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Alternate Bearing Considerations for the West

Two trucks transplant young pecan trees to this sandy field.

Site selection can play a major role in an orchard's tendency to alternate bear. (Photo by Catherine Clark)

Some fruit and nut tree species are famous for their alternate bearing, their tendency to swing back and forth every other season between really heavy (‘on’) crop loads and really light (‘off’) crop loads. You can see this kind of thing with apples, citrus, pistachios, olives, and avocados, among others. And, of course, you can see this tendency in pecans. One curious thing about this phenomenon is that it seems the majority of trees in a given orchard or even across a growing region are often synchronized in their alternate bearing so that most of the trees have their ‘on’ years together and ‘off’ years together.

With it being such a widespread phenomenon, alternate bearing probably serves some important purpose in nature. One suggestion is that alternate bearing in a forest setting helps keep the seed predator populations low compared to the number of seeds produced in an ‘on’ year (don’t forget that all fruit and nut trees are domesticated forest trees—and our pecan cultivars are barely a generation away from the wild forest!). In theory, the numbers of seed-eating animals (e.g., squirrels or weevils) drop to low levels during a year when most of the trees are producing few seeds. Then, the next year when there is a huge glut of seeds, the reduced seed-eating animal populations won’t be able to do as much damage to the seeds, and in theory, a greater number of seedlings will successfully grow and survive.

So, the tendency for alternate bearing might be a good thing for a population of wild forest trees. These trees are adapted to the forest, but it’s definitely not a desirable characteristic for an orchard. There are a whole host of reasons why pecan producers and the pecan industry should be anti-alternate bearing. Alternate bearing creates year-to-year unsteadiness for farm revenues and can further throw off farm revenues by reducing nut quality if the ‘on’ season crop loads are higher than the trees’ physiological carrying capacity. Moreover, it adds a kind of uncertainty for consumers and ingredient users with the fluctuation of product availability, and it reduces the efficiency of equipment and infrastructure for production, harvest, and processing. Last but not least, alternate bearing can negatively affect tree health. Huge crop loads are a source of severe physiological stress on the tree that can lead to tree decline or even tree death.

Managing alternate bearing of pecan orchards in the western growing region can be impacted by five factors: tree genetics, site selection, sunlight management, physiological stress, and nut thinning.

1. Tree Genetics & Alternate Bearing

As with most things in crop production, you must first have good plant genetics. As I mentioned above, the tendency to alternate bear is “hardwired” into pecan trees’ genetic code, but that does not mean that all pecan cultivars are created equal in this respect. Given identical ideal environmental conditions, some pecan cultivars will be more prone to severe alternate bearing than others. There is an “alternate bearing index” ranging from zero (equal yields each year) to 1 (absolute alternate bearing with no crop every other year) that can be used to quantify a particular cultivar’s severity of alternate bearing. Some studies show ‘Western’ having a moderate alternate bearing index of about 0.5-0.6, whereas ‘Wichita’ generally has a slightly higher index than ‘Western’ (e.g., Conner and Worley, 2000). My own observation has been that ‘Pawnee’ has a somewhat greater tendency for alternate bearing than both ‘Western’ and ‘Wichita’ under western growing conditions. And ‘Lakota’ seems to have much worse alternate bearing than ‘Pawnee.’ The original NPACT testing before ‘Lakota’ was released reported an alternate bearing index of 0.75 (Thompson et al., 2008), but I suspect it can be even higher in some conditions.

So far, I think ‘Waco’ has relatively low alternate bearing issues under southwestern growing conditions, which jives with some of the earliest data collected during its original release. Regarding other cultivars, I recommend that before planting, producers spend a little time looking at published reports if they’re available. If no data are available, you should talk with other growers who have already planted these cultivars or limit your trial plantings to a safe size to minimize the risks.

2. Site Selection

The site where you plant your orchard can impact alternate bearing too. It can pay to do your research on a particular area before planting—allowing you to possibly modify the site to make it more suitable for pecans ahead of planting or possibly re-think the idea of planting pecans there altogether.

