Pecan Growth & Development
Pecan trees grow, flower, and produce nuts according to patterns that growers and researchers have studied since the beginning of the 20th century.
This section describes those patterns beginning with the developing shoots, leaves, nuts, and roots. It also identifies the two characteristics of pecan growth that are especially important to producers: the pattern of male and female flowering among different pecan cultivars and the tendency of some older trees to bear fruit only in alternate years.
Shoot Growth
By carefully observing the growing shoots and nuts on pecan trees, a grower can know when to fertilize and spray, where and how to prune, what kind of yields to expect, and the general condition of the trees.
IN THE NURSERY
Commercial pecan trees are produced in nurseries from root stock and grafts. The root stock germinates from seeds in the spring, and the seedlings usually grow for two years before the
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shoots are large enough-3/8 to 3/4 inch in diameter- for grafting or budding. The grafted plants then remain in the nursery for another year or two before they are transplanted to the orchard.
IN THE ORCHARD
When transplanting nursery trees to the orchard, remove one-third to one-half of the top of each shoot to balance the top with the roots. Otherwise, the roots will not be able to provide adequate moisture and nutrients for the tree. Later, new sprouts may appear along the above-ground portion of the tree. Rub off all of these sprouts except those needed to shape the tree and to shade the trunk.
Pecan trees are usually the last trees to foliate in the spring, so they are seldom damaged by late spring cold snaps. Young, healthy trees in the orchard may have up to three separate flushes of growth during a growing season. The first flush in spring usually lasts three to five weeks. In late June or July, there may be another flush of growth, especially if the trees are young and vigorous and the supply of nitrogen is plentiful. Very young and vigorous trees might even have a third flush in late summer or fall.  |  |
During the first flush of growth in the spring, male and female flowers become visible (Figures 1-1, 1-2, and 1-3). Good shoot growth at this time is especially important because longer shoots, up to about 8 inches, tend to have more blossoms and carry more fruit to maturity than shorter ones (Figure 1-4). As a rule, eight or more compound leaves are needed per nut.
How much the shoots grow during the first flush depends largely upon the condition of the orchard during the previous season. If the nut crop the year before was heavy or if insects or diseases caused the trees to defoliate early in the fall, the new shoots will be relatively short.
Low fertility may also cause poor shoot growth, but if you determine your fertilizer requirements through soil tests and leaf analysis, you can get optimum shoot growth without applying exceptionally large amounts of fertilizer. (See Chapter 4 for guidelines on pecan nutrition.)
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MECHANISM OF BUD AND SHOOT GROWTH
Growth in pecan trees occurs only in certain tissues called meristems. Meristem tissue is part of every shoot tip, including those of dormant or inactive buds. From it develop vascular tissue, the flowers, leaves, and lateral buds.
During the warm days in late winter and spring, the apicalmeristems (those at the ends of the shoots) become active. The buds swell, the outer and inner scales break, and the shoot starts expanding (Figures 1-1 and 1-2). The catkins (the male flowers) unfold (Figure 1-3), followed by the middle leaves. Finally, the tip leaves and pistils develop.
The buds at the ends of the previous year's shoots dominate the growth of pecan shoots. Only these buds tend to develop while the lateral buds break and then abort or remain dormant. If the tip of the shoot is broken off, the buds closest to the end will begin to swell and form vegetative or fruiting shoots.
You can influence the growth of lateral buds to a certain degree. Additional lateral buds develop into shoots when growth regulators such as cytokinins, gibberellins, or dikegulac are applied. High nitrogen applications, high light intensity, and a change from water stress to adequate water levels may also stimulate lateral shoots to grow.
Shoot growth can also be inhibited, particularly the large amount of water-sprout growth that usually appears within a foot of large pruning cuts. Apply indoleacetic acid, ethylene, or abscisic acid to inhibit shoot growth.
TYPES OF SHOOT GROWTH
Classifications of pecan shoots depend upon the relative size of the shoots and the ability of different shoots to bear fruit. The most straightforward classification is the following one developed by H. L. Crane in 1931:
TYPE 1-Very short, weak shoots. These are less than 1 inch long and have small, pointed, very slender leaves. The few, if any, pistillate flowers that form on these shoots usually drop off.
TYPE 2-Short, weak shoots. These are less than 4 inches long and have short, slender internodes and small leaves. They produce many pistillate flowers, but most of them drop before the nuts mature.
