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The common garden asparagus (Asparagus officinalis altilis) is a perennial plant with much branched, annual, aerial stems which reach heights of 3 to 10 feet. These, like the long, somewhat fleshy, cord-like roots, arise from a rather short, much thickened, branched, and rather woody rootstock or crown which lies in a horizontal position a few inches below the soil surface. This rootstock grows 1 to 3 inches a year, either at one or both ends; hence, an asparagus plant once established will produce for many years. Asparagus is one of the most popular of the perennial vegetable crops. It is grown on a vast scale commercially and an asparagus bed is to be found in most home gardens. The plants are grown for their thick, soft, young shoots which appear early in the spring.
Early Development.--The early growth of asparagus has been thoroughly studied in California.
In the growth of the seedling, the single, primary root takes a direct course downward, developing numerous thread-like lateral rootlets. The chief function of this primary root with its laterals is absorption. It seldom attains a length of more than 5 or 6 inches. It is much more slender and fibrous than the storage roots which develop later. The single primary shoot takes a direct course upward and upon reaching the light develops a few side branches and leaves. This primary shoot seldom attains a length of more than 4 or 5 inches. Both the primary root and the primary shoot are temporary organs. They wither and die long before the end of the growing season . . . The primary root and primary shoot attain a length of 3 to 4 inches before connection with the reserve supply of food in the seed is severed. On the very young crown a brown sear may be observed at the point where the absorbing organ was attached 7l (Fig. 14).
Fig. 14.--Five stages in the development of an asparagus seedling. At the left a very young stage showing the short seminal root and the much shorter seminal shoot, both of which are attached to the seed and are deriving nourishment from the stored food in the endosperm. In the second and third stages the seed is still attached. In the fourth stage the plant has become independent of stored food in the seed, the seminal shoot has branched slightly, a second shoot has arisen from the crown, and a fleshy root has developed. In the fifth stage there is shown the seminal shoot, two well-developed, secondary shoots and one very short secondary shoot. The ages of the seedlings in days are 10, 14, 34, 54, and 75, respectively. (After Jones and Robbins, Calif. Agr. Exp. Sta., Bull. 381.)
Table 11 shows the rapidity of development of both roots and shoots. Both primary shoot and primary root were withered by Aug. 13.
TABLE ll.--DEVELOPMENT OF ASPARAGUS PLANT DURING FIRST SEASON 71 Numerical values are the average of 20 plants. Seeds planted Mar. 24 Length of Length Maximum Maximum Date primary of Number length Number length of shoot primary storage storage secondary secondary observation above root, roots roots, shoots shoots, seed, inches inches inches inches Apr. 27 3.0 4.1 0.0 0.0 0.0 0.0 May 19 3.8 5.1 1.6 0.2 1.2 1.1 June 9 4.0 5.2 4.1 5.5 2.1 5.7 June 30 4.2 5.4 6.4 7.5 3.7 7.6 July 24 4.3 5.4 16.1 18.6 5.5 12.1 Aug, 13 ... ... 28.3 .... 8.1 20.2 Sept.20 ... ... 42.0 .... 9.0 24.0
The rapid development of storage roots both in number and length is of interest. If they continued their growth in length at the same rate after midsummer as in July, i.e., nearly ½ inch per day, a depth of 3.5 feet would have been attained. Even an accelerated rate of elongation in late summer is not improbable, since the shoots are large enough to furnish much food. Although the plants produced but a single secondary shoot after Aug. 13, there was, nevertheless, a considerable increase in the number of storage roots and in the size of the crown.
The storage roots are clothed with fine, fibrous, often unbranched, absorptive roots which extend throughout their course. The fleshy roots increase in number, growing out from the sides and under surface of the rootstock. They continue their growth in length from year to year. Frequently, a definite scar may be found where the renewed growth occurred. Thus a very widely spreading and deeply penetrating root system is produced.
Mature Plants.--The common garden asparagus was excavated and studied near Lincoln late in June. The plants had been growing in the field about 6 years. The tops were 4 feet tall and the fruits fully grown but green. The black silt loam soil of loessoid origin was about 16 inches deep and underlaid with a very stiff clay. The clay continued to a depth of 3.5 feet below which it gradually gave way to loess. The soil was quite dry, at least to a depth of 9 feet, and made root excavations difficult.
