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The strawberry (Fragaria chiloensis) is an herbaceous perennial. The very short, thick stems occur close to the surface of the soil. Although not always included among vegetable crops, its wide distribution, the temporary nature of the crop, and its common occurrence in gardens warrant its inclusion here. It is the most valuable of the small fruits grown in the United States.150 It is indeed a cosmopolitan plant, growing and thriving under a wide range of conditions. The best crops are usually grown in rich, rather moist soil during a cool season.
Mature Plants.--A strawberry of the Dunlap variety, one of the most important commercially, was studied on an area of silt loam soil about a mile distant from the experimental field in Lincoln. The plants were excavated June 24 when blossoming and fruiting were nearly completed. They had grown 3 or 4 years in this field.
The underground parts are characterized by a dense network of fibrous roots which arise from the much-thickened, short, scale-covered stern (Fig. 49). Plants of average size possessed about 36 roots, each approximately 1 millimeter in diameter, and about 9 smaller ones. These fibrous roots radiated outward or downward in all directions from the horizontal to the vertical. Many spread outward somewhat obliquely and turned downward. A maximum lateral spread of 12 inches and a maximum depth of 37 inches were ascertained. The root system was characterized by a dense network of fibrous roots in the 10 to 12 inches of soil next to the surface. In the surface foot, and especially in the upper 6 inches, the roots were rebranched at the rate of 3 to 10 fine rootlets per inch. The branches were usually quite kinky And varied from 0.3 to 6 or even up to 10 inches in length. The longer ones usually- occurred on the older roots. The primary branches were rebranched at the rate of 3 to 8 per inch with laterals 0.1 to 1 inch long. The longer secondary laterals were again rebranched. Where the main root had been cut or otherwise injured, 3 to 5 long branches frequently arose from the root tip. Such branches usually extended only slightly horizontally but nearly always ran rather directly downward.
Fig. 49.--Underground parts of a 3-year-old Dunlap strawberry excavated in June after blossoming and fruiting.
Below the surface foot of soil the rate of branching was somewhat less, usually 3 to 6 rootlets per inch, and the branches were shorter, although rootlets 3 to 4 inches long were frequently found. The first 2 inches of roots near the stem were branched but little. Occasionally a root 1.5 millimeters in diameter, originating from the rhizome, was more profusely branched than the rest, the dense branching being largely confined to the surface foot of soil.
The young roots are white in color. As they become older, their color changes to a fight brown and finally to very dark brown or nearly black. Plants starting from runners have many long, young, light-colored roots.
Summary.--The fibrous root system of the strawberry arises from the short, thick stems near the soil surface. Just beneath the surface horizontal roots extend about 1 foot on all sides of the base of the plant. This delimits the lateral spread. The surface foot of soil is fully ramified by obliquely descending roots as well as many more or less vertically descending ones. The latter, especially, also ramifies the second and some of them the third foot of soil. Branches are mostly short but abundant, usually being more profuse in the surface 12 inches. The root system is relatively shallow and not extensive.
Other Investigations on Strawberries.--A plant of Triomphe de Grand variety of strawberry was washed from the soil at Geneva, N. Y., in midsummer. The horizontal roots were found to be few and short, the longest being traceable only 6 inches. The greater part of the roots extended nearly perpendicularly downward, and nearly, all the fibrous roots were found directly beneath the plant. The roots reached a depth of 22 inches. New roots were found growing from the rhizome about 1 inch above the old ones. The longest of these had attained a length of 6 inches. They were white and tipped at their extremities with a thickened point. 42
Strawberries of the Warfield variety were studied at Madison, Wis. The plants were 3 years old. They were washed from the soil late in May when the fruit was maturing. The roots were contained within a very small soil volume. The deepest roots extended a little less than 2 feet and the horizontal ones reached scarcely beyond the area covered by the leaves. Most of the roots grew downward and all but the merest fraction of them were contained within the first foot of soil. The soil was a light, clay loam which was underlaid at a depth of 1 foot with a subsoil of sandy clay. The soil had been cultivated about 3 inches deep. 45
It was found at the same station that certain herbaceous plants, including the strawberry, start growth extremely early in spring. As early as Mar. 22, when the ground was free from frost in places receiving the most insolation, the roots of strawberry had made considerable growth. Root growth in spring is most active near the surface of the soil which is first warmed by the sun's rays. "Thus the parts of the soil from which roots are in a measure excluded during the dry weather of summer may serve as a feeding ground for them in early spring." 46
That there is considerable difference in the root habits of varieties is indicated by the statement that some varieties which are usually a failure in dry climates because of their deficient root system are enabled by irrigation to flourish to such a degree as to be among the most profitable. 6
The root system of the wild strawberry (Fragaria virginiana) has been examined where the plants were growing in a spruce forest. The lateral spread and depth of penetration were very similar to that described and illustrated for the cultivated variety. In drier habitats it would undoubtedly be more extensive. 167
Root Habits in Relation to Cultural Practice.--The fact that native strawberries, from which the cultivated varieties have arisen, grow in nearly all types of soil from seashore to mountain top, indicates a wide range of soil and climatic conditions for cultivated plants. 19 The rather limited root range of the plant easily explains its well-known susceptibility to drought as well as its ready response to fertilizers. It emphasizes the importance of providing irrigation in regions of moderate and fluctuating rainfall if large and regular crops are to be obtained. The need of a rather constant water supply about the roots is indicated by the distribution of native strawberries in dry grassland areas. Here they are found in the better-watered soils in ravines and along creeks and river bottoms, and often fringe thickets and woodlands. Perhaps an ideal soil would be a sandy or gravelly loam underlaid with a pervious clay, i.e., one retentive of moisture yet easily tilled. By proper methods of tillage, mulching, and irrigation much can be done toward furnishing the roots with a constant moisture supply.
