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Avocado TreeThe avocado tree is grown primarily in California, to a lesser extent in Florida, and on only a few acres in Hawaii, Puerto Rico, and southern Texas. Crop production in 1970 amounted to 83,400 tons valued at $30 million. California produced 64,600 tons and Florida produced 18,800 tons.
On mature avocado trees, about 2 tons of fruit per acre are harvested, although productive orchards will yield 3 to 6 tons. Year-to-year production varies, depending upon many factors, but a year of high production is frequently followed by a year of low production. Weather has a strong impact upon production. Prolonged cool weather, subfreezing weather, low humidity, strong winds at flowering time, or tornadoes can all result in low set of fruit and low production. The most critical effect of temperature occurs during flowering.
The avocado is a tropical evergreen, upright shrub or tree that grows to 60 feet high, but usually between 15 and 30 feet in height (fig. 46). Its dark green leaves are 4 to 10 inches long and 2 to 3 inches wide. The avocado tree may exhibit two or more growth flushes during the year in contrast to the single growth period of most deciduous plants. It may flower in summer or in winter, and may have a flowering period lasting 6 months. It is less tolerant of cold than lemons or navel oranges and prefers high humidity and calm weather. The fruit, which can remain on the tree for several months after maturity, is a nutritious, fresh food rich in oil and high in calories and vitamin E. A few seedling dooryard trees are estimated to be 100 years old, but commercial trees last about 35 years (Goodall et al. 1970).
Hundreds of cultivars have been tried in the United States, but about two dozen are of commercial importance (Rowland 1970).
The cv. 'Fuerte' has for years provided the bulk of the avocado crop (Bergh et al. 1966, Rock and Platt 1968, Rowland 1970). Its fruit weighs 8 to 16 ounces and contains 18 to 28 percent oil. It is cold resistant and ripens over a long period - December to May. By comparison, the Florida cv., 'Pollock', weighs 30 to 50 ounces and contains only 3 to 5 percent oil. The 'Haas' cv. is second in importance to 'Fuerte.' Its fruit weighs only 6 to 12 ounces. Other important California cvs. include the 'Bacon', 'Zutano', 'Rincon', 'Nabal', 'McArthur', 'Anaheim', 'Carlsbad', 'Dickinson', and 'Puebla'. In Florida, the most important cultivars include 'Booth 8', 'Lula', 'Booth 7', 'Waldin', 'Pollock', end 'Hickson' (Rowland 1970).
Avocado trees can be grown from seed, but the plants are usually propagated by grafting. They are set in the grove 20 to 40 feet apart depending upon whether the type of growth is spreading or upright. Sometimes they are set at 15 to 20 feet with the alternate plants removed after a few years. Older orchards with spreading trees may have as few as 40 trees per acre. Orchards with upright trees may have 150 trees per acre. About 90 trees per acre is average (Lee and Burns 1967). Fruit bearing begins at 3 to 6 years of age and may continue for 50 or more years.
The honey bee is attracted to the plant for both the nectar and the pollen, although citrus, mustard, and many other plants that flower at the same time as avocado are much more attractive to bees than are avocado flowers. Pellett (1926, 1947*) reported that bees collect only a small amount of avocado honey. Vansell (1931) stated that avocados are visited moderately by bees for nectar and pollen. In general beekeepers consider the plant as a source of buildup for their bees rather than as a source of surplus honey.
A full-grown avocado tree may bear a million flowers in a season, the flowers occurring in panicles of severe dozen to several hundred on the ends of the numerous branches (Robinson and Savage 1926).
The relatively inconspicuous blossom is about one half inch in both width and depth. Three sepals and three similar-appearing green petals make up the perianth. The single pistil has a simple, bulbous, smooth ovary and a somewhat elongated style terminated by a slightly enlarged stigma. There are nine stamens inserted in two whorls. The inner whorl consists of three stamens, with three prominent, orange, nectar-producing staminodes (sterile or abortive stamens) alternating between them. Opposite each stamen and staminode of the inner whorl is one of the six stamens of the outer whorl. There is an orange nectary, slightly smaller than the staminode, on each side of each outer stamen.
The flower opens twice, on subsequent days or in two stages. In stage 1, the first day, the petals separate and bend outward. The stigma is whitish, fresh, and receptive to pollination (Hodgson 1930), but the stamens, bent out at right angles to the pistil, release no pollen. Some nectar appears on the staminodes. After a few hours, the flower closes.
