Pecan Roots—A Sight Unseen
Scientists also have instruments like minirhizotrons, which are literal windows beneath the orchard floor. Minirhizotrons are clear tubes buried into the soil, allowing us to nondestructively and non-invasively see the tree’s roots growing with a specialized camera. This tool can be highly informative, but you can imagine this has a drawback. Even those roots are not in a completely natural environment because of the glass or plastic tube they encounter (the camera can’t see roots away from the tube). Some other advanced technologies, like ground penetrating radar, show some promise for exploring beneath the soil surface of an orchard. The bottom line is that we know much less about the bottom part of the tree, the root system, than we know about the top, the canopy—even though the root functions are major contributors to the tree’s overall health and performance.
Structure and Function of Pecan Root Systems
There is a common idea that trees’ roots reach out to the “edge of the dripline,” and that’s it. Maybe this idea has merit in some situations where the only water source is rainfall. The dripline may be the best soil microenvironment for the roots to proliferate, whereas it is too dry both under the main part of the canopy and past the dripline. Here in semi-arid New Mexico (we usually get less than 10 inches of rainfall per year), the roots of a young pecan tree in an orchard or a freestanding mature pecan tree can reach out several times the distance of that tree’s canopy so long as the soil conditions are suitable (including irrigation).
An old scientific study (Woodroof and Woodroof, 1934) was conducted in Georgia, where they showed that even under rainfed conditions, a 12-year-old ‘Success’ pecan tree had a canopy spread of 17 feet, but the roots had a total spread of 47 feet! That might come as a surprise to a lot of people. In a flood-irrigated ‘Western’ cultivar pecan orchard in New Mexico, there’s usually only 15 feet or so from the tree trunk for the roots to explore before they start encountering (and competing with) roots from the neighboring tree in the orchard. I think this is probably the point when the roots stop exploring much further under our orchard conditions.
What about the depth of the roots? Sometimes drawings of tree root systems show a rounded root system that more-or-less mirrors the canopy. If you’ve ever seen a toppled tree, you will know that this is not really the shape of a tree’s root system. In reality, the vast majority of a tree’s roots are near the surface (to only a few feet depth) where the water, nutrients, and air are most abundant in the soil profile. This often gives the entire root system’s overall appearance more of a disk or flattened inverted dome shape than the globe shape you sometimes see in illustrations. Some roots do indeed go deeper, even much deeper, into the soil profile, but these roots make up a small minority in the overall root system. Pecan trees are naturally phreatophytes, having root systems well designed to access water from the top of the water table. This characteristic has allowed native pecan trees to survive in riparian areas even when there are extended periods of drought when precipitation and river flooding are insufficient to meet tree water needs.
A naturally-grown pecan tree has a big tap root that goes down to the water table. However, this may be absent from many of our commercial trees because this tap root was cut off when dug at the nursery. There are also some “sinker” roots that go down very deep at a 90-degree angle from the lateral scaffold roots and accomplish the same function of accessing deep water at the top of the water table.
A substantial percentage of the root system consists of fine roots whose primary function is to acquire water and nutrients. These roots can grow quite quickly but also tend to have short life spans of only weeks or months. This part of the root system, therefore, cycles in and out pretty quickly, which allows for these fine roots to respond rapidly to any changes in the soil microenvironment, like the depletion of nutrients in a patch of soil, a band of fertilizer that wasn’t there before, or damage due to cultivation. The growth of the roots occurs at the tips, which usually have a whitish color. It’s this area of newest growth near the tips of fine roots that has the greatest ability to absorb nutrients and water (although some level of absorption can occur several centimeters up from the root tip, it’s unknown whether that is the case for pecan roots). In most plant species, a zone along the length of a fine root just past its tip has a little “forest” of root hairs. Root hairs are not branch roots; rather, they are microscopic tubular extensions of the root epidermal cells that extend into the soil. Root hairs significantly increase the absorptive surface area of the fine roots. But guess what. Pecan trees do not have root hairs on their roots like most other plants.
