UNIVERSITY OF CALIFORNIA COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION CIRCULAR No. 273 Saving the Gophered Citrus Tree By ROBERT W. HODGSON A successful case of citrus inarching. UNIVERSITY OF CALIFORNIA EXPERIMENT STATION CONTENTS PAGE Introduction 2 Methods of Treatment 3 Promoting Natural Recovery 3 Bark Regeneration 3 Disinfectants and Treatment 4 Bridge-Grafting 4 Limitations 4 The Operation 4 Modified Method 6 Shoot-bridging 7 Root-bridging 7 Inarching 8 Function and Application 10 The Standard Method 10 Modified Method 15 Kind of Seedlings 15 Pruning Treatment. 16 Frost Protection 17 Cost of Treatment 17 Other Uses of Bridge-Grafting and Inarching 18 Bibliography 18 INTRODUCTION The pocket gopher is by far the most destructive rodent pest of citrus trees in California, the damage to trees and loss of crop result- ing from its activity annually amounting to several hundred thousands of dollars. The injury is produced by gnawing away the bark on the crown roots and the lower portion of the trunk, partially or entirely destroying the means of transferring the elaborated foodstuffs from the leaves to the roots, which results in the decline and, in some cases, the death of the tree. Few growers appreciate the full extent of the damage caused by the gopher for the reason that its activity is unseen and frequently never suspected. In the majority of cases its work is not detected until months after the injury is done, when visible indications of decline become evident in the appearance of the trees. The insidious and serious character of this pest may be indicated by the fact that the work of a single gopher in one night may result in serious damage to six or eight trees. Circular 273] SAVING THE GOPHERED CITRUS TREE 3 The hidden character of its operations and the difficulties of secur- ing the community cooperation necessary for a successful campaign of eradication render it highly improbable that the California citrus grower will ever be freed from the ravages of the gopher, although entirely satisfactory control measures have been developed. 1 The importance of every citrus grower being acquainted with methods of treatment calculated to aid damaged trees and of knowing how to apply them is, therefore, apparent. METHODS OF TREATMENT Since the effect of gopher injury is to destroy or restrict the down- ward circulation of elaborated foodstuffs to the roots, the problem is to reestablish the bark connection, immediately, either by means of bridge-grafting or inarching, or eventually, by encouraging the regen- eration of new bark tissue. The latter frequently involves supple- menting the nutritional functions of the tree either temporarily or permanently by means of inarching young seedlings to the trunk. The effectiveness and the character of the method of treatment are determined by the location of the injury, its extent, and the time elapsed since it occurred. Obviously these can be determined only by careful examination of the trunk and crown roots. The importance of immediate inspection and treatment cannot be overemphasized, since to delay treatment not only means prolonging the time of recov- ery, but also materially reduces the probability of complete cure. 2 Earliness in treatment is without doubt the most important factor in determining success or failure. PROMOTING NATURAL RECOVERY Bark Regeneration. — The well known facts that trees injured by gophers frequently recover without treatment, and that certain methods of treatment serve to promote recovery, are based on the regeneration of bark over the injured areas from the bits of live cambium tissue left. Certain environmental conditions apparently stimulate bark regeneration, and natural recovery, partial or com- plete, occurs where these factors are favorable. Methods of treat- i A comprehensive discussion of methods of control of the pocket gopher in California, is given by Joseph Dixon in California Agricultural Experiment Station Bulletin 340, 1922. 2 For a discussion of methods of tree inspection as related to orchard efficiency see California Agricultural Experiment Station Circular 266, 1923, by Robert W. Hodgson. 