222 WEEDS 



fallowing is often practiced to clear out weedy land by the method just 

 described ; but usually a cultivated crop may better be grown. 



For biennials, which also reproduce from seed, mowing them when 

 coming into flower or cutting the roots below the crown is usually 

 effective. Autumn is the best time for such grubbing. Biennial 

 weeds are readily killed by such tillage as is given to hoed crops. 



For perennials which reproduce both from seed and from surface 

 runners or perennial underground roots or stems, seed production must 

 be prevented and the underground part must be killed. Seed production 

 may be prevented by mowing when the first flower-buds appear. The 

 best methods for killing the roots or rootstocks vary considerably 

 according to the soil, climate, character of the different weeds, and the 

 size of the patch or the quantity to be killed. In general, however, the 

 following principles apply : 



1. The roots, rootstocks, bulbs, and the like, may be dug up and 

 removed, a remedy that can be practically applied only in small areas. 



2. Salt, coal oil, or strong acid applied so as to come in contact with 

 the freshly cut roots or rootstocks destroys them for some distance from 

 the point of contact. Crude sulfuric acid is probably the most effec- 

 tive of comparatively inexpensive materials that can be used for this 

 purpose, but its strong corrosive properties render it dangerous to 

 handle. Carbolic acid is less corrosive, and nearly as effective. Arse- 

 nite of soda and arsenate of soda, dangerous poisons, are effective, 

 particularly the former, applied as a spray on the growing weeds. 

 Fuel-distillate, a petroleum product, is very promising. 



3. Roots may be starved to death by preventing any development 

 of green leaves or other parts above ground. This may be effected 

 by building straw stacks over small patches, by persistent, thorough 

 cultivation in fields, by the use of the hoe or spud in waste places, and 

 by salting the plants and turning on sheep in permanent pastures. 



4. The plants may usually be smothered by dense sod-forming grasses 

 or by a crop like hemp, buckwheat, clover, cowpeas, or millet that will 

 exclude the light. 



5. Most roots are readily destroyed by exposing them to the direct 

 action of the sun during the summer drought, or to the direct action of 

 the frost in winter. In this way plowing, for example, becomes effec- 

 tive. 



6. Proper crop rotation is one of the best means of eradication. 