The long, fleshy storage roots arose from a rootstock or crown which, occurred at a depth of about 6 inches (Fig. 15). The fine, fibrous, absorptive roots occurred mostly as branches on the fleshy ones. The branched, rather woody rhizomes were about 1 inch thick and frequently many inches in length, the older portions having rotted. On a section of rhizome only 6 inches long, 140 roots were found (cf. Fig. 16). These were 4 to 5 millimeters in diameter and extended in all directions except toward the soil surface. It is quite impossible to show all of the roots in a small-scale drawing.
Fig. 15.--Rootstock and root system of a 6-year-old asparagus plant. Not all of the very numerous main roots are shown.
Some of the roots radiated horizontally from the rhizome 4 to 6 feet or more, at depths of only 3 to 8 inches. Although they were 4 to 5 millimeters in diameter, they gradually tapered toward their ends where they were only 2 millimeters thick. They were much more branched near their extremities than close to their origin. For example, in the first 2 to 3 feet of their. course, three to five rootlets per inch about 0.3 inch long and quite unbranched, were found. But on the distal half there were usually five roots per inch, most of which were rebranched. These roots were from 1 to 4 inches. long and formed quite a network in the surface soil. They were rebranched at the rate of two to five rootlets per inch, these secondary branches varying from 0.1 to 1 inch in length.
In addition to the horizontal, widely spreading roots, numerous others pursued an obliquely ouitward and downward course before penetrating straight downward. Still others ran almost vertically downward. They showed many kinks and curves in penetrating the clay soil layer. These roots were rather poorly branched at the rate of no laterals to five per inch. Most of the branches were short, 0.2 to 1 inch in length, and only occasionally did a branch several inches in length occur. The longer branches were rebranched at the rate of two to three rootlets per inch.
Fig. 16.--Crown and roots of a 3-year-old asparagus plant. (After Brooks and Morse, Mass. Agr. Exp. Sta. Bull. 194.)
Branching was most abundant in the surface foot of soil where the white tender branches formed a dense network. This is very difficult to illustrate in a small-scale drawing. In fact, it appeared that most of the absorption was carried on in the surface 3 feet of soil and especially in the surface foot. The working depth, however, as indicated by the abundance of roots, occurred at 4.5 feet, although some large roots extended deeper, and the maximum penetration was 10.5 feet.
Between 3 and 4.5 feet, roots with a diameter of 1 to 2 millimeters were abundant and a number of the larger roots (5 millimeters thick) ended here. The thicker roots were somewhat better branched than the finer ones, the branches were longer and more of them were rebranched. Below the working depth roots were not numerous. They were 1 to 2 millimeters thick, 0.2 to 1 inch long, and occurred at the rate of no roots to five per inch. Like other short branches on the main roots, they were simple and mostly horizontal but sometimes quite kinky. Occasionally, a rebranched lateral 3 to 5 inches long was found. Nearer their ends, i.e., below 9 feet, the main roots were only 1 millimeter or less in diameter. The smaller ones had few or no branches, the larger ones had fewer and shorter branches than those just described. The roots at this depth were usually found in earthworm burrows or in the holes formed upon the decay of previous roots. Frequently, they ran horizontally for a number of inches along these lines of least mechanical resistance in the richer and better aerated soil. Numerous cases were observed where the main roots had ceased elongating at various depths, especially those growing in the stiff clay. Then branches had arisen just above the large root cap and extended for several feet into the soil. These branches had a much smaller diameter than the main roots. The latter, arising from the rhizome, tapered very little toward their ends.
The finer roots were white in color, fleshy, and very brittle, thus being very difficult to excavate from the rather dry soil. Dead roots in all stages of decomposition were found, occasionally between depths of 8 and 10 feet. They were especially abundant in the surface soil, often consisting of the corky exterior and a small brown or black stele in the center, the food-stored, watery portion of the cortex decaying rapidly.