4 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION ment should therefore be designed to provide the injured parts with conditions tending to promote bark regeneration. That growers have long recognized this fact is reflected by the more or less common practice of binding up gopher injuries with manure or mud poultices, or of keeping damp soil about the injured area. Little exact infor- mation is available concerning the factors conducive to bark regener- ation. Disinfectants and Treatment. — Ample observational evidence is at hand to indicate that the application of standard disinfectants such as corrosive sublimate, bordeaux wash or paste, and carbolineum to areas gnawed by gophers, and especially to fresh injuries, is not to be advised since these apparently discourage bark regeneration. On the other hand the use of warm grafting wax, certain types of asphaltum compounds, and even thick lead paint, has repeatedly been noted to stimulate the regenerative process, especially in the heavier soil types. It is recommended in all cases of recent injury that the soil be removed from the trunk and crown roots and the injured areas be thoroughly coated with warm grafting wax or asphaltum paint. It must be remembered, however, that the regenerating cambium tissue is easily injured and dressings must not be applied too hot, nor should they contain any penetrating or caustic substances. Such treatment appears to be more important in soils of the heavier types than in the lighter soils where conditions are more favorable to natural recovery. BRIDGE-GRAFTING Limitations.^-'Wh.ere the injury is confined largely to the trunk and the problem is that of reestablishing bark connection with a practically sound root system, bridge-grafting may be used with ad- vantage (fig. 1). The use of this method, however, requires the pres- ence of sound bark on the trunk below the injured area. The Operation. — The method most commonly used is that of bridging the girdled or injured area with sour orange shoots, % to % inch in diameter, spaced from 1% to 3 inches apart and inserted in the bark above by means of an inverted T shaped (X) incision and below through the ordinary T-shaped cut used for budding. The shoots should be cut slightly longer than the distance between the points of insertion above and below in order that when in place they will form a slight arch, fitting snugly against the cambium with the ends well under the bark. Before being placed in position, the shoots should be beveled at each end with both beveled surfaces on the Circular 273] SAVING THE GOPHERED CITRUS TREE Fig. 1. — A successful case of bridge-grafting resulting in the saving of a tree, the bark on the trunk of which was destroyed by disease. In cases of gopher injury the areas bridged are usually much shorter and located lower down on the trunk, in many cases, partially underground. b UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION same side. The beveling should be done with a long sloping cut so that the wedge-shaped ends will be relatively thin and easy to insert in place. 3 Although not essential to success, it is a distinct advantage to thin the bark of the treated tree at the points of insertion and to fasten the ends of the bridges in place with small nails. On account of the danger of splitting, however, the nails should be inserted through holes made with a small drill and, to avoid bruising the tender bark, should be driven in place with a nail set. After insertion, all surfaces as well as the bridges themselves, should be coated with warm grafting wax to prevent drying. A handy means of treating the points of insertion is to apply a small rectangu- lar shield of waxed cloth split at one end in such a manner as to fit snugly against the trunk and about the end of the bridge. This forms an effective weatherproof covering. In order to protect the uniting parts against extremes of tempera- ture and humidity it is usually desirable to place damp soil over the bridged area, forming a conical mound a foot or more in height about the trunk of the tree. This is left for a month or six weeks until union has occurred, when it should be removed, and the waxed cloth shields pulled off. If the bridging extends high up on the trunk, burlap wrapped loosely about the trunk is generally used as a means of protection against sunburn. Bridge-grafting may be done whenever the sap flow is sufficient to permit of lifting of the bark. In southern California this con- dition obtains practically the year round. Modified Method. — A modification of the method above described for inserting the bridges has been successfully used in southern Cali- fornia for some years. Instead of using T-shaped cuts for inserting the ends of the bridges, strips of bark the width of the shoots used for bridging and four to six inches long are removed both above and below the injured area. A loose flap of bark an inch or so in length is left attached at the upper end of the strip above the zone 3 In bridge-grafting deciduous fruit trees in early spring before the bark will slip, it is necessary to make cuts for receiving the ends of the scions rather than trying to force them under the bark. For this purpose a short, quarter-inch chisel which has been ground down thin is a convenient tool. If the tool is driven into the wood for half an inch at the proper angle, the scions will always remain per- fectly in place. Even after the sap is flowing it is often impracticable to insert the scions beneath the bark especially when working among the crown roots. Under such conditions