UNIVERSITY OF CALIFORNIA agricultural Experiment station 
 : of agricul 
 BERKELEY 
 
 College of agriculture benj. ide wheeler, president 
 
 THOMAS FORSYTH HUNT, Dean and Directof 
 
 CIRCULAR No. 100 
 (June, 1913) 
 
 PRUNING FROSTED CITRUS TREES 
 
 BY 
 J. ELIOT COIT 
 
 It has previously been pointed out by this Station that it is wise 
 to defer pruning frost injured # trees until it is perfectly clear just 
 how far back the wood has been killed. After the line of demarcation 
 between living and dead wood becomes apparent, no time should be 
 lost in taking the necessary steps toward reconstructing the top in 
 the best way with the least possible loss of time. 
 
 The problems involved in mending and reforming the tops of 
 frosted citrus trees fall naturally into two classes, viz.. those having to 
 do with very young trees and those concerned with large mature trees. 
 
 REHEADING YOUNG TREES 
 
 When a one or two-year-old tree has been killed to a point near 
 the ground it should first be determined in each individual case 
 whether the dead wood extends below the bud union. If there remains 
 enough live wood above the union to grow a strong shoot, such a 
 shoot may be allowed to grow and become the trunk of the new tree, 
 all other shoots being suppressed. If on the other hand the trunk is 
 killed below the union it will be necessarj^ to set a new bud. The 
 opportunity is thus presented of reconsidering the relative profits to 
 be derived from oranges, lemons or pomelos. In case the root is sweet 
 orange the bud may be set immediately, provided there is sufficient 
 room to do so without excavating the soil, for in no case should a bud 
 be set below the soil. In case the trees have been killed to the ground 
 it is well to select the most vigorous sprout and bud into this two or 
 three inches above the ground. If the root is sour orange or pomelo 
 
it is best to place the bud six or eight inches from the ground on a 
 strong sprout. This will result in a somewhat hardier tree and one 
 more resistant to gum disease. 
 
 In all cases where a new trunk must be grown on trees already 
 planted in orchard form, it is necessary of course to stake the trees 
 and tie them every few inches, taking care to disbud often and remove 
 all sprouts from the root, eventually heading the tree at the desired 
 height. A stake larger than the customary building lath is advisable 
 in order to prevent injury by passing teams and tillage tools. 
 
 Where the tops of young trees are partly frozen it will be found, 
 especially with lemons, that the more rapid growing upright shoots 
 have been killed while the fine fruiting brush has been spared. 
 Instead of cutting back all of these injured shoots in proportion to 
 the injury, it is much better, especially with lemons, to take out most 
 of the very strong upright canes entirely. If all are merely cut back 
 and allowed to remain each one will throw up several similar shoots 
 which in turn may be cut back, the final result being a broom-like 
 growth with far too many shoots. The proper pinching and thinning 
 of such a multiplicity of shoots will involve much more labor than 
 the commercial orchardist can afford. It is better, therefore, to remove 
 most of the upright vegetative shoots entirely and rely on new shoots 
 from the horizontal wood to enlarge the framework of the top. All 
 water sprouts arising from the trunk and around the crotches should 
 be vigorously suppressed. It is a good plan to whitewash the exposed 
 trunks in order to prevent sunburning of the bark. 
 
 In many cases the bark of the trunks of young trees has been split 
 by the frost and has dried out resulting in a strip of dead bark on 
 one side, usually the southwest side. This condition is not necessarily 
 serious, provided there remains a strip of live healthy bark of sufficient 
 width to provide a flow of sap to the top. The living bark will 
 gradually increase in width and extend over the dead areas, in time 
 covering them entirety. In order to prevent the entrance of wood 
 decaying fungi it is well to paint the dead areas with white lead or 
 asphalt paint. 
 
 BEHEADING OF MATURE TREES 
 
 Orange trees are much more resistant to frost than lemon trees 
 Over certain large areas the recent freeze has ruined the fruit and 
 completely defoliated the trees without killing back the wood to any 
 appreciable extent. In such a case the trees need no extra pruning, 
 although while the leaves are absent a good opportunity presents 
 
itself to remove conflicting branches and thin the brush with greater 
 ease and efficiency than is possible when the trees are covered with 
 leaves. 
 
 Where the wood is killed back to any extent, however, too many 
 shoots are apt to start from the live wood and many suckers from 
 about the crotches. Theoretically the shoots should be thinned out, 
 leaving only what is needed. Those left will be stronger and bear 
 more fruit than if such a large number were allowed to struggle for 
 life. Practically, however, such a course would require more labor 
 than is available and large growers will have to adopt the next best 
 course, viz., that of cutting out all the dead wood and thinning the 
 new growth by taking out bodily a certain number of branches. 
 
 Where lemon trees have been killed back severely it is best to 
 withhold or decrease the fertilizer during the succeeding year in order 
 that a more moderate growth may take place. Frozen trees have been 
 thrown out of balance. They have the same amount of root as before 
 which is now pouring sap into a much reduced top. The result is 
 bound to be a tremendous new growth breaking out all over the trunk 
 and branches. At best a great deal of labor w r ill be required to 
 properly sucker the trees. If additional stimulation is brought about 
 by the application of quick acting fertilizers this trouble will be 
 increased. Wise lemon growers prefer to build the new head out of 
 the more moderate growth of fruiting wood rather than out of water 
 sprouts. It is not necessary or always advisable to root-prune frosted 
 trees, but if hard-pan or plow-sole exists it may be broken up with a 
 subsoil plow at this time with less injury to the trees from cutting large 
 roots. The number of roots which may be cut with impunity will of 
 course depend upon the amount the tops of the trees have been killed 
 back. 
 
 Whenever large limbs are taken out it is well to apply some good 
 antiseptic to the cut surfaces in order to prevent the entrance of 
 decay organisms and to prevent undue drying out and checking of 
 the wood. White lead paint and asphalt paint are good dressings, while 
 liquid grafting wax is often used. A good formula for the wax is 6 lbs. 
 resin, 1 lb. beeswax, and 1 pint of linseed oil. Another wound dressing 
 which is very cheap and has been highly recommended consists of 
 three parts melted resin mixed with one part warm crude petroleum. 
 
 "It seems reasonably certain that no injury to the tree can result 
 from any of the materials passing from the frozen oranges back into 
 the tree. An examination of those oranges which have been too badly 
 frozen to be fit for shipment, shows that most of them have been only 
 
partially killed; consequently they are presumably respiring carbon 
 dioxide. This loss of energy would be saved if the oranges were 
 removed from the tree rather than allowed to hang all summer. 
 There is no experimental evidence, however, to show whether this loss 
 is sufficient to warrant the cost of early removal by hand." In case 
 the fruit is picked off the trees it may safely be plowed under, thus 
 adding humus and some plant food to the soil. There is no danger of 
 the soil becoming acid as a result of this practice.