TYPE 3-Moderately long, strong shoots. These shoots are 4 to 20 inches long. They are relatively thick and have moderately long internodes and large leaves. They usually produce many pistillate flowers which develop into mature nuts.
TYPE 4-Long, very strong shoots. These are shoots over 20 inches long. They have large leaves and are usually too vigorous to produce pistillate flowers.
RAY WORLEY
University Of Georgia Bud, Leaf, & Nut Development
Management practices, such as spraying and pruning, usually are most effective if you do them at precise stages of pecan growth. Since these stages do not necessarily fall on the same calendar dates from year to year, it is important that you be able to identify them from the appearance of the tree itself. The following are proposed as standard names for pecan growth stages with illustrations of each.
| Figure 1-5. Bud and leaf development stages. |  |  |  |  |  |  |  |  |  |  |  | Esteban Herrera
New Mexico State University
Bill Goff
Auburn University |
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| Figure 1-6. Nut development stages. | Stage 1.
Post-Pollination |  |  |  | 1 week* after pollination
Early May
Stigmas turn brown. Catkins drop. First nut drop occurs. | Stage 2.
Early Nut Sizing |  |  |  | 6 weeks after pollination
Early June
Nuts grow slowly. Fertilization occurs. Second nut drop. | Stage 3.
Rapid Nut Sizing |  |  |  | 9 weeks after pollination
Mid June
Nuts grow rapidly, but no kernel development yet. Early water stage. Third nut drop. | Stage 4.
Late Nut Sizing |  |  |  | 12 weeks after pollination
Late July
Mid water stage. Shell hardening begins at tip. | Stage 5.
Early Kernel Filling |  |  |  | 13 weeks after pollination
Early August
Water stage. Shell hardening half complete | Stage 6.
Kernel Filling |  |  |  | 15 weeks after pollination
Mid August
Late water stage. Early gel and dough stages. Shell hardening complete. | Stage 7.
Late Kernel Filling |  |  |  | 19 weeks after pollination
Mid September
Late "dough" stage. Kernel development near completion. | Stage 8.
Shuck Split | |  |  | 24 weeks after pollination
Mid-Late October
Kernel development complete. Nuts can be shaken from shucks. |
| * Dates vary with season, location, and cultivar.
Diagram modified from Wolstenholme, B.N., and J.B. Storey, 1970. Pecan Quaterly 4(4):15-19. |
Root Growth
 The pecan root system anchors the tree, stores food reserves, and absorbs water and nutrients. If you understand how roots grow and function, more of the young trees that you plant will survive and grow, and you will be able to manage your older trees more efficiently.
At first, pecan roots grow faster than the shoot. In the second year, the shoot begins to catch up. By the fourth year, the trunk is usually higher than the root is deep although the root spread is about double that of the branches. Usually, the root system does not grow any deeper after five years; however, some taproots have been found growing 20 feet deep.
ROOT DEVELOPMENT
 During the first year after the pecan nut germinates, the seedling sends down a taproot that is two to three times longer than the shoot. In addition, the seedling develops feeder roots, the small roots that absorb nutrients from the soil. These feeder roots usually undergo four to eight cycles of growth during the growing season. Some develop into part of the permanent root system while others die and are replaced.
Pecan trees are frequently transplanted when the root system is three years old and the grafted top is one year old. At this stage, a good quality nursery tree will have a well-developed root system with many lateral branch roots (Figure 1-7A). Further root development is illustrated for 6-year-old and 12-year-old trees in Figures 1-7B and 1-7C.
After ten years, the height of the crown of the tree is typically about four times the depth of the taproot. The branch spread is about half that of the root spread.
In mature trees, the roots and branches divide in a similar way, and after about 20 years, a pecan tree is made up almost entirely of branched roots below the ground and branched limbs above. The lateral roots of mature pecan trees occupy an area much greater than that covered by the spread of the branches.
FACTORS AFFECTING ROOT GROWTH
Soil environment and cultivation influence root growth. Roots grow slowly in cold soil and may die at temperatures below freezing. In the spring, as the soil begins to warm up, the roots begin to grow just before the shoots. Pecan roots grow most rapidly when the soil temperature is about 85 degrees; they begin to die when the soil temperature is above 100 degrees.