this method of inserting the scions is recommended. It also has the further advantage that in its use even an amateur cannot fail of having the cambium surfaces in contact with each other. Circular 273] SAVING THE GOPHERED CITRUS TREE 7 of injury and at the lower end of the strip below. Long bridges are used and these are cut an inch or two longer than the distance between the ends of the strips of bark removed. The bridges are split so as to fit into the strips from which the bark has been removed, and the ends beveled to a thin edge so as to fit snugly under the flaps of bark. The bridges are then inserted and fastened in place by means of small nails or brads. After a coat of warm grafting wax is applied, strips of waxed cloth are laid over the sections where the bark was removed extending over the bark flaps above and below, and are securely held in place by another coat of wax, which fills in all crevices and makes a weatherproof covering. This method is somewhat more complicated than the one described above and does not appear to have sufficient advantages to warrant its recommendation. Shoot-bridging. — Still another modification is that of selecting shoots produced from the bark below the injured area and when these are large enough, cutting them back to the proper length and inserting the upper ends in the bark above the zone of injury. On account of the lower end of the shoot being attached, rendering manipulation rather difficult, the modified method of insertion described above is recommended for shoot-bridging. The advantage of having to make only one union which this method possesses is more than offset in most cases, however, by the delay in treatment necessitated by the time required to produce such shoots large enough for use. Root-bridging. — Root-bridging has been done to a limited extent and although sometimes successful is ordinarily not recommended on account of other and more satisfactory methods being available. Bridging may be done between roots and trunk although usually not advised on account of mechanical difficulties. 4 Scions containing several buds may be grafted into the bark of the main roots and the shoots produced from these scions later used for bridging. Roots may be dug out while still attached and the tips bent up and inserted. While these methods are of interest, none of them compares favorably with inarching and hence they cannot be recommended for general use. 4 With deciduous fruit trees its use is rather common by means of the chisel method previously referred to. UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION INARCHING Inarching as a means of saving injured trees has been known to horticulturists and plant propagators for centuries but has been employed only occasionally as a means of assisting fruit trees to recover from bark diseases and injuries. It was not until compara- tively recently that its applicability to citrus trees injured by gophers Fig. 2. — Two-year-old inarches on badly injured orange tree. The injuries were of long standing when the inarching was done. The tree has made a remarkable recovery. Note the size of the inarches and the ridge of growth above the one on the left. was discovered. Credit for the demonstration of this fact is due largely to W. M. Mertz, formerly connected with the Citrus Experi- ment Station, Riverside, who in 1919 successfully treated some 200 gophered lemon trees at Alta Loma. It should be stated, however, that citrus trees in California had been inarched before that time, but Circular 273] SAVING THE GOPHERED CITRUS TREE in a small way only and principally as an aid to recovery from the brown rot gum disease. As an evidence of the rapidity with which this method of treating gopher-injured trees has come into favor, it is reported that during the past three seasons one individual has inserted approximately 10,000 inarches. During this same period nurserymen report the sale of more than 25,000 sour orange seedlings presumably for inarching purposes. At the present time the demand for seedlings suitable for use in inarching exceeds the supply. %E^Jii3S!ffiR Fig. 3. — Thinning the bark at the point of insertion of the inareh is an ;ii i to the operation as discussed in the text. The inarching method of treatment appears to offer many derided advantages over all methods previously used. Most of the trees treated by this method have shown satisfactory recovery. When applied soon after injury, in nearly all cases the trees have suffered no appreciable nutritional disturbance and no diminution of crop whatever. Even in very bad cases where months had elapsed since the injury occurred and where the trees had shown pronounced indi- cations of disorder, they were well on the way to recovery within two seasons after treatment (fig. 2). It is recognized, however, that in such cases recovery must necessarily be slow and in a certain per cent of cases so delayed as to be commercially unsatisfactory. 