Carya illinoensis (Wangenh.) K. Koch
by Liming Shen
Other common names include: Illinois nut
Pecan is considered as the most valuable of the North American nut species. The word 'pecan', means "a nut too hard to crack by hand." Its scientific name, Carya illinoensis (Wang.) K. Koch(Wolstenholme and Storey, 1970), is from karyon (an ancient Greek name for nut) and a Latinized version of Illinois. Illinois nut (meaning the nut that grew in the territory occupied by the Illinois Indian tribe) was an early common name for pecan. Native stands of pecan trees were very common in early times. We still have approximately 840,000 acres in eight states after 200-300 years of continuous elimination of native pecan groves to provide new cropland, large lakes, roads, etc. (Thompson, 1984). It is among the few native plant species of North America to be developed into a sizable agricultural crop and very profitable business in several southern states reaching from Georgia in the east as far west as Arizona (Manaster, 1994).
Native Range (map) : Pecan was found native on the moist bottom lands along streams from Indiana south to Kentucky and Alabama, and from Iowa south to Texas, principally along the Mississippi and its tributaries, the Colorado river in Texas, and along some of its tributaries into Mexico (see map).
Climate: The appropriate climate for pecans is quite wide-ranging from 760 mm (minimum average annual rainfall) to 2010 mm (maximum annual rainfall). The weather must be warm enough to sustain an adequate growing season, with a chill spell during the winter dormancy period. Pecan trees need one to two inches of water a week to initiate nut growth, increase the size and to prevent the nuts' drying out and dropping prematurely during the growing season which occurs between April and October, about fifty-five inches a year. While in the native range pecans are adapted to the weather's vagaries, improved varieties are more finicky. The preference is a summer mean temperature ranging between 750 F and 850 F with extremes of 1050 to 1150 F. In the winter, trees may require at least four hundred hours with the temperature falling below 450 F and a
mean range of 450 to 550 F, with extreme of 00 to -200 F, from December to February (Manaster, 1994).
Soils: Pecan trees generally favor alluvial flood plains where they can compete successfully for light and space amid the other plant growth. Their preferred soil is a fine, sandy loam to a depth of about twenty-four inches and below this a sandy clay subsoil which secures the extensive taproot. A permanent, static water table, ideally ten to twenty-five feet below the surface, is the foundation for such soil. The trees are well nourished with the humus and organic matter produced by generations of seasonal inundation that carries all the major and minor nutrient elements that ensure the tree's long life (Manaster, 1994).
Reproduction and Growth
Flowering and Fruiting: Pecan is monoecious and flower in the spring. The staminate flowers develop from axils of leaves of the previous season or from inner scales of the terminal buds at the base of the current growth. The pistillate flowers appear in short spikes on peduncles terminating in shoots of the current year. Since two kinds of flowers are produced on the pecan, pistillate and staminate, the pollen must be transferred from the latter to the former in order that pollination may take place. Wind is the carrying agent for pollination. It is difficult to propagate the pecan either by budding or by grafting, but it is used in some cases (Young, and Young, 1992).
Seed Production and Dissemination: Fruits are ovoid, globose, or pearshaped nuts enclosed in husks developed from the floral involucre. Fruit usually in clusters of 3 to 12. Husks are green prior to maturity; they turn brown to brownish black as they ripen. They become dry at maturity and split away from the nut into 4 valves along sutures and at the base at maturity, usually releasing the nut. The bulk of the edible embryonic plant is cotyledonary tissue (Young, and Young, 1992). The nuts range from 1 to 2.5 inches in length and are deep brown in color. For seed purposes, pecan nuts are kept as they are dried out. Before they germinate, they should be soaked in water for two or three days. The nuts should be planted three or four inches deep, depending upon their size and character of the soil. The ground may be mulched with pine-straw, grass, leaves, or other suitable material in order to keep moisture (Hume, 1912). The minimum seed-bearing can be ranged from 2 to 20 years and the maximum seed-bearing can be as many as 300 years. The cleaned nuts average about 200 to 350/kg. Good crops are produced at intervals from 1 to 3 years. Seed dispersal is principally done by flood water and animals such as squirrels.
Yield (MAI) and Rotation Length: Grown commercially for nuts with single trees was known to produce over a half ton of nuts in a good year, but 500 pounds was average for most mature trees. Rotation length is not known. Because pecan trees are generally used to
produce pecan nuts, little information is known about MAI (mean annual increment of wood product) and Rotation Length.
