Although knowledge of the art of budding, or grafting, goes back several millennia , it was only in the late 1800’s that budding of citrus became a general practice. The change was partly due to the introduction of superior varieties like the Washington navel orange, the Valencia orange and other named cultivars in which it was a disadvantage to grow seedlings or marcots, and partly because of the susceptibility to soil pests and diseases . The height of budding in most citrus areas has been somewhat standard and probably varies from a height of 5 to 15 cm, although it is now changing. However, the height of budding in some instances has varied from the ridiculous to the sublime. Nowhere are trees budded higher than they are in Italy, where sour orange seedlings may be budded at a height of 90 to 150 cm in attempts to escape gummosis, which so severely devastated their seedling orchards in the 1800’s . On the other hand, the author years ago observed a nursery near Fairhope, Alabama, in which the nurseryman hoed the soil away from the base of trifoliate orange seedlings and fall budded them to Satsuma mandarin just above the crown roots.In California, height of budding has generally tended to be directly related to the inherent vigor of the kind of root stock seedlings grown. Vigorous seedlings like Rough lemon tended to be budded somewhat high; low vigor seedlings like trifoliate orange, somewhat low. Seedlings of sweet orange, grapefruit and Cleopatra mandarin were budded at more of an intermediate height.
When trees were in great demand, even the vigorous seedlings might be budded low,vertical growing systems and occasionally nurserymen have selectively budded the largest seedlings in the nursery row and left the slower growing seedlings for a later budding. In addition, those root stock cultivars which tend to branch and sucker excessively and thereby develop a bushy character were often budded lower than cultivars with a strong central leader merely because the nurseryman didn’t want to go to the expense of continually removing the lower suckers and branches. Figure 4 [Image could not be located] is a composite picture of six typical root stock cultivars used in California citrus nurseries in the early 1950’s. The sweet orange, grapefruit, and Sampson tangelo seedlings were characteristic of the slow growing category which tended to branch and sucker excessively, requiring a great deal of pruning. They were not conducive to high budding, although the trunk caliper was fairly good. The Cleopatra mandarin seedlings grew more upright, but tended, in the nurseryman’s terms, to be “leggy”, of slender caliper, and often times in need of staking for support. Cleopatra does grow more rapidly and obtain better caliper under tropical conditions. The Rough lemon and the Troyer citrange tend to send out a strong central leader and a minimum of lateral branches. Troyer seedlings, of course, do not attain the caliper of Rough lemon seedlings, but can easily be budded to desired heights. Within the trifoliate orange, there is also considerable variation in seedling types comprised of the small-flowered, large-flowered, and of course the flying dragon. Figure 5 shows a nursery comparison of Rubidoux trifoliate on the left, compared to Pomeroy trifoliate on the right. The Rubidoux seedlings do not have the strong central leader and are quite prone to lateral branching. Whether all small-flowered trifoliate strains have the same nursery characteristics as the Rubidoux, or all large-flowered strains have the same nursery characteristics as the Pomeroy, has not been verified by adequate observation, but it would seem to be a likely possibility. The flying dragon trifoliate is something else.
It is more spindly than the other trifoliates, with a twisting, corkscrew type of trunk growth, making bud insertion difficult at some points on the trunk. The retrorse, or strongly recurved, spines hinder the budding procedure. The first significant experiment on height of budding was reported by Murray in Trinidad. Included in a series of root stock trials was the height of bud insertion of grapefruit and Jaffa sweet orange scions on sour orange root stock. Six heights of bud insertion were used: 5, 13, 25, 38, 51, and 64 cm. There were eight trees of each combination and the trees were completely randomized. There was some incidence of gummosis in these experiments, and while the incidence was low, it was obvious that to avoid gummosis the trees should have been budded at 38 cm or higher. The effect of bud insertion on average number of fruits per tree, average total weight of fruit per year over a ten year period, as well as final stock and scion girth measurements in 1945 are presented in Tables 3 and 4. Several things deserve to be pointed out. The largest trees were those budded at the 5 cm height, and the size of the tree became successively smaller with each increment increase of height. The sour orange stock was noticeably larger than the scion at the 5 and 12 cm insertion, the stock and scion girth was about equal at the 25 cm insertion, but above the 25 cm insertion, the girth of the scion was larger than the sour orange root stock. The greatest number of fruits per tree per year was at the 5 cm level, but there does not appear to be a significant difference in number of fruits at the different heights of budding. The average yearly weight of fruit seemed to be less at the 5 and 13 cm levels and was greater at 25 cm and above. Somewhat of a parallel situation existed with the Jaffa sweet orange . While there was no reversion in bud union character, the largest trees were those budded at 5 and 13 cm, with a break in size at 25 cm, and the smallest trees were at the 64 cm level as in grapefruit. Highest yields were at the 5 cm level of insertion, and a rather marked reduction in yield at the 64 cm level. Fruit weight gradually increased from the 5cm level up to the 51cm level, with a sharp drop at the 64 cm level.