Pecan roots are most dense 1 to 2 feet below the surface of the ground, but cultivation practices can cause their depth to vary. Deep cultivation tends to force roots to lower levels. Shallow cultivation, a high water table, and mulch litter all encourage the roots to grow near the surface. In addition, lime worked into the subsoil of the orchard markedly increases tap root and lateral root growth.
TRANSPLANTING EFFECTS
Trees can be transplanted with their roots exposed or in containers. When practiced correctly, either method can be successful. Bare-Root Transplants
The bare-root method of transplanting is the more common way to move young trees because it is more economical and, if carried out correctly, can produce a healthy tree with a straight taproot. However, bare-rooted nursery trees undergo considerable shock because they lose most of their lateral roots. For this reason, the transplanted trees often die.
When transplanting bare-rooted trees, be careful to protect the small lateral roots on the transplant. The first new roots that form on the transplanted trees emerge from these lateral roots and from the lower end of the taproot.
Prune taproots to no longer than 10 to 20 inches, rather than leaving them as much as 30 inches long. New roots form better and have a better chance of surviving on a shorter taproot.
In addition, you can stimulate additional lateral roots to grow from pecan taproots by inserting toothpicks dipped in indole-3-butyric acid into holes drilled in the roots. (Romberg, 1938; Taylor, 1976).
Container Transplants
 | | Figure 1-8. Encircling root system of container-grown plant. |
Many pecan growers are turning to container transplants because bare-rooted transplants are relatively fragile until they can develop enough feeder roots. Container-grown transplants suffer less transplanting shock than do bare-rooted trees, but they do tend to develop severely kinked or encircling root systems (Figure 1-8).
It is too soon to know whether the denser root systems of container transplants affect the stability and productiveness of mature trees. However, the evidence so far suggests that the two methods result in similar trees. ADOLPH J. LAICHE, JR.
Mississippi State University Flowering
Pecan trees produce two kinds of flowers, male and female. The male flowers contain pollen and are located along new branches. They appear in clusters or catkins of two or three stalks. The female flowers usually are at the end of the new branches in clusters of three to seven flowers.
As a rule, the two types of flowers do not mature at the same time, so a tree usually does not fertilize itself. Since only fertilized flowers produce pecans, a grower must be careful to match varieties in an orchard that can pollinate and fertilize each other.
EARLY DEVELOPMENT OF FLOWERS
Both the male and female flowers begin to form during the season before the flowers appear. The developing male flowers can be seen with a microscope early in the previous season. The female flowers, although they cannot be seen at all, are thought to be induced the previous August or September. For this reason, the same factors that affect the health of the tree during one growing season, particularly the health of the foliage, will also affect flowering and nut production the following spring. Staminate Flowers
The male or staminate flowers are the first of the two flowers to appear after spring growth begins. The catkins that contain them usually grow on either side of the elongating vegetative shoot (Figure 1-9).
As both the shoots and catkins grow longer, the staminate flowers spread out enough to be seen. There are two catkins per primary bud, three stalks per catkin, about 110 flowers per stalk, and three to five anthers per flower. Each anther may contain 2,000 pollen grains (Figure 1-10).
Pollen shed is said to begin on the day that the catkins are shaken and the pollen falls out. It is complete when the catkins can be rolled in your hand without leaving any pollen. Temperature, rain, and fog all affect the mechanisms that control pollen shed.
Pistillate Flowers
 The female or pistillate flowers appear soon after the male flowers and farther toward the end of the new shoot. Usually, three to seven female flowers appear in a cluster (Figure 1-11). Often, these flowers are green and inconspicuous.
Stigmas on the female flowers are receptive to pollen when they are glossy. They are past receptivity when they begin to dry and turn brown.
Not all of the female flowers become pecans. Some never mature and some fall off the tree. Flowers tend to fall either because they are not pollinated or because the tree is not vigorous enough to support flowering. FLOWERING HABITS
In theory, pecans could be largely self-pollinating since they bear both male and female flowers on the same tree. However, the male and female flowers in most cultivars are not mature at the same time, so self-pollination usually does not occur. This is fortunate because self-pollination often produces smaller, poorer quality nuts and may increase premature nut drop.
Since pecans are not generally self-pollinating, you must plant at least two cultivars in the same orchard-one that will be shedding pollen at the same time that the other is receptive to pollen, and vice versa.
Cultivars that release pollen before the female flowers on the tree are receptive are called Type I or protandrous (Figure 1-12).
