Take your head out of the canopy, focus on the orchard floor
One key piece to this management puzzle is orchard floor management. With so much focus on the tree itself, it’s easy for a grower to forget how the ground that impacts other decisions. But by thinking about one’s end goal and examining the other factors within their operation, a pecan grower may discover how their orchard floor can augment the other pieces of their management practices—irrigation, pest management, and harvest decisions.
Orchard Floor Management Options
A number of options exist with regard to orchard floor management systems and there are numerous considerations involved in deciding which is the right choice for your orchard. The most basic considerations for all pecan producers in the western growing region revolve around these four things: annual cost per acre of the orchard floor system, compatibility of the floor system with other routine orchard activities, the relative convenience of the floor system under the environmental conditions of the orchard, and the floor system’s sustainability for both the farm operation itself and the broader environment. As with most things in pecan production, these most basic considerations for choosing an orchard floor system can vary a great deal from orchard to orchard or even year to year—so, the “right” choice will certainly not be the same for everyone and may not always be clear cut.
Several secondary considerations can also sometimes play a role in deciding on an orchard floor system. One example of these is aesthetics. It might be true that beauty is in the eye of the beholder, but it is also true that growers may select a particular orchard floor system simply because she or he really likes the way it looks—and there is nothing wrong with that so long as the four most basic considerations are dealt with first. The aesthetic appeal of the orchard can even have an indirect or direct impact on orchard income (for example, those with direct marketing outlets located right by the orchard).
A very popular orchard floor management system in the Mesilla Valley, New Mexico continues to be an orchard floor completely free of vegetation. This system is usually accomplished by some combination of herbicide sprays and cultivation. Increasingly, in the western growing region, however, producers are adopting a system with some kind of strip vegetation control, where a vegetation-free strip is maintained, usually with a herbicide, in the tree row and a vegetated strip (often called the “mow strip”) is maintained in the space between the tree rows. Often, the floor vegetation in this sort of system is composed of weeds and whatever else happens to grow there (i.e., “resident vegetation”) and is managed through regular mowing. Some producers plant either an intercrop or a cover crop in the vegetated strip. An intercrop is a crop planted on the orchard floor for the purposes of harvesting that crop. For example, some growers plant alfalfa between the tree rows with the goal of cutting and baling a hay crop. A cover crop, on the other hand, is a crop intentionally planted with some other goal in mind, usually for the benefit of the orchard soil.
Irrigation System Considerations
When formulating one’s orchard floor management, a producer must also consider which irrigation system will work best with one’s orchard floor system. A large number of pecan producers across the western region still use some variation of a basin flood irrigation system. Both completely vegetation free and vegetation-free strip orchard floor systems can be used successfully with flood irrigation, but for most orchards, the vegetation-free strip system may present some advantages over the completely vegetation free system. Especially where there is a heavy reliance on cultivation or tillage, flood irrigation has the potential to push large quantities of soil to the far end of the irrigation run, which will have negative consequences for the uniformity of applied irrigation water. Furthermore, depending on soil texture, a completely vegetation free orchard floor can require a significantly longer dry down time after a flood irrigation before heavier equipment like tractors, sprayers, or mowers can safely drive in the orchard again, than in orchards where a mown vegetated strip is maintained. Driving in the orchard before the soil has dried down sufficiently not only presents some risk for getting equipment stuck (in a worst-case scenario), but also presents an increased risk for soil compaction (a problem for water percolation, salt leaching, and root health) and formation of tire ruts in the orchard floor (a potential problem at harvest).
With pressurized irrigation systems (sprinklers, micro-sprinklers, or drip), there are also some important considerations for orchard floor management.
First, if the goal is to maintain a living vegetated strip between the tree rows, most areas in the western growing region will require a pressurized irrigation system that wets the entire orchard floor. With drip systems and some micro-sprinklers, parts of the orchard floor will have living vegetation only during the rainier parts of the season. That’s not necessarily a problem if a mown “resident vegetation” cover is in mind, but it is certainly a major need with most kinds of intercrops or cover crops in the semi-arid West.
Second, for pressurized irrigation systems with above-ground parts (e.g., PVC risers or polyethylene tubes) it is essential to keep the orchard floor around those above-ground parts completely weed free at all times. Most often, the above-ground parts of the irrigation system are placed in or near the tree rows, so they are already within the vegetation-free strip. The important point here is that growers with these kinds of systems cannot ever let the weeds get out of control in the vegetation-free strip. Any weeds present can block the uniform distribution of the water, and once present, these weeds can be extremely difficult to remove, especially since mechanical weed control approaches that will not damage the irrigation system are limited. The most effective weed management systems for orchards with above-ground pressurized irrigation systems typically involve a rotation of pre-emergence herbicides in the spring with a judicious rotation of post-emergence herbicides as needed throughout the rest of the growing season.