Population differences: A large genetic diversity within population and a high degree of variation between breeding populations were reported in a number of articles for pecan. The significant interaction between genotype and environment was observed. The pecan has two sets of chromosomes producing pistil and stamen flowers on the same tree. The tree blossom in the early spring with the male flowers appearing at budbreak, and the female flowers as the new growth reaches about six inches. Instead of allowing natural or sexual reproduction, all cultivated pecans are propagated asexually by one or another grafting techniques so that each clone is genetically stable. Pecan cultivars have been selected for a wide range of climatic conditions. Southern cultivars were developed for a long growing season, northern cultivars were developed for a short growing season and intermediate season cultivars were developed for high elevation of New Mexico and northern Oklahoma (Sparks, 1992; Manaster, 1994).
Commercial Cultivars: A lot of cultivars have been deveoped for comercial purposes and four of them have become the standards of the pecan industry. They are Desirable , Schley, Stuart and Western Schley. These cultivars are planted in large acreage throughout the pecan regions of the world. They proved to be profitable over a wide range of conditions and remained profitable as mature trees (Sparks, 1992). Other cultivars released commercially by the US Department of Agriculture in cooperation with State Experiment Stations are Oconee (which is a progeny from a 1956 cross between Schley and Barton), Houma (derived from a 1958 cross between Desirable and Curtis) and Osage (derived from a 1948 cross between Major and Evers). Oconee has large nuts, a high (56) kernel percentage, and adequate disease resistance to be grown in the southeastern USA as well as good precocity and yield potential. Houma has excellent kernel color, good nut quality and yield and is recommended for its superior disease resistance. Osage has its early nut maturity, small nut size and high yield and high kernel percentage to be grown the northern USA (Thompson et al, 1990). Natural interspecific hybridiztion taken place with Carya aquatica (C. x lecontei Little), C. cordiformis (C. x brownii Sarg.), C. laciniosa (C. x nussbaumeri Sarg.), C. ovata, and C. tomentosa (C. x schneckii Sarg.).
Tree Improvement: The tree reaches such a large size that, except during the first fifteen or twenty years for its life, it does not lend itself readily to regular pruning treatment. Even during its initial period of growth, little pruning appears necessary except to cut back a branch here and there, that the trees may develop well-rounded, symmetrical tops. All the dead or injured branches should be removed, and all wounds should promptly and carefully treated. The natural life of the pecan tree covers a period of several hundred years, and to have it live out its usual period of time, it is well to give it the most careful
treatment. The germs of decay, entering through dead branches or exposed wounds, may subject it to further and increased injury from high winds and storms. New grafting techniques, finding especially promising trees, and cross-breeding the cultivars are the other way to make tree improvement (Hume, 1912; Manaster, 1994).
Propagation Methods: During some seasons, nearly every bud or graft inserted grows, and in others nearly all fail. The difficulty may be due in part to lack of skill, in part to lack of judgment in selecting good material with which to work, or in part to untoward weather conditions and improper condition of the stocks. But, notwithstanding all these drawbacks, pecan trees can be, should be, and are propagated in large numbers by budding and grafting, and the seedling is becoming more and more a thing of the past ((Hume, 1912).
Pests: The pests attacking buds and leaves are the bud worms, the bud moth, the case worm, the catocalas, the fall web-worm and the pecan caterpillar. Pests attacking the trunk and branches are the twig girdlers and the oak pruner. Pests attacking the fruit is the pecan weevil and the hickory-shuck worm.
Seedling Costs: The quoted price for one- and two-year-old stock of standard varieties varies from 50 cents to $ 2 per tree, in small numbers, with considerable reduction for trees in lots of one hundred or more. It is possible that these prices may be changed often depending on efficiency of propagation and due to economies in growing.
Site Preparation: If the land is covered with a growth of timber, this should be cleared away, and the ground cultivated for a year at least before the trees are set. Corn is probably the best crop to grow on new land, and cowpeas should be sowed thereafter. On poorer ground the land should be continued in cultivation another year, sowing it in beggarweed, cowpeas, soja bean, or velvet beans. These crops should be plowed into the soil in autumn or early winter, after they are dead and dry. Every effort should be made to insure a good stand and a good growth. Inoculation of the seed with nitrogen-gathering germs will help, and a good fertilizer, such as the one recommended for these crops elsewhere, should be applied. Nitrogen, humus and stable manure added to the soil will insure a good growth in the young trees. It is suggested to break and loosen the soil to a depth of twelve or fifteen inches, or even more to improve soil condition.
