, but 

 in the case of one of the commonest and most harmful 

 weeds, namely charlock, the j-eUow-flowered weed so 

 obvious on badly managed farms, spraj-ing is resorted 

 to. Here crop and weeds are sprayed with a solution 

 of copper sulphate (blue vitriol), which scorches and 

 blackens the charlock as it will most plants. The 

 question at once arises, " Why does it not kill the wheat 

 or oats ? " For the very simple reason that it runs off 

 the upright glossy leaves of the grain, and lies on the 

 hairy broad leaves of the weed. Sulphuric acid and 

 ferrous sulphate (green vitriol) are also used, bracken 

 being eradicated from grassland by the former. 



Lawn sand recommended for the removal of weeds 

 from lawns works on the same principle. It consists of 

 sand and ammonium sulphate mixed together, and on 

 being scattered over the lawn, lies on the broad-leaved 

 weeds, scorching them. .-Vmmonium sulphate, more- 

 over, is a valuable nitrogenous manure, and when in 

 solution, reaches the roots of the grass and stimulates 

 growth so that grass is enabled to crowd out the 

 exhausted weed. 



Deep-seated and thick-rooted weeds, such as docks, 

 dandelions, thistles, etc., must either be dug up where 

 this is possible, or else continuall}' cut down. Gradually, 

 if the leaves which supply the food to the rootstock 

 are not allowed to develop, the weed must die when it 

 has used up its stored food-material. Such weeds are 

 extremely tenacious of life, or they would have become 

 extinct long ago. 



Some weeds, especially sorrels, are characteristic 

 _ of acid soils, and these will tend to disappear when land 

 is neutralised by lime. Other weeds can be kept under 

 by sowing down arable land to grass for a few years. 

 {For Books of Reference, see p. 50 ) 