As pecan harvest approaches, the standard practice in most areas of the West has been to eliminate all orchard floor vegetation. Sometimes that simply means mowing very closely with a flail mower. For others, it might mean light cultivation followed by rolling/packing and an irrigation—to create a firm, smooth surface from which to harvest. In some soil conditions where there are many big cracks in the soil surface (typically heavy clay soils and soils with high sodicity) or the soil is very soft (typically deep sandy soils), it may be best for harvest efficiency if there is a short-mown grass cover crop to hold the nuts up off of the soil surface and provide greater support for the harvest equipment. Additionally, dust produced by shakers, sweepers, and harvesters at nut harvest may be somewhat lessened if there is a short-mown cover crop present. In any case, it is important that the orchard floor be managed for the whole season with harvest in mind. If there is excess residue from mown vegetation on the orchard floor at the time of harvest, this can dramatically slow down the harvest operation and the amount of trash that ends up in the harvester.
While much of what goes into choosing an orchard floor management system surrounds the grower’s ability to carry out other orchard activities, the overall health of the trees and the efficient use of production inputs can also potentially be affected by our orchard floor management. And this, in turn, affects our ability to farm sustainably.
Competition occurs when the orchard floor vegetation “steals” resources like fertilizer nutrients, water, or even sunlight (in the case of very big weeds and very small trees) before the tree can get it. When this happens, either the tree will function below optimum or the producer has to apply more of the resources that are being “stolen” by the weeds so that the tree can get enough. Either way, the grower loses in profit.
In the case of fertilizers, the actual nutrients may not be lost from the orchard system for the long term when the weeds get it first. Over the long term, the nitrogen, phosphorus, etc. that the weeds get first will simply be recycled back into the soil when the weeds are mown and mulched back in the soil. Nevertheless, there can be important short-term consequences of weeds outcompeting smaller trees for mineral nutrients.
For water, on the other hand, when weeds outcompete the trees for irrigation water, the water is lost from the system forever. In young trees this water loss is likely to be particularly important to the tree’s health and wellbeing both because the tree root system is smaller and the tree canopy casts a shadow on a smaller part of the orchard floor. As the tree ages, the root system goes deeper and wider; it is then able to extract water from parts of the soil profile where the weeds’ roots don’t reach, plus the shade on the orchard floor significantly reduces the total water used by the weeds.
As was already suggested above, the impacts of resource competition are a much greater concern for small trees than for larger, bearing trees. Several studies have shown how floor vegetation growing near young pecan trees has dramatic detrimental effects on the growth and establishment rates of those trees. And the economic impact of this is considerable because of the delay in production due to weeds growing near the young trees. Nevertheless, work conducted here at New Mexico State University could not detect any negative effects of competition by orchard floor weeds for mineral nutrients or irrigation water in a bearing/mature flood-irrigated ‘Western’ orchard. Season-long maintenance of a completely weed-free area of at least 3 or 4 feet on all sides of immature trees is recommended.
Orchard floor management affects soil properties and health, which can then also, in turn, affect tree health and performance. First, orchard floor management affects the risks for wind erosion and, to a lesser extent water erosion of soils in the western growing region. In our environment here in the West, tons of soil per acre may be lost due to wind erosion each year if special care is not taken to mitigate this. Cultivation greatly increases the amount of soil loss to wind erosion, especially in orchards with sandier soils. Keeping strips with some sort of floor vegetation—even dead vegetation—between the tree rows greatly reduces this. Second, frequent cultivation and low organic matter cause soil structure to deteriorate. Soils may become compacted which decreases the soil’s ability to hold and move water and nutrients. Maintaining vegetation in the row middles also may help mitigate these problems.
Most pecan arthropod pests in the West live only on pecan trees and will not live on the orchard floor plants, but orchard floor vegetation can nonetheless change the dynamics of the arthropod communities in an orchard. Stink bugs and leaffooted bugs are pecan pests that actually will take up residence in orchard floor vegetation; in fact, they tend to prefer living and feeding in certain cover crops than in the pecan trees. As long as these bugs stay out of the trees, they don’t really present a serious threat, but when the floor vegetation dies or is mown, the bugs may be forced to go into the canopy and start causing issues. This movement is something to watch closely as you are managing the orchard floor vegetation. Alternatively, growers may also use certain cover crops plants (e.g., various legumes) as “trap crops” where the cover crop, rather than the tree canopy, is treated with insecticides to reduce stink bug populations (always be sure to carefully read and follow all insecticide labels).
On the other hand, orchard floor vegetation can also provide food and shelter for numerous beneficial arthropods that can help keep aphids and pecan nut casebearer populations in check. Some of these beneficials include parasitoid wasps, lady beetles, lacewings, and spiders. In the same way that the stinkbugs don’t impact the trees when they’re in the cover crop, the beneficial arthropods aren’t of much consequence to the pecan tree when they stay on the orchard floor. Some growers will mow the orchard floor vegetation at critical stages of pest lifecycles in order to force the beneficials up into the trees. This seems like a reasonable strategy for an IPM program, but, to my knowledge, there is no research about how effective this practice is.
Before looking up into the trees this summer, growers should take a moment to analyze what’s happening underneath the canopy. The orchard floor serves as a base for not only the pecan trees themselves but also for one’s management system in general. By fitting all the pieces together, pecan growers in the West can make the most of an essential part of pecan growing: orchard floor management.