CITRUS Specialists Find New Methods of Propagation
The stock problem in the growing of citrus fruits has come to be recognized generally as one of capital importance, and the experimental work of the last few years has shown that the commonly used seedling stocks, such as our orange, rough lemon, sweet orange, and grapefruit, are often exceedingly variable.
Now that selected bud wood of the best varieties of citrus fruits available for nursery use, it becomes vitally important, to have uniform, high-grade stocks, well adapted to the varieties propagated and well fitted to the soil where the orchard is to be planted, as well as resistant to root rot and other stock diseases.
The amount of variation exhibited by seedlings of the common sour orange, one of the most commonly used root stocks for oranges and lemons, has been shown by H. J. Webber’s experiments at the Citrus Experiment Station at Riverside, Calif., to be very great; and he has likewise demonstrated that the growth and the yield of standard varieties of oranges budded on such variable stocks is also extremely variable, even when selected buds are used. Abnormal stocks produce abnormal trees even when uniform buds are used.
For some years past the Office of Horticultural Crops and Diseases in the Bureau of Plant Industry has been giving especial attention to the vegetative propagation of citrus fruits and their wild relatives. During these investigations new methods have been worked out which have surprisingly good results with almost all forms of citrus fruits and all of the wild relatives that have been tested.
The simplest of these new methods is the rooting of cuttings with leaves attached, in a propagating bed, the air above the cuttings being kept moist and the soil below kept about 10° F. hotter than the moist air above.
Terminal twigs are best for making such cuttings; they must have reached a certain degree of maturity so that they are no longer soft, i. e., they can not be pinched off easily with the fingers, but on the other hand they do not need to be round and thoroughly hard. Such cuttings, watered freely, with little or no ventilation, will usually begin to strike roots within two to four weeks, and by a month or six weeks after being put in the frame they will usually be so well rooted that they can be removed and planted out in nursery rows or put into pots for culture under glass or in a lath house. The sour orange, the rough lemon, and citranges (hybrids of the common sweet orange and the trifoliate orange), all of them excellent stocks for citrus fruits, root freely by this method. Such cuttings develop much faster than young seedlings because of their greater leaf area, and if rooted during winter or early spring and set out in the nursery, by the time warm weather begins they will often grow large enough to be dormant budded by fall, thus saving a year over seedlings.
It is not necessary to have a greenhouse to do this kind of propagation. By using a solar propagating frame, described in detail in Department of Agriculture Circular No. 310-C, it is possible to get fairly good results, especially in the citrus-growing States of the Gulf coast. This solar propagating frame differs from the ordinary coldframe chiefly in being elevated from the ground, providing a closed-in air space underneath the rooting bed which is warmed by the sun’s rays through a glass window on the south side. There is also another closed-in air space above the rooting bed, kept very moist to prevent wilting of the leafy cuttings before they form roots, and shaded if necessary to prevent excessive heating from the sun’s rays.
In the Gulf States such a solar propagating frame may be used for six to nine months of the year without artificial heat. Farther north its use is restricted to the warmer summer months unless some auxiliary heat is provided. Where electric current is available it is easy to arrange a small electric heater controlled by an automatic thermostat.
Observations made in Florida by one of the writers and in South Africa by H. J. Webber have proved conclusively that healthy citrus trees can be grown to great age and continue to produce heavy crops of fruit when grafted on stocks grown from cuttings.
Where for any reason especially well-developed tap roots are needed, there is another possibility of securing uniform citrus stocks by planting the seeds of the Rusk citrange and other sterile hybrids which often produce a fair crop of seed which reproduces the hybrid without any variation, because instead of a true embryo in the seed its place is taken in such hybrids by a mass of cells from the mother plant growing into the developing seed.
Another method of obtaining uniform citrus stocks is by making root cuttings. Vigorous young roots of the sour orange, rough lemon, and some other citrus stocks can be propagated fairly easily from root cuttings. Roots taken from older trees do not seem to propagate rapidly, but F. F. Halma, of the Citrus Experiment Station at Riverside, Calif., has shown that if scions of lemon or some other of the vigorous growing citrus species are inserted into such root cuttings growth starts very quickly in both root and scion and vigorous grafted plants are produced in a short time. By making sure that well-nourished roots are taken from cuttings, it will probably be possible to reproduce the sour orange, rough lemon, and other citrus stocks on a commercial scale from ungrafted root cuttings. Such cuttings would, of course, reproduce exactly the particular individual plant from which they were cut.
During the last few months a new discovery has been made that makes possible the rooting of all citrus fruit trees and almost, if not ite, all of the wild relatives of citrus fruits, even those that up to now have resisted all efforts at vegetative propagation by known methods. By using this new method the Satsuma orange, a variety hard to propagate vegetatively, and other citrus fruit trees grown in regions subject to very severe cold weather can be grown literally “on their own roots,” so that if frozen to the ground they will sprout up again, and the sprouts from the base of the trunk or from the roots will not need to be budded or grafted to reproduce the variety originally planted. By this same method stocks hard to propagate vegetatively by ordinary methods can be propagated from a single superior individual plant and perfectly uniform rootstocks secured.
This new method consists in approach-grafting a small seedling or a vigorously growing rooted cutting to one branch of a Y-shaped twig of the tree to be propagated, the nurse graft having its roots inclosed in rooting or sphagnum moss wrapped in waxed paper.to prevent drying out, or in a special marcottage box, the free use of which to the public is guaranteed by Government patent No. 1655731.
As the nurse graft after about two weeks begins to unite with one of the side branches of the Y, the twig is “ringed”’ or girdled at the base of the Y about 6 inches below the fork. Finally, after about six to eight weeks, when the nurse graft is thoroughly united to one side of the twig, the Y is cut off at the girdled base and the whole Y cutting together with its nurse graft is plunged into a propagating bed or solar propagating frame having bottom heat and a relatively cool humid chamber above. The roots of the nurse graft are at this time taken out of the moss and spread carefully in the soil.
After the nurse graft gets well established, in a fortnight or so, the twig to which it is grafted is “nicked,” i. e., given a shallow cut just below the graft union. Every few days this nick is deepened, and after six to ten weeks, when the roots have developed well on the base of the Y cutting, the grafted twig is severed entirely below the union, leaving a well-rooted cutting.
This Y nurse-graft cutting method produces, therefore, two well-rooted plants by one operation; the one a rooted cutting, the other a graft on the seedling or rooted cutting used as a nurse graft. Both plants can be utilized if the nurse graft selected is a suitable stock for the variety propagated. In actual tests of this method for propagating the Wase Satsuma orange, each Y cutting yielded one Wase Satsuma cutting on its own roots and one Satsuma plant grafted on Rusk citrange or Yuzu stock.