In stage 2, the second day, the flower opens again. This time, nectar on the six true nectaries is secreted more profusely than occurred on the staminodes. The pistil is shriveled and dark and no longer receptive. The stamens are longer and larger, the inner three overtopping the stigma but facing away from it, and the outer stamens at about a 45 deg angle from the style and facing it, and both sets releasing sticky clumps of pollen. Each stamen has four pollen sacs, the valves of which hinge at the top.
When the flower closes the second day, it never reopens. It is therefore, structurally bisexual but functionally unisexual. This dichogamous condition was first noticed by Nirody (1922) and enlarged upon by Stout and Savage (1925) and Peterson (1955a, b, 1956).
The unusual part about the avocado flower is that in some cultivars stage 1 occurs in the morning of the first day and stage 2 in the afternoon of the second day. These cultivars are referred to as type A. In other cultivars, referred to as type B, stage 1 occurs in the afternoon, and stage 2 occurs the following morning. If cultivars of both types are interplanted within the same orchard, pollen should always be available when the stigmas are receptive (Stout 1932, Robinson 1930, 1933, Ward 1933, Bergh and Gustafson 1958, Bergh and Garber 1964). At least one cv., 'Collinson', produces no pollen; therefore, it is incapable of setting fruit unless pollen is transferred to it from other cultivars that release pollen when its stigmas are receptive (Anonymous 1930).
If the temperature is too low, some flowers, for example, those on the 'Fuerte' cv., may fail to open in the female stage, making fruit set impossible. On the other hand, hot weather and low humidity are not conducive to fruit set. Also, too much disturbance of the flowers by wind can cause shedding. A mild climate with calm humid days is best for the flower.
Bergh (1968) showed that trees set more fruit when there are flowers of different avocado cultivars nearby. This may not be true for all cultivars or all years, but such effects have been thoroughly demonstrated. For example, he showed that the 'Fuerte' and the 'MacArthur', which are considered to be self-fertile, increased production as much as 50 percent when exposed to pollen of other interplanted cultivars.
Peterson (1955b) showed that the pistillate stage, or stage 1, of the 'Rincon' cv. was open for 3 hours 40 minutes, the maximum time in which pollination of this cultivar could take place. He showed that the flower was incapable of selfing because first flowering began at 7:25 a.m. and ended by 11 a.m.; whereas the second stage of the 2-day-old flower did not begin until 11 a.m., by which time the current-day stigma had withered and was no longer receptive. In the 'Zutano' cv., stage 1 extended from 2:50 p.m. to 6:20 p.m., and stage 2 (the next morning) from 8:40 a.m. until after 11 a.m. Therefore, when the flowers of type A, for example, 'Rincon' cv., are receptive to pollination, the pollen is being shed by flowers of type B, for example,'Zutano' cv.. and when flowers of the 'Rincon' are shedding pollen, flowers of the 'Zutano' are receptive to pollination. This condition is considered by horticulturists to be highly fluid and influenced by the cultivars involved and various environmental conditions.
Peterson (1955a) showed that at least the 'Zutano' and the 'Haas' cvs. were capable of setting fruit when isolated from other cultivars if honey bees were present in abundance. He caged four individual trees, two of each cultivar with one tree of each group in a cage with honey bees during the flowering period. When flowering was over, the bees and cages were removed and the fruit counted. The results concerning the treatment and fruit produced were as follows:
Whether the pollen was carried over on the bees from the normal time of anther opening until the time of stigma receptivity, whether the opening phases overlapped, or whether the bees forced open the anthers when the stigma was still receptive was not determined, but in any event the effect of the bees was striking.
The evidence is clear that avocados must be insect-pollinated, and that production is best when varieties are interplanted. Bees usually transfer avocado pollen no greater distance than two avocado rows (Bergh 1961). The varieties should intermesh in their blooming dates so that pollen is available on one cultivar when the stigmas on another are receptive, and vectors should be available to move the pollen to the receptive stigmas. Maximum set can only be achieved through adequate provision for cross-pollination - the interplanting of appropriate flowering types and the availability of adequate pollinating agents (Bergh 1969).
Various pollinating agents visit the avocado flowers for nectar and pollen. These include the honey bee, various species of wild bees, wasps, flies, and hummingbirds (Chapman 1964*).