Soil for strawberries should be thoroughly prepared. Deep plowing and thorough pulverizing a considerable time before transplanting encourages the roots to penetrate more deeply, thus lessening the danger from drought and injury from cold. Soil thus prepared will catch and retain more moisture than a poorly and recently plowed soil.
In transplanting, only large, sturdy, young plants with a vigorous root system should be selected. These plants have thick, turgid, light-colored roots quite in contrast to the dark, less vigorous appearing roots of older ones. A plant with a healthy root system and a comparatively small top is much to be preferred to one in which these conditions are reversed. The transplants make a better growth if both tops and roots are pruned before setting. Very little pruning of the tops will be required if the plants are secured early. The roots are frequently cut back to a length of 3 to 5 inches. The removal of a portion of the root system permits better spreading of the roots and facilitates transplanting. 147
Great care should be exercised so that the roots do not dry out in the process of transplanting. It is also important to spread the roots in the soil and to press the moist soil firmly against them, thus establishing good contact. Care should also be taken to set the plants at the proper depth. When the roots are too deep and the stern is buried under the soil the crown or terminal bud will be covered with soil and the plant may not grow. Exposing too much of the stem, thus placing the roots too near the surface where they will become dry, is also harmful (Fig. 50). The fact that new branches from the perennial stem appear above the older ones explains the tendency for the short stern to become more and more above the soil as the plant becomes older.
Fig. 50.--A strawberry plant correctly pruned and set at the proper depth.
Because of the method of propagation by rooting aboveground stems (stolons or runners), commercial growers generally prefer the matted-row system of planting since it is the simplest and easiest to maintain. Moreover, it almost invariably gives the highest yields. 147, 114, 177 In one experiment 43 varieties were employed. The transplants are often set 10 to 30 inches apart in rows 3 to 3.5 feet distant. The runners are allowed to form plants 6 to 12 inches on either side of the row thus leaving sufficient space between the rows for cultivation. Sometimes the offshoots are spaced 6 toll inches apart. Under such conditions the roots thoroughly ramify all of the soil and extend a considerable distance between the matted rows. In fact all of the space between the rows may he well occupied by roots. They are disturbed by cultivation unless it is very shallow. Plants in hills or hedge rows often develop more extensively, according to the root and shoot competition imposed upon them, and the fruits are usually larger.
Proper cultivation is said to be the most important factor in strawberry production. The root system needs thorough aeration and a constant supply of moisture. Both of these conditions. are attained by preventing the formation of a soil crust due to driving rains or irrigation. Keeping the soil mellow also encourages the rooting of the runners. Strawberries will not stand competition of weeds; the tops are easily overshadowed and the shallow roots are relatively not extensive. It is important, therefore, that cultivation should be thorough and timely but shallow. The roots are not exhaustive of soil nutrients. Loosening and aerating the soil seems quite as important as manuring it. Indeed, the saying "tillage is manure" applies very well to the root system of this plant.
Mulching in the fall is an essential practice with such a superficially rooted, perennial crop. It not only protects the aboveground parts from sudden temperature changes and drying winds in winter and spring but also tends to prevent the soil from heaving and breaking the roots, Moreover, it conserves moisture and inhibits the growth of weeds. By leaving the mulch intact in the spring the soil is kept cool and damp during the season when the fruit is being produced. Because of the lower soil temperature, absorption and growth are slower and blossoming is delayed. The root relations also explain why a mulch is usually more beneficial in regions of light and precarious snowfall than in those in which a blanket of snow lies on the ground all winter.
Among the hundreds of varieties of strawberries grown in the United States there must be considerable variation in rooting habit and adaptability of the root system. In selecting, adapting, or breeding varieties for particular regions, the underground parts of the plants should be given consideration.