10 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION Function and Application. — The function of inarching is to sup- plement the nutritional processes of the tree either temporarily, in the case of recent injury, while bark regeneration is taking place, or permanently, in the case of old injury, by substituting new and actively functioning roots for those lost by starvation or death follow- ing the severing of bark connection. Its use is therefore recommended in all cases of injury, whether recent or old, involving any consider- able area of root bark, or trunk bark adjacent to the crown roots. Pig. 4. — Showing the cut end of the seedling tree (inarch), ready for insertion. The Standard Method. — As commonly used, this method consists in planting sour orange seedlings, three to five in number and % inch to one inch in diameter, around the injured tree, at the same time inserting the trunks of the seedlings into the trunk of the tree — as low down as is possible, by means of inverted T-shaped (X) incisions, in much the same manner as described for bridge-grafting. The first step in the operation is the examination of the trunk and crown roots to determine the number and proper placement of the seedlings. This may be done conveniently by means of a trench shovel Circular 273] SAVING THE GOPHERED CITRUS TREE 11 and a gardener's trowel. The points selected for the insertion of the seedlings should be as close to the ground as is possible, and not more than two to three inches above the injury as there is evidence to indi- cate that the lower down the inarch, the more rapid the union and development of the seedling. Fig-. 5. Prepared seedling pushed well into place. Beady for nailing-. Note upright position. The holes for the seedlings should then be dug in such a manner that the seedlings may be planted close to the trunk in a relatively upright position so as to avoid their being pulled loose later by culti- vating tools. The holes should not be larger than necessary to accommodate the roots of the seedlings, and should be slightly deeper than required when the seedlings are in final position, in order to permit of some manipulation in pushing the seedlings into place. 12 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION The bark of the injured tree at the points of insertion should then be thinned somewhat either by paring or by scraping, the latter being conveniently done with the back of the knife-blade (fig. 3). The thinning of the bark accomplishes several objects; it renders it pliable so that separation from the cambium does not occur except in the Fig. 6. — The cut end of the seedling is fastened tightly against the cambium layer of the tree by means of a small nail inserted through a previously drilled hole. immediate vicinity of the incisions, a matter of importance; and it cleans the bark so that dirt is not likely to be smeared on the cut surfaces of the seedlings. Inverted T-shaped (X) incisions are then made in the back on the thinned areas, the vertical cut being not longer than 1% to 2 inches and the horizontal cut just long enough to permit of easy insertion of the cut ends of the seedlings. Circular 273] SAVING THE GOPHERED CITRUS TREE 13 The seedlings are next shortened to approximately the correct length and prepared for insertion, each being trimmed with a single long sloping cut of proper angle to provide for the contact of con- siderable cambium tissue and a relatively upright position when in place (fig. 4). A slight arc to the cut may be used to insure good cambium contact and it is sometimes desirable to trim the bark si do of the tips slightly to facilitate insertion without pulling the bark loose. The corners of the bark at the incisions are then raised and the seedlings pushed into the slits so that the tips of the cut ends are well past the upper end of the slit, and cambium contact is secured all round (fig. 5). Where the bark is thick or the sap flow poor, n sharpened tooth-brush handle or shoot taken from dormant growth greatly facilitates lifting it and assists in inserting the seedlings. When securely in place, the cut surfaces of the seedlings are fastened tightly to the trunk of the treated tree by means of % to % inch nails inserted one to each seedling through a hole bored with a small hand drill or gimlet just before insertion (fig. 6). The nails should be driven in carefully and forced into final position with a nail set in order to avoid bruising the tender bark, a common cause of failure after union occurs. The nails should never be driven through the ends of the seedlings without holes having previously been drilled ; if not done, splitting of the seedlings is likely to result. Damp soil should then be filled in and tamped about the roots, taking special pains