WHY WEEDS ARE INJURIOUS. 



A weed has been defined as any plant out of place ; and, in that 

 sense, a wheat plant in a field of turnips is a weed. 



Most weeds do considerable, and some very much, injury to the crops 

 in which they are found. They produce these effects in several ways : 



1. They absorb soil moisture. The amount of water which is taken 

 up by weeds and evaporated from the surface of the leaves is very great. 

 For instance, an average Mustard plant pumps from the soil about four- 

 teen ounces, or seven -tenths of a pint, per day ; a sunflower, thirty-three 

 ounces ; and so on. The transpiration is generally in proportion to the 

 surface of the leaf : but thin leaves transpire, or throw off water, more 

 freely than fleshy ones. Consequently weeds having large leaf-surface 

 draw from the soil and give off' through the leaves a large amount of 

 water, and thereby rob the surrounding plants. Many botanists consider 

 this waste of moisture the most serious injury done by weeds. 



2. They use plant food. Weeds naturally make use of the same 

 food as the cultivated plants among which they grow. Consequently 

 they deprive a crop of a large amount of the available nourishment ; and 

 they rob the succeeding crop as well. For example, an analysis of the 

 Russian Thistle by Snyder showed "that it contains from 12 to 17 per 

 cent, as much nitrogen as there is in clover ; and an ordinary thistle of 

 this kind covering a square yard takes more potash and lime from the 

 soil than two good crops of wheat from the same area." 



3. They shade, crowd, and choke useful plants. Weeds often grow 

 more vigorously than useful plants ; and, as a consequence, they shade, or 

 crowd, or partially choke the seedlings of the desired crop. Black Bind- 

 weed (Fig. 19), for instance, often covers completely a large part of the 

 plants among which it grows. 



4. They increase the labour and expense of cleaning seed. At best, 

 it is difficult to clean many of the smaller seeds, such as clover, grass, and 

 rape seed ; and the difficulty is greatly increased when they are grown on 

 a dirty farm. It is almost impossible to clean clover seed by winnowing. 

 Hence the necessity that the land on which it grows be clean. 



5. They interfere with a regular rotation of crops. A well balanced 

 rotation of crops conserves the fertility of the soil ; but it is often neces- 

 sary to depart from such a rotation when noxious weeds get possession 

 of the farm to give undue attention to the growth of hoed crops, for 

 instance, or almost omit a certain crop altogether for a time, as in the 

 case of the oat crop on a farm overrun with wild oats. 



6. They harbour the spores of injurious fungi. Many of the rusts 

 which attack grain crops find a resting place on weeds of the grass and 

 other families, which preserve them through the fall, winter, and spring. 



7. Lastly, they offend the eye, or are, as we say, an eye&r e to good 



