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Common rhubarb or pieplant (Rheum rhaponticum) is a coarse, perennial herb, a new growth arising each year from strong rhizomes. The plant is usually propagated by transplanting portions of the rhizomes and attached roots. It is a cool-season crop, cultivated for its large leafstalks which are available early in the growing season. The plant will withstand the heat of summer, however, and the underground parts are unaffected by severe winter freezing.
Mature Root System.--Four-year-old plants were excavated in early summer near Lincoln, Neb. The large, leafy tops were well developed. About 16 broadly expanded leaves with blades 15 to 23 inches in length and only slightly less in width grew in clumps of average size. A rich, black, silt loam soil of loessoid origin occurred at a depth of 27 inches. It was underlaid with mellow loess subsoil. A sharp line of demarcation separated these two layers of the soil profile. The upper foot of the deeper layer contained numerous pockets or nodules of calcareous material, the remainder to great depths was fine grained and quite free from concretions.
The crowns on selected specimens were about 6 inches in diameter and composed of three to five stems. Each of these short, thick stems was surrounded by dead and partly decayed leaf bases which formed a compact structure extending 5 to 8 inches below the soil surface (Fig. 18). From the base of these stems or rhizomes numerous large roots arose. The largest originated directly beneath the stem cluster and penetrated rather vertically downward; the remainder pursued more horizontal or oblique courses. The largest had a diameter of 3 inches, the others ranged from 1 to 1.5 inches in thickness. In addition a number of smaller roots ranging from 1 to 5 millimeters in diameter arose from the base of the crown and spread horizontally in the surface 12 to 18 inches of soil. These smaller roots branched and rebranched profusely and carried on considerable absorption in the surface soil.
Fig. 18.--Root system of rhubarb four years old. Note the large number of absorbing roots near the surface of the soil. Some of the roots reached depths of more than 10 feet.
Six or more strong laterals and numerous smaller ones arose from the main root, originating to a great extent in the second foot of soil. The course of the major branches was vertically or obliquely downward. Many were traced to a depth of 8 feet; the maximum depth exceeded 10 feet.
The thicker portions of the main laterals were nearly always poorly branched with only scattered young rootlets densely covered with root hairs. The characteristic gradual tapering of the fleshy roots to fine extremities, as well as the course pursued by the laterals arising from them, may be best understood by a study of the drawing. The wealth of fine branchlets and the thorough occupancy of the soil within a radius of 3 to 4 feet of the base of the plant and to a depth of 8 feet or more afforded an excellent absorbing system. A maximum lateral spread of 56 inches was ascertained. The finer roots rebranched at a rate, which although somewhat variable, was strikingly uniform at all depths and in the different layers of soil. Usually 4 to 12 branches per inch of root were found. Many of these were simple and only 0.1 to 1 inch long. Many others were 1 to 4 inches or even more in length and furnished with rootlets at the rate of 2 to 8 per inch. The latter were usually short and simple but not infrequently branched. Sometimes, especially on the coarser branches, 2 to 3 inches of root length were quite free from laterals. Young root systems were better supplied with smaller branches than older ones.
The youngest branches were white but the rest of the root had a characteristic reddish or reddish-brown color without and a yellow-colored interior. The roots were brittle and of a rather watery consistency. Even the largest and oldest possessed very little woody tissue. The large branches appeared to increase greatly in diameter with age. The soil beneath the plants was filled to a depth of 8 feet with holes formed by roots now molded away and with many dead roots. These dark-brown to black root remains showed very plainly in the yellowish, loess soil. As the older roots die they are replaced by newer ones. The latter may be readily recognized by the lighter brown or yellow color.
Summary.--The perennial root system of rhubarb is characterized by a thick, fleshy, main root which soon divides into numerous thick branches. These, like the other strong laterals, attenuate gradually and end in very fibrous rootlets. The main roots and their major branches pursue various courses from almost horizontal to nearly vertically downward. Long, slender, much rebranched laterals occur throughout and thoroughly occupy a soil volume with a radius of 3 to 4 feet and extending from the soil surface to a depth of 8 feet.
Relation of Root System to Cultural Practice.--A study of the root system explains why rhubarb flourishes in a deep, rich, mellow, well-drained soil rather than in one that is shallow or underlaid with a hardpan. Since it is an early spring crop, a soil that warms rapidly, such as a sandy loam, is best.
A crop should not be harvested until the roots have become well developed and have a reserve supply of food. The practice of encouraging the growth of a leafy plant but of preventing the growth of the flower stalks is directly concerned with the root activities. The extensive early spring growth of the plant is made at the expense of food stored in the roots during the precedIng year. Hence, sufficient nutrients should be furnished the plants so that they will make a good growth of foliage after the early leaves are harvested and thus be enabled to manufacture this reserve food supply for the roots. In fact, there is usually a direct correlation between a good yield of rhubarb during any year and the growth of the leaves the preceding one. If the coarse flower stalks, often 4 to 6 feet tall, are permitted to grow they utilize much of the food that would otherwise be stored in the roots.
The large supply of reserve foods in the roots is shown by the practice of forcing rhubarb in cellars. In the absence of light, all of the food used by the growing leafstalks comes from the supply already stored in the underground parts before planting them in the forcing bed. The fleshy roots are placed in a shallow layer of moist soil which is provided chiefly for the purpose of supplying water to the plant. In fact root growth is usually very slight.
The common practice of spacing the plants 3 to 4 feet apart in rows 4 to 5 feet distant affords sufficient room for root development, although even at this distance there is considerable overlapping in absorbing territory. Where there is a single row, as in most home gardens, closer spacing is permissible.
After 4 or 5 years the plants often appear to be "running down." This is thought to be caused by too great a root growth. Growers cut away a part of the roots by plowing closely to the plants in the fall or by spading around the clumps. Thus, the overcrowded mass of roots is reduced and new growth of roots stimulated. This should be done only every 4 or 5 years when the plants seem to require it. 24
The transpiring surface is very large and the roots must absorb and transport to the leaves large quantities of water. Hence, thorough cultivation to keep out competing weeds and to maintain a soil mulch is necessary for the best development. Because of the proximity of many roots to the soil surface, cultivation should never be deep. Mulching the plants with a heavy application of barnyard manure in the fall of the year protects the roots during the winter and also affects them favorably by enriching the soil. Soils do not freeze so deeply when covered with a mulch, and root activity may be resumed earlier the following spring, especially if a part of the mulch is removed. Rhubarb roots are well fitted for extensive absorption and require a rich soil.