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The pecan’s journey from flowering to nuts

Catkins on a 'Wichita' pecan tree.

Catkins on a 'Wichita' pecan tree. (Photo by Josh Sherman)

It’s that time! Have you seen them? Most of you reading this probably so, but I’ll bet that the majority of the world has not seen pecan flowers. I used to think those long “tassels” hanging from the trees were somehow just part of the leaves. Those “tassels,” or to botanically correct the catkin, are just a small part of the magnificence of pecan flowering.

Stop and ponder a little about the flowering development and maturity, the way male and female flowers form on the same tree, the origination of the male flowers from the female flowers, the absence of synchronicity between the male and female flowers development time and maturity, and between cultivars, the various shapes, sizes and characteristics of the flowers.

Historically, researchers, horticulturists, and botanists have studied pecan flowers. I cannot say for sure the dates when this focus originated, but performing a light search reveals some familiar researchers who gave extensive work in this area, including L.J. Grauke, Tommy Thompson, Bruce Wood, Morris Smith, Ray Worley, George Ray McEachern, and Richard Heerema.

Pecan flowering has been given relatively more attention in the past decade. Still, there is vast information to be gained from studying these developments across cultivars and regions. What we do know from research, observation, and experimentation is:

  • The pecan tree is monoecious with a dichogamous flowering habit that exhibits heterodichogamy.
  • For maximum pollination, two (or even three or four) types of pecan cultivars should be planted.
  • It takes a certain number of pollinators and distance to effectively pollinate others.
  • The flowers are wind-pollinated (no bees required).

Monoecious, Dichogamy, Heterodichogamous – What in the world?

All these terms relate to the “sexual system” or reproductive morphology and mechanism of pecan. Monoecious stands for “mono” (single) and “ecious” (house). A single house for the male and female flowers. In other words, the pecan tree contains both the male and female flowers on the same tree. However, they are not perfect flowers where the pistillate (female organs) and staminate (male organs) flower parts are contained in one structure. The male flowers are on separate parts of the tree from where the female flowers originate.

Dichogamy refers to the period of flowering of male and female flowers. Specifically, the timing of maturity of the flowers. Within a single pecan tree, the male flowers may develop and mature first, thus releasing their pollen prior to the female flower becoming receptive. This type of pecan is termed a protandrous, or Type I. If you flip the pattern to where the female flower becomes mature and receptive before male shedding pollen, this is the protogynous, or Type II, pecan.

Another definition of dichogamy is explained by Dr. Richard Heerema, who simply states, “…the period of pollen shed by male flowers occurs either a little earlier (Type I) or a little later (Type II) in the spring than the period of pistillate (female) flower receptivity.”

Within this dichogamy of flowers, there can be pecan trees where the male flower’s maturity and development occur significantly earlier than the female flower’s maturity. Thus, a truly complete dichogamous tree or some cultivars (most cultivars) may overlap, in which the male flowers mature and are still shedding pollen when the female flower comes into maturity and is receptive. The latter would be termed incomplete dichogamy. One common cultivar used in the Southwest which displays incomplete dichogamy is the ‘Western.’

Heterodichogamy is used in reference to the species as a whole in which there are differences of the dichogamy, or flowering interval, in Type I and Type II pecan tree cultivars. Heterodichogamy is phylogenetically widespread, occurring in 9 orders, 12 families, and 18 genera of flowering plants. Some of the best work investigating the heterodichogamy of 80 pecan cultivars and an in-depth documentation of the flowering intervals amongst cultivars to improve selection for maximizing cross-pollination was done by Wood, Smith, Worley, and others (Table 1). L.J.Grauke also has an excellent resource on pecan flowering located online.

a table showing the flowering classification of pecan cultivars at different ages

Wood, B. W., Smith, M. W., Worley, R. E., Anderson, P. C., Thompson, T. T., & Grauke, L..J. (1997). Reproductive and Vegetative Characteristics of Pecan Cultivars, HortScience HortSci, 32(6), 1028-1033.

This explanation may or may not get confusing for some, but the one thing that is most interesting about this mechanism of heterodichogamy that pecan demonstrates is that it prevents outcrossing or self-pollination from occurring and encourages genetic diversity. It has been documented that self-pollinated pecan nuts are lower in quality, exhibiting smaller size, and poorly filled kernel. Thus, the reason to plant two or more cultivars.

Two or More Cultivars for Maximizing Cross-Pollination

Due to the avoidance of self-pollination and climate and micro-climate effects on the male and female flowers’ maturity timing, it is recommended, if one is to ensure maximum pollination, to plant two to three different cultivars with overlapping flowering intervals. Planting will take careful consideration of the Type I and Type II cultivars and a closer look at the table provided.

A little anecdotal observation: there are some 30-year small acreage producers in my service area of Arizona who have planted five different cultivars. They have stated to me that most years in their experience, there is little early fruit shed due to incomplete fertilization. When they have noticed excessive early fruit drop, it was because the year had some abnormal weather patterns.