to firm the soil about them so as to establish good contact between soil and roots and to prevent settling after irrigation. A good coating of warm grafting wax: 5 should now be applied to the points of insertion (fig. 7) and as an extra precaution it is some- times desirable to coat the bark surface of the inarches as well, to prevent excessive evaporation. It is very important not to apply the wax too hot as the bark and cambium are easily injured by heat. Soft and sticky waxes are also objectionable as they apparently retard healing. Although not required for success.it is recommended that as an additional precaution the points of insertion be further protected by s Many formulas for making grafting wax are in common use. Two of the best are as follows: Formula 1. Melt together, taking care not to allow to boil, one pound of bees- wax and two pounds of rosin. Formula 2. Melt together one pound of beeswax, five pounds of rosiu, and one-half pound of tallow or one pint of raw linseed oil. 14 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION the application, while the wax is still warm, of small shields of waxed cloth 6 split at the lower end so as to fit tightly to the bark of the tree and snugly about the inserted ends of the inarches. The waxed cloth should be held in place by another coat of warm wax (fig. 8). Fig. 7. — After tamping the soil about the roots, the point of insertion is thoroughly covered with warm grafting wax. A shallow basin should then be constructed about the trunk of the tree and irrigation water applied immediately. For some months after inarching it is desirable to water the trees in this manner at intervals of ten days to two weeks in addition to the usual orchard irrigation schedule. 6 Waxed cloth for inarching may be purchased from seed houses, nurserymen, and budders but may easily be made at home. A fairly good grade of bleached muslin should be used, such as Daisy or Hope, wound in six-inch strips on a stick and thoroughly soaked in a solution of warm wax composed of one pound of beeswax and five pounds of parowax. Circular 273] SAVING THE GOPHERED CITRUS TREE 15 As a means of providing conditions of temperature and moisture favorable to union it is the common practice after the first irrigation to mound up damp soil about and over the inarches and to irrigate thereafter in a circular ditch or furrow at the base of the mound. After a month or six weeks when union has taken place the mounds are pulled down and the waxed cloth shields removed. Fig. 8. — As an additional precaution a waxed cloth shield is used, held in place by another coat of warm wax. Modified Method. — A modification of the method just described has been used successfully by L. H. Dawson of Corona for some years and consists in the removal of a strip of bark leaving a flap at the upper end and splitting the seedling and nailing it in place as de- scribed for the modified method of bridge-grafting. Kind of Seedlings. — The use of sour orange seedlings for inarch- ing is recommended on account of the greater resistance of this variety 16 UNIVERSITY OF CALIFORNIA EXPERIMENT- STATION to trunk and root diseases. It is generally cheaper, more satisfactory, and convenient to use seedlings with bare roots instead of balled seed- lings, although the latter may be preferable under certain conditions. It is important, however, that the seedlings used be vigorous and thrifty and not discarded stock unsuitable for nursery trees. Fig. 9. — Complete equipment need in inarching. The trench shovel and gar- deners' trowel are used in excavating about the tree for examination and for placement of the inarches. The lower part of the tool box contains the seedling trees packed in sphagnum moss and the tray carries the brush for cleaning the bark, the knives, also drill, hammer, and nail set and other tools frequently found to be convenient. The wax is kept warm by a one-burner kerosene stove in the five -gallon oil can used for heater. PRUNING TREATMENT Pruning treatment for gophered trees is determined by the extent of injury, the period elapsed since the injury occurred, and the character of the treatment given. Even where trees have been vir- tually girdled, if early treatment has been provided, little or no pruning is required. If the injury is not extensive and natural recovery is relied upon, a moderate thinning of the top is desirable in order to restore as nearly as possible the balance between the root system and the parts above ground. CIRCULAR 273] SAVING THE GOPHERED CITRUS TREE 17 If the injury is old, and the appearance of the trees indicates nutritional disorder, it appears to be desirable to remove a part of the top. This removal of parts, however, should be accomplished by a thinning process rather than by heading or ' ' stubbing. ' ' Where the injury is extensive and the conditions of the trees