Summary.--Asparagus is a perennial plant with a very extensive root system, which arises from a thick rootstock which grows from year to year. The primary root is only a few inches long and is short lived. It is soon replaced by the thick, long, storage roots which are clothed with short, absorbing laterals. The root system, under favorable conditions, may extend to a depth of 3 feet or more during the first season of growth. With the growth of the rhizome, the roots greatly increase in number. Many radiate into the surface soil 4 to 6 feet even in compact silt loam. Others grow outward and downward and then pursue a vertically downward course. Finally, numerous roots grow almost vertically downward. Thus a very large soil volume is occupied to a working depth of 4.5 feet. Some roots are 9 to 10.5 feet long. Branching is, however, most abundant, and undoubtedly absorption is very active in the surface foot of soil.
Other Investigations on Asparagus.--Asparagus plants about 3 years old were examined at Geneva, N. Y. The soil was a clay loam underlaid at a depth of 6 to 10 inches with a tenacious, gravelly clay subsoil. The longest roots were more than 2 feet deep and others extended horizontally an equal distance.
The greater part of the feeding ground of the roots seemed to be within 15 inches of the surface, though many roots extended below this . . . Roots penetrated beneath an area 5 feet in diameter. 43
These findings are scarcely in agreement with the preceding or the following investigations.
Three-year-old asparagus plants, grown at Concord, Mass., on coarse, sandy loam were taken for chemical examination and without particular reference to securing the entire root system. They had an average root length of 4 feet and a few roots 5.5 feet long were recovered. 16
Another writer states:
. . . many of the individual storage roots, in a plant 8 or 9 years old, may extend to a depth of 6 to 8 feet and laterally to a distance of 8 to 10 feet. The great mass of storage roots, however, is confined to the surface 3 feet. 118
Habit of Underground Parts in Relation to Cultural Practice.--A few of the more important ways in which the roots and rhizomes of asparagus are closely related to the common practices in the successful production of the crop will be briefly discussed.
Soil Preference.--That asparagus grows well in almost any kind of soil is indicated by its success as a weed when it escapes from cultivation. A consideration of its root and rhizome habit makes clear why a deep, loose, light type of soil is best. Open, porous soils permit of easy penetration and elongation of the rather thick storage roots. Such soils, moreover, warm earlier in spring and promote a more rapid, early growth. These soils are well drained, an essential environmental factor for good development, since asparagus roots are very sensitive to an excess supply of water which reduces soil aeration.
Another important factor is that connected with the practice of propagating the plant by resetting the crowns and attached roots from plants grown from seed in the nursery. In light soils the crowns may be dug with a minimum of injury. Heavy soils become packed and it is difficult to dig the crowns without injuring many of the rootstocks and losing a large percentage of the fleshy roots. 71 These roots contain the reserve food supply and loss of them lessens the early growth of the plant.
Transplanting.--The preference by practical asparagus growers for 1-year-old plants, rather than more mature ones, is undoubtedly related to the extent of root development. With asparagus, as other plants, transplanting checks growth. If the growing point of a fleshy root is uninjured in transplanting it will continue to elongate. Transplanting, however, permits of the selection of the most vigorous plants and also has certain other advantages, especially that of getting a full stand of the crowns started at a proper depth. In the process of transplanting, the roots are spread out evenly on the bottom of the trench or over a small mound of earth placed thereon and moist soil packed firmly about them. 49
Sowing the seeds in pots and starting the plants in the greenhouse or hotbed, repotting into larger containers, and finally transplanting into the asparagus bed without root injury should promote vigorous development of the plants and increase the succulent stem development earlier in the life of the plant. 154 Experiments in Pennsylvania, extending through a period of 6 years, have shown that the size of the transplanted roots has a direct relation to subsequent yield. The larger, more vigorous plants bore crops with a 26 per cent greater value than the smaller ones 4 (Fig. 17). In the process of transplanting, the roots and crowns should not become desiccated or the resumption of growth will be greatly delayed.
