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Horse-radish (Armoracia rusticana) is a coarse, hardy perennial with clusters of large leaves somewhat similar to those of the dock. The flower stalks are 2 to 3 feet tall. It is a member of the mustard family. The plants are grown for their roots which are grated or shredded and used as a condiment. A few clumps are found in nearly every home garden where the plants are allowed to grow from year to year and the roots are dug when needed. In commercial gardens it is grown as an annual crop, the plants being propagated from cuttings of the small side roots.
Mature Plants.--Vigorously growing horse-radish plants were excavated at Lincoln, Neb., in October. The plants were old, formed large clumps, and had grown undisturbed at least 10 years. The soil was a moderately rich, mellow silt loam grading into a well-drained loessoid subsoil.
The plants were characterized by large fleshy taproots which penetrated deeply but were not widely spreading in their habit of growth. The great clumps of leaves were found to arise from two to several separate stalks which, although quite distinct 1 or 2 inches below the soil surface, were attached to the same root. After several plants had been excavated, a typical specimen was selected for detailed description and drawing.
This plant had a crown consisting of five distinct parts as is shown in Fig. 45. The main roots below the crown frequently reached a diameter of 2 inches. The maximum spread of laterals from these roots in the surface foot of soil was 30 inches. Although the average diameter of the soil column occupied by the roots was only about 3 feet, the root system penetrated so deeply that the soil volume ramified by the roots was really very large. Frequently it exceeded 125 cubic feet.
Fig. 45.--Mature root system of a 10-year-old plant of horse-radish.
Many fine horizontal roots 1 to 12 inches in length arose in the surface soil. Intermingled with these smaller roots were found several roots 1 to 5 millimeters in diameter. They ranged in length from 1 to 3 feet. The larger of these roots turned downward at a distance of 1 foot or less from the base of the plant; the smaller ones usually pursued a more horizontal course.
The main root divided at a depth of 6 to 12 inches into six strong branches, thus reducing the taproot to a diameter of only a few millimeters. In fact it was not as large as some of the main branches, nor did it penetrate so deeply, ending at the 11-foot soil level. The main branches, which were 0.4 to 1 inch in diameter, ran either obliquely downward or outward 3 to 12 inches and then downward. Beyond a depth of 1 to 1.5 feet they pursued a generally vertically downward course, although often curving and turning in the characteristic manner shown in Fig. 45. As regards size and appearance, as well as manner of branching, the taproot and the stronger laterals were very similar.
The larger branches of the major roots were fine and numerous and may be grouped either as horizontal or vertical, although rootlets pursuing intermediate directions were not infrequent. Many of the horizontal branches turned downward after pursuing their outward course for a few inches. At levels of 6 to 12 feet where the soil was quite moist, laterals were often found which ran horizontally throughout their entire course. The horizontal laterals were rather uniformly 0.7 to 1.3 millimeters in diameter but varied from a few inches to more than 3 feet in length. They occurred. irregularly. In the soil above the 6-foot level these larger horizontal branches arose at the rate of one on each inch or two of the main root. Below 6 feet they were sometimes separated by distances of 12 to 24 inches. Although 30 to 40 inches was the usual length of these branches, a maximum lateral spread of 42 inches was found at a depth of 12 feet. Long laterals, however, were not common at this depth. At the 4.5-foot level a layer of hard, lime concretions was found in the soil. Here branches were very abundant. Not infrequently 4 or 5, approximately 1 millimeter in diameter, occurred on only 1 to 2 inches of the main roots. They grew outward a short distance and then turned downward.
More or less vertical major branches of the main roots occurred with less frequency. They were found on all of the main roots, however. These branches usually followed the same course as the main root and not infrequently twined about it, becoming tightly pressed against it as they increased in size. This phenomenon is not shown in the drawing.
The main roots were well covered with fine, absorbing laterals. Their course, as already indicated, was rather tortuous at least to a depth of 9 feet. Here the soil was continuously moist and much more easily penetrated, and the roots responded by pursuing a straighter, downward path. A maximum depth of 15 feet was attained.
The smaller laterals on both the main roots and their major branches were very fine and usually simple, although some were branched. They did not exceed 0.1 to 0.3 millimeter in diameter. Although somewhat irregular in occurrence, they were always abundant, frequently 16 to 24 being found on each inch of main root. In the first 3 feet of soil this number was often exceeded. Here it was not unusual for the branches to occur in clusters of 2 or 3. The number of laterals on the small branches and on the main roots at depths greater than about 6 feet was 16 or less per inch. These absorbing roots were frequently 3 to 4 inches in length in the first 8 feet of soil. But the root length rather gradually decreased from the surface-soil layers to within a few inches of the root tips where rootlets had not yet formed. At a depth of 12 feet they averaged only about 1 inch in length. Branches were also shorter on the smaller laterals. The roots were white in color and could be easily identified at any depth by their characteristic taste.
Summary.--Horse-radish is a perennial with a very thick, fleshy taproot system which penetrates to depths of 10 to l4 feet but does not spread widely. The taproot gives rise to many fleshy laterals in the surface foot of soil, where also frequently the main root divides into several rather equally prominent branches. These penetrate more or less vertically downward, or, more usually, first run obliquely outward 4 to 12 inches and then pursue their very tortuous downward course. A diameter of 4 millimeters is often retained even at a depth of 6 feet. Supplemented in the surface soil by numerous fine horizontal roots and profusely branched throughout, the main roots ramify a very large soil volume. The larger of the fine laterals are usually either horizontal or vertical in direction of growth and are from a few inches to over 3 feet in length. Their distribution is somewhat irregular. Smaller branches are numerous. Thus the absorbing system as a whole is both extensive and profuse.
Root Habit in Relation to Cultural Practice.--The great extent and rapid development of the horse-radish roots explain why the plants grow best in a very deep, moist, fertile loam soil; why deep plowing is beneficial; and why a good soil structure is an important environmental factor for growth. Soils for vegetables should always be deep. A depth of 8 to 12 inches is desirable for most soils. But if they are shallow, they should be deepened gradually and not all at one plowing or in a single year. Too much subsoil brought to the surface at once is usually not beneficial. In planting the roots, they are placed with the upper end of the cutting 2 to 5 inches below the soil surface. The soil should be packed firmly about them to insure good contact and resultant prompt growth. Weeds should be kept out by thorough cultivation so that sufficient water and nutrients will be available late in the season when the plants make their best growth. On hard, dry, or shallow soil the roots are very likely to be crooked, unshapely, and scarcely fit for use. The growth of long, straight roots of more uniform size is promoted by preparing and maintaining a deep, mellow soil. To aid in securing shapely roots
. . . some growers remove the side roots early in the season. This is done by removing the soil and stripping the side roots from the upper part of the main root. The soil is then replaced. This treatment results in the production of large, compact roots, but unless the work is carefully done, serious injury may follow. The earlier in the season the trimming is done the less check there is to growth. 155
Since plants several years old have a lateral spread of roots scarcely exceeding 18 inches, it would seem that the usual spacing for plants grown a single year is sufficiently great to allow a good root development. The usual distance is 10 to 18 inches in rows 3 to 4 feet apart. If allowed to grow more than 1 year, the roots are apt to become hollow and undesirable for market.