A plant like the pigweed, which lives only one year, is called an _annual_ and is one of the easiest weeds to destroy. Mustard, plantain, chess, dodder, cockle, crab grass, and Jimson weed are a few of our most disagreeable annual weeds. The best time to kill any weed is when it is very small; therefore the ground in early spring should be constantly stirred in order to kill the young weeds before they grow to be strong and hardy. [Illustration: FIG. 57. WILD CARROT] The wild carrot differs from an annual in this way: it lives throughout one whole year without producing seeds. During its first year it accumulates a quantity of nourishment in the root, then rests in the winter. Throughout the following summer it uses this nourishment rapidly to produce its flowers and seeds. Then the plant dies. Plants that live through two seasons in this way are called _biennials_. Weeds of this kind may be destroyed by _cutting the roots below the leaves_ with a grubbing-hoe or spud. A spud may be described as a chisel on a long handle (see Fig. 58). If biennials are not cut low enough they will branch out anew and make many seeds. Among the most common biennials are the thistle, moth mullein, wild carrot, wild parsnip, and burdock. [Illustration: FIG. 58. A SPUD] [Illustration: FIG. 59. HOUND'S TONGUE] A third group of weeds consists of those that live for more than two years. These weeds are usually most difficult to kill. They propagate by means of running rootstocks as well as by seeds. Plants that live more than two seasons are known as _perennials_ and include, for example, many grasses, dock, Canada thistle, poison ivy, passion flower, horse nettle, etc. There are many methods of destroying perennial weeds. They may be dug entirely out and removed. Sometimes in small areas they may be killed by crude sulphuric acid or may be starved by covering them with boards or a straw stack or in some other convenient way. A method that is very effective is to smother the weeds by a dense growth of some other plant, for example, cowpeas or buckwheat. Cowpeas are to be preferred, since they also enrich the soil by the nitrogen that the root-tubercles gather. [Illustration: FIG. 60. CANADA THISTLE] Weeds do injury in numerous ways; they shade the crop, steal its nourishment, and waste its moisture. Perhaps their only service is to make lazy people till their crops. =EXERCISE= You should learn to know by name the twenty worst weeds of your vicinity and to recognize their seeds. If there are any weeds you are not able to recognize, send a sample of each to your state experiment station. Make a collection, properly labeled, of weeds and weed seeds for your school. SECTION XXI. SEED PURITY AND VITALITY Seeds produce plants. The difference between a large and a small yield may depend upon the kind of plants we raise, and the kind of plant in turn is dependent upon the seeds that we sow.