decidedly weak, skeletonizing may be necessary if they are to be saved. Where such is the case, however, it is questionable whether the trees are worth saving, even though it be possible to do so. FROST PROTECTION For two or three seasons after insertion, bridge-grafts and in- arches, and especially the thinned bark areas about the points of insertion, are quite susceptible to injury from low winter tempera- tures. In districts where damaging frosts are likely to occur it is desirable to protect the treated trees against frost injury. Mounding dry soil around the trunks is recommended as the simplest and easiest method of affording satisfactory protection, although open to the objection of increasing the danger of infection with the brown rot gum disease. This may be minimized, however, by coating the lower parts of the trunks with bordeaux paste prior to mounding the soil about them. The treated trees should be provided with suitable protection in late fall and the mounds pulled away in early spring as soon as danger from frosts is past. COST OF TREATMENT The cost of treating gophered trees varies according to the method used, and ranges from twenty to thirty cents each for slight injuries, to $1.50 to $2.00 for extensive injuries that require the use of three or four inarches. Labor is usually the principal factor in determin- ing the cost of treatment although in the case of inarching at the present time the cost of the seedlings used practically equals the labor costs. A statement of costs for the present season for a treatment requir- ing nearly two thousand seedlings and extending over two months, furnished by a commercial operator, is as follows : 1859 sour orange seedlings (in different lots) $435.70 Average cost per seedling $.235 Labor (operator and one assistant) 441.00 Average cost per seedling .237 Total cost per seedling inserted .472 Average of three seedlings used per tree 3 Total average cost per tree treated $1.42 18 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION OTHER USES OF BRIDGE-GRAFTING AND INARCHING The methods of bridge-grafting and inarching described in this circular have already been used in a limited way in the treatment of bark and root injuries of other kinds and will doubtless receive more extended use in the future. Bridge-grafting has been employed very successfully for saving trees partially or entirely girdled by field mice or the brown rot gum disease (fig. 1). Inarching has also been used with advantage in treating similar injuries as well as for the restoration of roots lost by smudge-oil injury and various forms of fungi producing root decay. Inarching is now receiving considerable attention as a means of supplementing the root systems of stunted or declining trees, with young and vigorous roots of more congenial stocks, in the hope that thereby they may be invigorated or brought back into productive condition. In numerous cases such treatment has proved decidedly beneficial. The inarching of citrus trees for this purpose has been rather widely used in Florida and with success. In California, how- ever, this practice is at the present time purely experimental and some years will be required before its value in such cases will be definitely determined. With slight modifications according to the requirements, all the methods described herein are applicable to the other classes of fruit and nut trees grown commercially in California. BIBLIOGRAPHY 1879. The Art of Grafting and Budding, by Charles Baltet. English translation, pp. 54 and 213. Macmillan and Company, London. 1896. Methods of Propagating the Orange and Other Citrus Fruits, by H. J. Webber. Yearbook, U. S. D. A., pp. 488-489. 1911. Citrus Fruits and Their Culture, by H. H. Hume, ed. 4, p. 196. Macmillan and Company, New York. 1916. Bridge-grafting of Fruit Trees, by W. F. Fletcher. Farmers' Bull. No. 710, U. S. D. A., pp. 1-8. 1916. Inarching Citrus Trees, by H. D. Eundle. California Cultivator, Vol. 46, No. 18, p. 540. 1917. Saving the Girdled Fruit Trees, by W. C. Gillespie. Extension Circular No. 54, Penn. State College, pp. 1-8. 1919. Bridge-grafting on Citrus, by E. H. Paddock. Mo. Bull. Calif. State Com. of Hort., Vol. 8, No. 2, pp. 72-73. CIRCULAR 273] SAVING THE GOPHERED CITRUS TREE 19 1921. Saving the Gophered Tree, by Eobert W. Hodgson. Calif. Citrograph, Vol. 6, No. 10, pp. 352, 353. 1922. Improved Method of Inarching or Bridge-grafting, by C. S. Pomeroy. Calif. Cultivator, Vol. 59, No. 1, p. 3. 1923. Experiments in Bridge Grafting and Inarching in Connection with Gum- mosis of Citrus, by H. S. Fawcett. Calif. Citrograph, Vol. 8, No. 3, pp. 68 and 95. 1923. Gum Diseases of Citrus Trees in California, by H. S. Fawcett. Bull. No. 360, Calif. Agr. Exp. Station, p. 390. 1923. The Pruning of Citrus Trees in California, by Eobert W. Hodgson. Bull. No. 363, Calif. Agr. Exp. Station, p. 528.