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During the past decade, sunflowers (Helianthus annuus) have been frequently grown for silage in the United States. Their wide distribution as a weed, especially in the semiarid Western States, adds further to the interest and value of a knowledge of their root habits. Plants of the Russian variety were seeded on both upland and lowland soil near Lincoln, Nebr., and their root habits studied at different stages in their development.
Early Growth.--On June 2, when the plants were about 5 weeks old, they had reached a height of 15 inches in the rich silt loam of the lowland. The strong taproots penetrated quite vertically downward to a working depth of 18 inches, some extending slightly into the third foot of soil. Numerous rather widely spreading branches occurred, being especially abundant in the surface 6 inches. These spread laterally, some nearly parallel with the soil surface, to a maximum distance of about 12 inches.The older ones were well furnished with smaller branches. In the drier and less fertile upland plots, the plants were only 10 inches tall, but here, also, the extent of roots exceeded that of the tops, the deepest roots ending in the clay subsoil at the 15-inch level.
Later Development.--A second examination was made when the crop was 2.5 months old. The plants, spaced only 20 inches apart, had made a vigorous growth. They were nearly 7 feet tall and had a stem diameter near the base of almost 1.5 inches. Each individual was furnished with 35 to 40 active green leaves. The flower heads were fairly well formed, and a week later, the plants would have been in bloom.
The major portion of the root system is shown in Fig. 111. The roots occurred in such great numbers that it was quite impossible to represent all of them in the drawing without confusion. Hence, the front portion of the plant, with its accompanying roots, was removed before the penciled draft was made in the field. The bulk of roots occurred in the surface 1.5 feet of soil,
Fig. 111.--Root system of a 2.5-months-old sunflower.
The enlarged taproot gave off so many laterals and tapered so rapidly that, at a depth of 8 to 10 inches, it was only 4 to 5 millimeters in diameter and, in fact, no larger than some of the major branches. The taproots penetrated nearly straight downwards to a depth of about 5 feet. In the surface 6 inches of soil, 28 large laterals originated. Some of these ran obliquely, at various angles with the taproot, spreading rather widely, and reached depths of 2 to more than 3 feet. Numerous others took a course more or less parallel with the soil surface and ran to distances of 3 to 4 feet, where they ended in the surface 6 inches of soil or, more rarely, turned downward. One large lateral was traced to a distance of 5.5 feet from the base of the plant. The main laterals gave off few or no large branches but were thickly clothed with smaller ones. The surface 10 inches of soil, especially the first 2 feet on all sides of the plants, were so densely filled with great masses of branched and rebranched roots of all sizes that they formed a complete network. Indeed, a more profusely developed absorbing system can scarcely be imagined. The roots were less abundant and more poorly furnished with laterals below the first foot. Even in the third foot, however, glistening white, poorly branched roots were quite abundant.
The vigorous transpiration rate and the high water requirement of sunflowers are well known, 600 to 700 pounds of water being required to produce a pound of dry matter aboveground. A, single vigorous sunflower plant may produce 2 pounds of dry substance and use 150 gallons of water. 118 A comparison of the root habits of corn (Figs. 84 and 85) with those of sunflower shows that the sunflower absorbs water and nutrients from the same soil levels as the corn. In addition, the broadly expanded leaves of the sunflower shade the corn plant. Clearly, there is no place for sunflowers or other weeds in cultivated crops and especially in semiarid regions where lack of sufficient water is the chief limiting factor to crop production. The results of competition with weeds are dwarfed plants and decreased yields.
Variations in Root Habit under Competition.--Roots of sunflowers, like those of other plants, are greatly modified in their development as a result of competition with other species or individuals. In one experiment, plants were grown 2, 8, and 32 inches apart, respectively, in three different plots. Competition for light occurred in all three fields, but it started earlier and was more severe in the thicker plantings. Repeated soil moisture determinations showed that the more vigorous, widely spaced plants were drawing upon the soil moisture to a degree very similar to that of the thicker plantings.
On July 21, when the plants were over 2 months old, the roots were excavated and the data given in Table 11 obtained.
TABLE ll.--EFFECT OF COMPETITION UPON DEVELOPMENT OF SUNFLOWERS Spacing Height Diam. Aver. Total Aver. Aver. Max. Working Max of plants inches at base no. of leaf dry wt. depth root depth spread in inches mm. leaves area of tops of tap depth of of sq. in in gm. root, feet later- later- feet als, als, inches inches 2 30 4.5 8 18 1.5 5 6 12 10 8 45 12 16 328 17 6 7.3 37 27 32 50 29 32 3,426 148 8 9 47 42
A representative root from each of the thicker plantings is shown in Figs. 112 and 113, and Fig. 111, except for its lack of depth of both taproot and laterals and slightly greater lateral spread, is very representative of the 32-inch plantings. This illustrates the fact that rate of planting of cultivated plants or their competition with weeds results in pronounced modifications of both aboveground and belowground parts. Ample development of roots as well as tops is imperative for maximum yields.
Fig. 112.--Root of mature sunflower where plants were spaced 2 inches apart.
Fig.113.--Root of sunflower where plants were spaced 8 inches apart.
The sunflower has a pronounced taproot which develops rapidly and penetrates nearly vertically downward. Usually, its depth exceeds the height attained by the plant, and this holds true for all stages of growth. Numerous strong laterals appear early in the development of the plant. These mostly originate from the enlarged portion of the taproot which usually occupies 4 to 6 inches of surface soil. Often, 30 to 45 large laterals occur on a single plant. These spread widely, usually 2 to 5 feet, some running in so nearly a horizontal direction that they end in the surface foot of soil. Others turn downward but seldom reach depths greater than 3 to 4 feet. The taproot, however, clothed throughout with small branchlets, reaches depths of 5 to 9 feet. The very numerous but mostly small branches clothing the laterals furnish an extensive and efficient absorbing system, especially in the surface 2 feet of soil. Since the same soil volume is also occupied by the roots of most cultivated plants, sunflowers as weeds strongly compete with crops for water and nutrients. The roots are quite as much modified as are the tops by thick or thin planting.