The soils or irrigation water quality at some sites might be ill-suited for pecans, guaranteeing poor root health, mineral nutrient deficiencies, water shortages, or other kinds of physiological stress. These issues, in turn, can cause poor return bloom following even a moderate ‘on’ year. Other sites might be marginal because of the risks for springtime freeze injury. Spring freezes can trigger severe alternate bearing in pecan orchards by killing all the pistillate flowers in an orchard so that there is a 100% ‘off’ crop. After a year like that, the following year will invariably show huge return bloom with massive amounts of pistillate flowers, triggering very severe alternate bearing. In some locations, installing wind machines to counter springtime freezes could be worthwhile. If they happen at the wrong time, other weather events, like hailstorms, can also cause a 100% ‘off’ crop year and similarly trigger severe alternate bearing.

3. Sunlight Management

Orchard sunlight management is a hugely complex subject that involves planting patterns, planting density, tree vigor, and tree training and pruning. We won’t get into all of that here, but suffice it to say one of the things that makes the southwestern growing region such a great place to grow pecans is the abundance of high-intensity sunlight that we are blessed with. Pecan trees are highly responsive to sunlight, as well as to the lack thereof.

Without a good, effective sunlight management plan, the sunlight distribution in the tree canopies will become less uniform due to self-shading and shading of adjacent trees. The most obvious impact of this on pecan production will be increased alternate bearing over time, along with declines in nut size and percentage kernel during the ‘on’ seasons. One important study conducted in Australia showed that when a program of light management through mechanical pruning was established in pecan orchards, the index of ‘Western’ orchards dropped from 0.5 to 0.2, meaning the swings in yield between ‘on’ and ‘off’ years were significantly reduced (Wood and Stahmann, 2004). In that same study, the alternate bearing index of ‘Wichita’ trees dropped from nearly 0.8 to about 3.5. Similar results were shown in a study conducted here in New Mexico on a ‘Western’ orchard, where the index for unpruned trees was over 0.5, but mechanical pruning reduced this to below 0.3.

4. Careful Management of Physiological Stress

As with site selection, physiological stresses of all sorts can potentially hurt the tree’s ability to produce pistillate flowers in the springtime after even a moderately productive year. This, of course, includes insufficient light availability and distribution but also includes salinity, water stress, nutrient deficiencies, feeding by aphids, diseases, competition by weeds, and so on. It is crucial to be diligent in collecting soil and irrigation water samples for lab analyses, monitoring soil or plant water status, collecting leaf tissue samples for nutrient analyses, and scouting for pests. More than that, it is essential to diligently respond to what you learn from these tests by adjusting your management promptly to keep plant stress to a minimum.

5. Nut Thinning

Currently, it’s not common practice in the Southwest to thin out immature pecan nuts during heavy ‘on’ years. In fact, I only know of a small handful of southwestern pecan producers who have ever done it. That being said, it is well established that fruit thinning is useful for reducing alternate bearing intensity, as well as for improving nut quality during the ‘on’ year.

With pecans, some of the immature nuts of trees with excessive crop load should be mechanically removed by shaking the trees with a trunk shaker when the nuts are heavy enough to be shaken out but before the “dough stage” or kernel fill begins. For ‘Western’ and ‘Wichita’ pecans in New Mexico, that timeframe will probably mean that it needs to be done sometime before mid-August to be most effective. After the beginning of the dough stage, any benefits of fruit thinning drop off pretty rapidly.

I don’t expect fruit thinning by trunk shaking will ever become a standard practice for managing alternate bearing in the Southwest. Still, I think it should be strongly considered as a tool to bring back severely alternating orchards to a more moderate yield alternation pattern. What you may not have thought of previously is that mechanical pruning is actually a form of fruit thinning in addition to its role as a light management tool. When you hedge and top a dormant pecan orchard, you are removing some portion of those trees’ fruit wood for the following season. To be used effectively as a fruit thinning technique for managing alternate bearing, mechanical pruning should be done most aggressively in the winter ahead of an ‘on’ year.

By considering these five factors, growers can better manage alternate bearing in their orchards and, ultimately, protect their farm revenue.

Sources Cited:
Patrick J. Conner and R.E. Worley. 2000 Alternate Bearing Intensity of Pecan Cultivars. HortScience 35(6):1067–1069.
T.E. Thompson, L.J. Grauke and W. Reid. 2008. ‘Lakota’ Pecan. HortScience 43(1):250–251.
B.W. Wood and D. Stahmann. 2004. Hedge Pruning Pecan. HortTechnology 14(1):63-71.
Author Photo

Richard Heerema

Dr. Richard Heerema is the Extension Pecan Specialist at New Mexico State University, Las Cruces, New Mexico.