Fig. 17.--Grades of crowns: "Number 1" (bottom), "Number 2" (center), and "Number 3" (top). It should be the aim of the grower to produce "Number 1" crowns to plant in the permanent field. (After Jones and Robbins, Calif.Agr. Exp. Sta., Bull. 381,)
The practice of planting the crowns deeply, although not covered to the extreme depth until the shoots are well up, is directly connected with the root habit. Many of the old fleshy roots die each year and are replaced by new ones. The new roots originate from the rootstock at points somewhat higher than the older ones and thus each year approach nearer the soil surface. To compensate for this so-called "lifting of the plants," the crowns are placed at least 8 to 10 inches deep. Otherwise in only a few years they would be injured by the process of cultivation. Beds planted too shallowly may last only a few years; plants in deeply planted and properly cultivated and fertilized beds retain their vigor for 10 to 20 years. 151 In all tillage operations constant care must be exercised to prevent injury to the rhizomes and roots.
Growing the Seedlings.--In planting asparagus seeds they should be spaced a few inches apart. Where the seeds are dropped in groups, fleshy roots and rootstocks become so interwoven that they are separated with difficulty and often with considerable injury to the plants. If thinning is necessary it should be done before the second aerial shoot has appeared and before the development of the fleshy root system has begun.
After the fleshy root system has begun to develop, the shoot usually breaks at the crown when an attempt is made to pull the plant . . . after the fleshy roots have once started to develop the only way to thin is to dig out the crown, but in doing this there is danger of injuring adjacent plants. 71
The Permanent Bed.--In preparing the asparagus bed the soil is deeply tilled and thoroughly pulverized before planting. This insures a more intimate root-soil contact and promotes vigorous growth. Subsoiling heavier soils and plowing under considerable quantities of manure is also a common practice. This furnishes a better soil structure and at the same time places stores of available nutrients, upon which asparagus draws heavily, in proximity to the growing roots.
Food Storage in Relation to Harvesting.--A direct root relation exists in the common practice of permitting the plants to grow 2 full years before the shoots are closely cut. Until the plants have had time to manufacture sufficient food for a good crown and root development, they are materially weakened by removing the tops. Even on older plants the shoots should not be harvested too late in the season but an abundant growth of tops encouraged. The large amount of reserve food required for early spring growth is manufactured by the tops the preceding yeare before their death and stored in the roots and rhizome. The reserve materials stored in the roots in autumn have been shown to be principally sugars. The synthesis of sugar in the tops and its translocation to the roots appear to continue until the tops are killed by frost.
The fertilizing constituents which were stored in the roots over winter appeared to be nearly, if not quite, sufficient for the full development of the succeeding spring crop . . . The production of young stalks drew most heavily on the sugar contained in the roots, but there was no approach to exhaustion of that constituent. 108
Relation to Mulch and Humus.--The dead tops, if not removed, afford protection for the roots in winter, holding the snow and preventing sudden fluctuation in soil temperature. This is very important in cold climates where the tops are usually supplemented or replaced by a mulch of straw or other material.
When the old roots die and decay, they furnish the soil a considerable amount of humus. This supplements the supply afforded by the decaying tops. Although the plants are widely spaced--usually in rows at least 4 to 6 feet distant and the plants set 1.5 to 2.5 feet apart--yet the extensive root systems fully occupy the soil. The amount of humus is so great that the continued use of manure as a source of humus is sometimes apparently not beneficial. It is thought that the fleshy roots of asparagus may live and function only 1 or 2 years. The constant replacement of older roots by new ones is well known. The older ones then decaying
. . . contribute largely to the humus content of the soil and would seem, therefore, to be a highly important consideration in accounting for the lack of favorable influence of manure on the humus content of asparagus beds. 16
Summary.--In conclusion it may be said that the rather thorough studies on the root relations of asparagus have thrown much light upon the activities of the whole plant and have led to a far better understanding of its needs and requirements for successful crop production. A knowledge of the root development of the seedlings has permitted more intelligent practice regarding the preparation of the seed bed, methods of planting, spacing, and thinning; preparation of the permanent bed and the time and manner of transplanting. A study of the older underground parts has thrown much light upon the care, cultivation, and harvesting of the crop and its needs as regards fertilizers, water, mulching, etc. Similar studies on other vegetables will help advance the practice of crop production far beyond the empirical stage.