Work done in the 1920s and 1930s by Woodroof and Woodroof documented how the male flower pollen is shed and how weather affects both the release of pollen and the duration of that pollen shed. These scientists discovered that the anther (where the pollen develops on the male flower) is composed of only two cell layers, an inner and an outer layer. The anther’s opening is related to a drying and contraction of the outer and inner cell layers, thus the release of pollen.

Now, it makes sense that increasing drying weather conditions or even increasing humid conditions will likely affect the timing and duration of pollen shed and female flower receptivity. In the Southwest, we often have high wind conditions associated with dry air during the flowering period. These conditions not only shorten the duration of pollen shed but also shorten the period of pistillate (female flower) receptivity due to drying out the flower, no matter the dichogamy type.

The tips of pollinated 'Pawnee' flowers appear a dark purplish colors amidst bright green leaves.

‘Pawnee’ flowers turn a darker color when pollinated. (Photo by Josh Sherman)

If you have ever examined the pistillate flowers when they are mature, they all have different shapes, sizes, and colors based on the cultivars but, usually, always have some sheen to them. This sheen is the moisture on the stigmatic surface, an exudate that ensures pollen will secure itself once in contact. Then, when pollinated, the sheen disappears, and they appear darker. Other cultivars, like ‘Pawnee,’ turn a very dark bronze to purplish color when pollinized.

How many pollinizers does it take?

In 2000, Bruce Wood published an article in HortTechnology on Pollination Characteristics of Pecan Trees and Orchards. Wood’s article gives a complete description of pecan flowering and also eludes to how to achieve maximum pollination and reduced self-pollination that negatively affects yield and quality.

Basically, we need to re-think the use of the recommendation of the 5:5 rule. If you have not heard of this rule, it is described as planting a pollinator tree every 5th tree, every 5th row in an orchard design. However, it is my opinion there is a better recommendation than this. One suggestion by Dr. Heerema, at least for the Southwest’s 30-foot spacing designs, is to “plant eight rows of the main desired cultivar followed by two rows of the pollinizers, with one cultivar pollinizer in one of these rows and another cultivar pollinizer in the other row.”

Wood’s research suggests that the greatest pollination is gained by having adjacent trees of both types of dichogamy, and the amount of pollen needed to penetrate target trees is more important than the distance the pollen travels. In other words, it can be considered to plant one type in one row followed by an adjacent row of the other type and repeat the design throughout (or even consider adding a 3rd or 4th cultivar in the mix) for best pollination to occur.

figure that shows the flowering phenogram comparing temporal aspects of pollen dispersal and receiving periods for "southern" pecan cultivars, cultivated in Byron, Georgia

Wood, B. W., Smith, M. W., Worley, R. E., Anderson, P. C., Thompson, T. T., & Grauke, L..J. (1997). Reproductive and Vegetative Characteristics of Pecan Cultivars, HortScience HortSci, 32(6), 1028-1033.

Secondly, producers should give the selection and design much thought prior to planting, and this 2-class system of simply selecting a Type I and Type II needs more attention. Wood suggests looking at the 30-class system in which the different dichogamies of pecan cultivars are separated by Type I and Type II, but then further divided into Very Early, Early, Mid, Late, and Very Late, and even further separated by tree age effects on the flowering intervals (Table 2). Thus, this division makes a higher potential for selecting the cultivars that will cross-pollinate one another.

Options to Correct Pollination Issues

Do not fret if you observe negative impacts on your yields based on inadequate pollination due to cultivar and dichogamy type selection not overlapping in their flowering maturity intervals. You have options to consider and need careful consideration at that.

One option is to remove and transplant the desired cultivars; however, this would obviously be an expensive endeavor.

A second option may be to graft some of the desired cultivars into the top of established trees. There are many grafting options to train crews on and even specialized manufactured tools (grafting nursery shears) available on the market now that make the process faster and more consistent.

A third option may be collecting pollen and performing artificial pollination in the orchard. Dr. Heerema touched on this in his article on pecan pollination in the May 2017 Pecan South. The most difficult and laborious part of this method is collecting the male catkins at the early onset of pollen shed. Then they are to be dried for several hours, protecting them from excessive temperature (no more than 95 degrees Fahrenheit) and too much humidity. After drying, they can be sifted through a sieve into an airtight container and stored in the refrigerator until ready for dispersal or kept in the freezer for up to two years. As for the dispersal of the pollen when it comes time, the date industry has developed some methods that should prove effective. A simple internet search on artificial pollination in dates will show you many examples.

Lastly, other factors can affect pecan flowering. Some other considerations are the previous season stresses for instance—especially in terms of potassium levels, other nutritional disorders, and water management. Here again, no specific generalization can be made due to the variability across our pecan production regions and even variability within a single region, but these experiments’ hard work and results have at least gotten us closer to some recommendations to ensure better pollination and fruit set, specifically in the careful selection of these cultivars and their dichogamy patterns. However, there is still a lot of work to be done in the area of pecan flowering and other relationships, even hormonal, and that I think merits more focus; after all, it is the genesis of our pecan nut.

Author Photo

Joshua Sherman

Joshua Sherman is an Area Extension Horticulture Agent with the University of Arizona, Willcox, Arizona.