The consensus of various research workers who have studied the flowering and fruiting of the avocado is that only honey bees are sufficiently abundant on the blossoms at all times to set satisfactory crops of fruit (Clark 1923,1924; Clark and Clark 1926; Boyden 1930; Traub et al. 1941; Lemmerts 1942; Lesley and Bringhurst 1951; Winslow and Enderud 1955; Lecomte 1961; Popenoe 1963).
Many observers have noted that a bee tends to visit a single tree and thus fails to afford the cross-pollination desired. This can occur when the trees are separated by some distance, for example, when they are small or spaced too far apart (Bergh 1966). It also occurs when there is an insufficiency of bees in relation to the number of blooms available.
When the flowers per bee ratio is low, the bees are required to visit many flowers to obtain a load of food and their efficiency as cross-pollinating agents is increased. Ruehle (1958) stated that good crops are set consistently in groves a considerable distance from any bee hives hut that the presence of trees would increase production. Wolfe et al. (1942, 1946) stated that it is quite possible that a hive of bees per acre with sets of five in the middle of each 5-acre tract would materially increase production. Popenoe (1963) stated that honey bees are probably necessary for good pollination unless there is an abundance of wild bees in the area.
In an excellent survey of the reasons for low yield of avocados in California, Bergh (1967) unequivocally stated: "Practically every avocado fruit set means that a honey bee transferred pollen to that flower from some other flower. Gravity or wind may act, but they are so rare they can be ignored by the practical avocado grower." Further on, he stated, "At the present time the California avocado industry is dependent upon the honey bee. The greater the bee population, the more likely the bees are to travel from flower to flower and so make the best of such inter-flower overlap in male and female stages as may be present. This is probably the chief source of avocado set in California."
Peterson (1955a) stated that there was no evidence that addition of bees to the "natural population of wild bees and other large insects" would increase fruit set. He gave no indication as to the population of wild bees honey bees, or other large insects present on the trees. Wolfenbarger (1954) showed that honey bees were more abundant within 375 feet of a 64-colony apiary than at more remote distances, and more avocados were harvested per tree within 250 feet of the apiary than at a distance of 1,000 feet. Wolfe et al. (1946) and Ruehle (1958) recommended that one colony of bees per acre be used with five colonies set in the middle of each 5-acre tract. Stout (1923) recommended providing "bees in abundance" and control of other plants in the area that might attract the bees. LeComte (1961) suggested one colony per acre. Stout (1933) went even further by stating that one hive per acre for other fruit is satisfactory, but the flowering habits of the avocado make it desirable to employ more than one hive per acre to supply the honey bees in abundance.
Bergh (1967) stated that the average California avocado grower would have better crops if he would use more honey bees. He recommended that growers use two to three strong colonies per acre, the colonies placed in groups no more than one-quarter mile apart with 0.1 mile being preferable.
Bergh (1967) made the following recommendations: (1) Place hives or have them placed by the beekeeper after the avocados begin blooming so the bees will "get the avocado habit" right away; (2) place hives in the grove if possible, at least avoid locations where the bees must fly past citrus or other attractive pasturage; (3) control other blooms, such as mustard; (4) avoid use of insecticides during the blooming season, (5) and for cross-pollination, interplant types A and B to increase production 50 to 150 percent.
Thus, after careful study of the research by these scientists, one must conclude that for commercial production of avocados bees are essential, that honey bees are the primary pollinators, and that two to three colonies per acre should be used, the colonies placed within or alongside the groves, and that steps should be taken to insure protection of the bees and discouragement of associated plants attractive to them.
The majority of avocado growers only passively encourage the keeping of bees in the area of their groves. Few if any actively contract for the bees or pay any type of pollination fee to insure the presence of adequate numbers. Many of them know that beekeepers usually move the colonies to the avocado growing areas to obtain nectar and pollen for buildup of the colonies. The bee population the beekeeper desires on the flowers for colony buildup, however, is far short of the population needed for maximum avocado pollination. Colonies vary enormously in strength and pollination effectiveness. Also, unless contracted for, the colonies may be transported to avocados when forest, range, or desert conditions are unfavorable for beekeeping, but may be placed elsewhere at avocado flowering time if the other flora is more favorable. For dependable pollination and maximum avocado fruit set, the grower should see that his trees are amply supplied with strong colonies of honey bees.