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Weeds may be grouped with reference to their duration under the following classes: Annual, biennial, and perennial. An annual plant is one which germinates from seed in the spring, produces flowers and seed the same season, after the accomplishment of which it usually dies. Examples of annual weeds are foxtail, ragweed, and smartweed. The members of this class vary greatly. Some annuals approach biennial in habit and are called winter annuals.
Seeds of winter annuals germinate in the fall and produce a good growth until checked by frost. In the succeeding spring they make rapid growth, mature fruit, and die. Examples of winter annuals are speedwell, shepherd's purse, and chickweed.
The biennial plant, during the Fig. 1. Canada thistle first season, produces vegetative seed; a common source of growth only, this often consist- Breading the weed. (C. ing of a rosette of leaves close to M. King.) the ground. In the second season, a flower stem is produced. Examples of biennial weeds are bull thistle, mullein, burdock, parsnip and carrot. Biennial weeds do not appear when the ground has been properly plowed.
The perennial plant has a natural existence of more than two years. These plants produce stems and roots which send up flower stalks year after year. Examples of this class are morning glory, milkweed, horse-nettle, and horse-radish.
How Weeds Spread.—Weeds are spread by means of seeds, by vegetative reproduction, or by both seeds and vegetative reproduction.
Reproduction by Seeds.--Most weeds reproduce themselves by seeds. One of the exceptions to this rule is the horse-radish, which does not, so far as we know, seed in Iowa.
It has been our observation that the Canada thistle usually does not seed in Iowa, although specimens of heads containing seed have been received from different parts of the state. It is also probable that the Canada thistle does not seed so frequently in Iowa or in the United States as in
means of roots, stems, or both. In - quack grass, one means of multiplication is by stems commonly called “roots,” which are divided into a series of joints at which new shoots are produced. The same structure occurs in germander or wood sage. Horse-radish may be propagated by roots exclusively. In another type, like the
Fig. 2. Woolly thistle, a type of perennial r00t.
weeds multiply by
Canada thistle, morning-glory, and horse nettle, a small part of the underground portion is stem, the rest being
true root. On these roots buds are produced which send
up new shoots each year. Some plants, like wild onion, produce bulblets. In
others, as crab grass, the stem above the ground may strike root at the nodes, or roots may be produced at the joints, as in purslane. These roots and stems, capable of producing new plants, are widely scattered in fields by means of the cultivator and plow. They may be dispersed with undecomposed manure, packing materials, or imported fruit trees. Mice and gophers may scatter roots to different parts of the field. Character of Root Systems of Weeds.-The root systems of weeds vary greatly. The term root, as ordinarily used by the farmer, may mean a rootstock, as in the case of quack grass or nimble will. A great many weeds, especially perennials, have not only perennial roots but rootstocks also. A rootstock is simply a stem growing beneath the surface of the ground. Many weeds have strong tap roots, this being especially true of biennial weeds like the Canadian lettuce, mullein, hemp, cocklebur, wild carrot, ragweed, prickly lettuce, pigweed, mayweed, lamb’s quarter, bull thistle, and field thistle. The roots of many annual plants are fibrous and without any distinct tap roots. Moreover they are shallow, like those in the buckhorn, foxtail, plantain, yellow Oxalis, and bootjack. The roots of plants in the same order may differ greatly, but their general habit depends a little on the character of the season. During moist seasons they become quite shallow, while after the season becomes drier, they descend obliquely. The common spurge (Euphorbia Preslii) has a straight taproot with horizontal roots near the surface of the ground which descend obliquely later. The common field thistle (Cirsium discolor) has a straight tap root with portions frequently enlarged bearing several more or less prominent lateral roots. The cocklebur, which belongs to the same family, has a tap root which is considerably thickened near the surface of the ground, and which has large lateral roots. It
RINDS OF WEEDS AS TO DURATION II
may likewise produce a few nodal roots, but these are generally small and fibrous. The large ragweed (Ambrosia trifida) has a straight tap root with numerous small fibrous roots that descend obliquely into the lower strata of the soil. The horseweed of the same family has a straight tap root, numerous small fibrous roots, and one or more prominent lateral roots. These are at first horizontal, descending obliquely later. The Spanish needle (Bidens frondosa) and its ally, stick-seed (B. discoidea), are frequently found in moist places. Although they belong to the same family as the sunflower and ragweed, they do not ordinarily produce tap roots, but large lat
1 - d Fig. 4. Root of common milkweed, era. roots instead, single root 14 feet long, with numerous
which soon descend lateral roots. The points indicate
- e where new roots and stems are formobliquely into the ing. Root method of spreading the ground. The Canadian weed. (C. M. King.)
canadensis), with a horizontal rootstock, produces small lateral roots which soon begin to descend. The ox-eye (Heliopsis scabra), related to the goldenrod, also produces a rootstock from which grow fibrous roots, horizontal at first and then gradually descending. In the common mayweed of the same family the straight tap root produces numerous fibrous roots which descend obliquely. The buckhorn and plantain produce a large number of whitish, fibrous roots which are at first horizontal and then descend gradually into the soil. In some plants, as in cowbane, the roots are fascicled. In the wild parsnip and the garden parsnip the