Planting Techniques: The number of trees which may be set to advantage on any given piece of ground is governed by the quality of the land, the amount of plant food, the amount of moisture, and the grower's objects. On the rich soil, the trees should be set
farther apart than on poorer soil, for they will grow to larger tree size and should be given sufficient room for their best development without crowding either tops or roots. The best time to plant pecan trees is during the months of December, January and February. It is necessary to set a stake for each tree. The hole should preferably be dug six or eight inches wider than the extended lateral roots and eight inches deeper than the length of the tap-root.
Insects and Disease: Disease and insects are often the limiting factors in the production of pecans. The crop requires about 7 months to develop and during that time is subject to attack by a variety of pests. A good disease and insect control program is important, not only in protecting the maturing crop, but also it is essential to the production of high yields year after year. Trees, which are prematurely defoliated by insects, mites, and diseases, frequently set a light crop the following year. Pecan varieties differ in resistance to scab and other diseases. Scab susceptible varieties will require more fungicide sprays than resistant varieties. Even the best spray program can be improved if the following cultural and sanitary practices are followed:
1. Knock shucks from branches. Turn under, or rake and burn, culls, shucks, leaves and leaf stems, to prevent build-up of insects and diseases. for shuckworm control, disc 3 inches deep during late February or early March.
2. Provide better air circulation in orchard; mow or disc weeds, and prune low hanging limbs. Keeping the tree row weed-free by use of herbicides is recommended.
3. Maintain tree vigor by following recommended fertility practices.
Perhaps the three most important factors in a spray program are timing, coverage and rate.
Weed Control (herbicides used): Herbicides advocated for use include trifluralin, diuron, cycloxydim, glyphosate, proprop [dalapon] and paraquat. Tabulated information with notes are given by Vincent (1992) on the options available for weed control in pecan orchards.
Establishment Costs: The establishment costs may vary substantially due to:
1. differences in fertility,
2. weed control requirements,
3. number and size of trees,
5. land preparation, and
6. other areas.
According to the report from the University of Georgia (1992), total establishment costs was about $700/acre.
Wood Properties and Uses
Specific Gravity: 0.6 (green); 0.66 (dry).
Weight: 47 lbs/ft3 (dry).
MOE: 9800 psi (green), 13697 psi (dry).
MOR: 1,365,000 psi (green), 1,735,000 psi (dry).
Common Uses (wood value): The pecan tree is a large Northern American tree that bears sweet edible nuts. A pecan tree usually ranges from 70 to 100 feet in height, but can grow as tall as 170 feet. In addition to the nuts, pecan trees yield strong hard timber used in flooring and furniture.
Cooperative Extension Service, College of Agriculture, The University of Georgia. Estimated costs of producing pecans, 1992.
Hume, H. Harold. 1912. The pecan and its culture. Mount Pleasant Press. p. 1-195.
Manaster, Jane. 1994. The Pecan Tree. University of Texas Press. Austin. p. 11-65.
Sparks, Darrell. 1992. Pecan cultivars, the orchard's foundation. Pecan production innovations, watkinsville, Georgia.
Thompson, T.E. 1984. Pecan cultivars: Current use and recommendations. Pecan Quart. 18(1):20-26.
Thompson, T.E., E.F. Jr.Young, H.D. Petersen, R.E. Worley, R.D. Bar, R.S. Sanderlin R.S. and L.J. Grauke. 1990. Oconee, Houma and Osage: new pecan cultivars. 80th Annual Report, Northern Nut Growers Association. 117-123.
Vincent, A. P. 1992. Herbicides in pecan orchards. Inligtingsbulletin Navorsingsinstitut vir Sitrus en Subtropiese vrugte. No. 238: 17.
Wolstenholme, B.N. and J.B. Storey. 1970. Agreement at last - It's Carya illinoensis. Pecan Quart. 4(4):15-19.
Young, J. A. and C. G. Young. 1992. Seeds of Woody Plants in North American. Dioscorides Press, Oregon. p. 74-77.
Texas Pecan Growers Association
Pecans in Florida