Cultivars whose female flowers are receptive to pollen before their male flowers release pollen are called Type II or protogynous (Figure 1-13).
There are also a few cultivars that release pollen at the same time that their female flowers are receptive. These are called incomplete.
COMBINATIONS FOR GOOD POLLINATION
As a rule of thumb, you can plant a combination of most Type I trees with most Type II trees and achieve reasonably good pollination for both types. Pollination types of common pecan cultivars are listed in Table 3-1 on page 26. RICHARD H. MULLENAX
Mississippi State University Alternate Bearing
Alternate bearing in pecan production means that a tree bears a relatively heavy crop of nuts one year and a lighter one the next. It is a characteristic of pecan trees, but it creates serious problems for the grower who must depend on consistent yields from season to season.
To reduce effects of alternate bearing, you should choose cultivars that tend to be consistent annual bearers and then practice good orchard management. Healthy trees of any cultivar are better able to bear pecans consistently from year to year.
CAUSES OF ALTERNATE BEARING
For a tree to grow vigorously enough in the spring to produce the leaves and flowers it needs for a good harvest, it must carry enough assimilated food reserves over the winter to support its first flush of growth. For that reason, anything that interferes with a tree's ability to produce and store nutrient elements, carbohydrates, etc., also interferes with its ability to produce nuts. Among these are the nut crop itself, the age of the tree, and the tree's health. Nut Production
Nut production is a considerable drain on a tree's resources. A pecan tree carries its nuts for most of the growing season. There is little time after the nuts mature for the tree to restore its food reserves before the leaves fall. If the tree is carrying too many nuts, it will begin the next years growth with its reserves depleted. It will then be unable to support adequate nut production and will use the year to build up its reserves for the following year.
Age Of The Tree
The physiological age of a tree also influences the consistency of its yields. As a tree ages, more and more of its shoots begin to produce nuts, and the crop load becomes likely to stress the tree. Cultivars that produce heavy nut crops when young tend to become severe alternate bearers when they reach 10 to 15 years old. 'frees that are alternate bearers when young will become increasingly more so as they age.
Pests, Diseases, And Weeds
Pests that damage foliage and remove nutrients also stress a tree's resources and contribute to its cycle of alternate bearing. Among the most damaging pests are three pecan aphid species and the pecan leaf scorch mites. Aphids are especially harmful because they remove vast amounts of energy, plug the phloem cells of shoots, and reduce photosynthesis.
Diseases and competing vegetation also stress a tree. Pecan scab, zonate leaf spot, downy spot, and vein spot all damage and can kill leaves, reducing tree vigor. Weeds compete for water and soil nutrients PREVENTION
Because the severity of alternate bearing varies with tree type and tree health, it can be partially overcome by planting cultivars that are relatively consistent bearers and by maintaining a healthy orchard. Cultivars
Some cultivars are more likely to be alternate bearers than others, largely because they carry relatively heavy nut crops during their "on" seasons. The more consistent annual bearers tend to thin some of their fruit each season, leaving a greater proportion of leaves to nuts. Table 1-1 lists the tendencies of some common cultivars.
Table 1-1.
Tendency Of Pecan Cultivars To Be Alternate Bearers. | | Relatively Consistent Bearers | Desirable
Stuart
Sumner | | Severe Alternate Bearers | Moore
Van Deman
Mahan
Mobile
Cherokee | Shoshoni
Chickasaw
Mohawk
Success
Wichita |
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Pruning
Trees that have 30 to 40 percent vegetative shoots-the young shoots that feed the tree without producing nuts-are much more likely to be annual bearers than those with fewer vegetative shoots. Less precocious cultivars, such as Stuart, tend to retain a high percentage of vegetative shoots longer than a faster maturing cultivar like Wichita. You can sometimes artificially maintain the percentage of vegetative shoots in trees by selectively pruning some of the mature limbs or by hedging.
Pruning will also open the canopies of mature trees, increasing the light that reaches the lower limbs and thereby improving nut production. 'frees should not be allowed to become crowded nor should canopies overlap.
Soil Fertility
By maintaining soil fertility at near-optimum levels, you can boost a tree's efficiency and offset somewhat its tendency to alternately bear. Chapter 4 discusses in detail the need to take leaf and soil samples and to follow recommendations for fertilizer and lime applications. BRUCE WOOD
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