PLANT PKOBLEMS 71 



germinate the first year than if they are plowed under deeply. 

 Some weeds of this class chick weed, peppergrass, shepherd's- 

 purse germinate late in the summer or fall, live through 

 the winter, and are known as " winter annuals " in latitudes 

 where this occurs. 



A biennial. These plants complete their life cycle in two 

 years and hence rapidly disappear if prevented from maturing 

 seed. Burdock, bull thistle, mullein, teasel, and wild carrot, 

 all immigrants from the Old World, are familiar examples. 



A perennial. Couch grass (a native American), Canada 

 thistle, curled dock, plantain, wild onion, milkweed, bind- 

 weed, most of them importations from the Old World, are 

 examples. Most of the hard problems relate to weeds of this 

 class. The plants, besides producing seeds, live from year to 

 year in the ground by means of perennial crowns, tubers, and 

 burrowing rootstocks. Cultivation which would eradicate an- 

 nuals or biennials may serve merely to subdivide and increase 

 these pests of the soil. 



The fact that certain weeds tend to thrive best with certain 

 crops suggests rotation as a means of control. Land should 

 have a radical change of treatment, if badly infested with 

 weeds. Annual weeds of gardens and cultivated fields die 

 out if the land is seeded to grass. This applies to grainfields 

 which have become seeded to chess, cockle, or ragweed. Pas- 

 ture lands " run out " to perennial weeds should be planted 

 to annual crops. Thus a system of field rotation and general 

 management may be adopted which shall give weeds the least 

 possible chance to survive. 



Chemical weed killers, herbicides. Salt or strong brine, arse- 

 nate of soda, coal oil, crude sulphuric acid, carbolic acid, and 

 sulphates of copper or iron are the chemicals usually employed 

 and suggest interesting experiments for the lawn or garden. 



Medicinal plants. Many crude drugs and aromatic herbs 

 used as condiments are now imported, although they grow in 