Since these plantings were replicated and the experiment ran for 13 years, Murray concluded that the combinations gave the overall best performance budded at a height of 38 cm. Blondel , in Algeria, budded Clementine mandarin on trifoliate orange stock at essentially three height groups. There were 24 trees budded 0-10 cm; 102 trees at 10-30 cm; and 65 trees budded over 30 cm. Observations were made on the presence or absence of exocortis, character of the bud union,vertical greenhouse volume of the trees, and yield over a four-year period. He found that at the 0-10 cm budding only 13 per cent of the trees showed exocortis; at 10-30 cm, 58 per cent of the trees showed exocortis; and those budded over 30 cm, 86 per cent of the trees showed exocortis. This variation is a little difficult to explain merely on the height of budding and also the knowledge now available on mechanical transmission of the disease by contaminated budding knives . Blondel also found a marked variation in the size of the trees at the different budding heights, which he divided into three classes; large, medium, and small. At the 0- 10 cm level, 70 per cent of the trees were large, 13 per cent were medium, and 17 per cent were small. At the 10-30 cm level, 31 per cent of the trees were large, 16 per cent were medium, and 53 per cent were small. However, at the 30 cm level and above, only 10 per cent of the trees were large, 1.5 per cent of the trees were medium and 89.5 per cent were small. Obviously, the trees became smaller with increased height of budding and this might be due in this experiment to the increased incidence of exocortis. The character of the trifoliate stock also changed with the height of budding. At the 0-10 cm level, 91 per cent of the stocks were ribbed and 49 per cent were smooth. However, at the 30 cm level or higher, only 15 per cent of the trifoliate stocks were ribbed and 85 per cent were smooth. The author has also noticed that the bud unions of trifoliate stock and trifoliate hybrids become smoother with increased height of budding . Yields declined with height of budding. For the four-year period 1948-51, yields on the 0-10 cm height averaged 25 kg, at the 10-30 cm height it declined to 21 kg, and at the 30 cm level plus, it averaged only 13 kg. While it is unfortunate that Blondel’s experiment was confounded by the presence of exocortis, the results still agree pretty well with what Murray found in Trinidad, and with subsequent observations the author has made in California. Batchelor and Webber set out very extensive root stock plantings with all the major scion varieties at the Citrus Experiment Station, Riverside, and also with grower co-operators, during 1927, 1928, and 1929. The trees were budded at from 15-30 cm, somewhat higher than commercial practices at that time. The reason for this was to provide full bud union expression, and permit the girth of the trunk to be measured 10 cm above the union and 10 cm below the union as an index of scion or root stock volume, and avoid any measurements near the union where there might be over growths or under growths or flaring of the crown roots.
A comparison of these old experimental root stock trees and trees in commercial orchards , suggested that the greatest single difference between the two groups of trees was the height above ground at which the trees were budded . Further observations indicated that while bud union malformations are greatest with lemons , it might also occur with other scions to a lesser degree, and also differ with root stocks. This fact is consistent and obvious. The higher the trees are budded once a critical height is reached, the smoother the bud union. The lower the trees are budded, usually the more malformed the bud union. The incidence of gummosis was lower on the higher budded trees. With lemons affected by shellbark , shellbark is slower in expression on the high-budded trees and the severity of the symptoms is much less . Lemon shell bark would not be a problem if the trees were budded around 90 cm. The author felt that around 30 cm was a fairly critical height and, pending more observations and research, recommended to nurserymen in California that this height be considered as a bare minimum, especially for lemons. Reasons for the variation in tree response and bud union reaction with the height of budding are unknown. Some researchers say that it changes the top-root ratio, but it is difficult to understand how a few cm of trunk more or less can have such an effect on bud union and scion reaction. Neither can it be explained by the root-stem transition area. While in some plants the transition area is completed at leaves above the cotyledonary nodes, In the subgenus Citrus, the seeds are hypogynous in germination and the cotyledons remain below ground. Hayward and Long found that in Citrus, the transition area was completed at the cotyledonary node. In 1937, L. D. Batchelor planted out a small experiment with different heights of budding. The experiment consisted of two trees each of Eureka and Lisbon lemon budded on Rough lemon root stock at heights of 30, 45, 60, 90, 120 and 150 cm. Unfortunately, they served as guards for another experiment, were next to a road adjacent to a eucalyptus windbreak, and suffered some pilferage. Consequently, the trees were somewhat neglected and observations were few and sporadic. However, 20 years’ yield data is available. It must be pointed out that lemon trees in California are usually pruned annually. The trees were finally pulled in 1960 when the trees were 23 years old. Some final measurements of height and width and resultant top volume was done at that time. The data are presented in Table 5.