With regards to absorbing nutrients and water, the lack of root hairs probably puts pecan trees at a bit of a disadvantage compared with other plants. On the other hand, pecan roots are unusually good at forming associations with mycorrhizae, which might make up for this disadvantage. Mycorrhizae are fungi that can infect pecan roots, but instead of causing disease, these fungi have a symbiotic relationship whereby the tree “shares” carbohydrates from photosynthesis with the fungi in exchange for nutrients, like phosphorus, that the fungi have extracted from the soil. The mycorrhizal fungal strands can be thick and extensive so that in a similar fashion to root hairs, the amount of soil that is explored and extracted on behalf of the tree is greatly increased by mycorrhizae. Research at New Mexico State University funded by the Specialty Crops Block Grant program is investigating how cover crops, compost application, and inoculation can be used to manage the pecan root microbiome, including mycorrhizae.
Fine roots additionally function in hormone production for a tree. Plants produce cytokinin-type hormones in the root tips, which affect the growth patterns of the roots themselves, but can flow long distances through the xylem up to the top of the tree and probably have important effects on shoot growth/development patterns there, too, as well as regulating a wide range of other physiological processes like photosynthesis and senescence.
Another interesting plant hormone produced in roots is abscisic acid (aka ABA). Like the cytokinins, ABA created in the roots might have some effects over both short distances (i.e., within the root) and long distances (i.e., up in shoots). Still, the impacts of ABA on plant growth, development, and physiology often seem opposite to that of cytokinins. One brilliant function of ABA may be the early signaling of impending drought. In some plants (maybe pecan too), roots in drying soils may produce ABA that “tells” the stomata up in the canopy to begin closing to conserve water even before the physiological water status of the leaves significantly declines.
Somewhat courser and higher order roots have longer lifespans than the fine roots, on the scale of years instead of weeks or months, but are not necessarily a permanent part of the tree root system. These courser roots have two primary functions: 1) conveying water and nutrients from the absorptive fine roots up to the larger perennial (more permanent) roots and then, from there, onto the trunk and the top of the tree and 2) storage of carbohydrates, like sugars and starch.
That second function is, I think, an underappreciated function of the root system. Each season, deciduous trees have to store up all the food energy needed to survive the winter plus enough to get growing again in the spring until photosynthesis is able to meet all the demands for carbohydrates and growth. This period of springtime dependency on carbohydrates stored over winter from the previous season no doubt includes bloom and the early stages of fruit set, which are so critical to final crop determination. The above-ground perennial structures, including the trunk, limbs, and branches, store much of the wintertime carbohydrates within the wood and bark tissues, but the course roots and perennial roots also represent a major storage pool for carbohydrates. Indeed, some research has shown relatively strong relationships between nut production and root carbohydrates. For example, Wood (1989) showed a significant positive increase in nut yields of 80-year-old pecan trees with increasing starch concentration in roots in the 0.75-to-1.25-centimeter range during the previous dormant season (i.e., in January of the same year). But, as is often the case with tree physiology, some other studies have shown no such association between carbohydrates and yield (e.g., Smith et al., 2007), showing us that the whole story is probably much more complex than just carbohydrates in the roots.
In addition to transporting water and nutrients to the tree top and storing carbohydrates, the larger perennial roots have an important structural support and anchorage role. The smaller course roots and even the fine roots also contribute to this to some degree, but ultimately it’s the framework of large roots that holds the tree upright when the wind blows.
Maintaining Healthy Root Systems in Pecan Orchards
Where should a grower start to manage for a healthy pecan tree root system over the long term? As usual, the beginning is a perfect place to start. The beginning, in this case, means site selection.
When it comes to site selection and root health, the orchard site’s soils are at the forefront of considerations. In their natural habitat, native pecan trees thrive in well-drained ridges having loam soils but generally not in poorly drained, clayey areas (Sparks, 2005). In managed, improved cultivar pecan orchards in the western United States, the same principles apply to soils that are poorly drained due to heavy soil texture (i.e., a lot of clay) or some barrier to drainage (e.g., compacted or impermeable layers, stratified soil textures in profile, sodicity issues) it can be a tricky balancing act to supply enough irrigation water to the trees while maintaining enough soil oxygen for a healthy root system. On the other hand, native pecans are also less abundant in excessively drained sandy places. The problem with very sandy soils is different and more closely associated with the trees’ actual water stress levels (and maybe some nutrient issues too). But this problem is associated with root health, as root health does begin to suffer when soils become excessively dry. In one sense, this issue is a bit easier to handle in a managed, improved orchard by irrigating more frequently, but that assumes sufficient water is available at the right time.
If a site is not currently ideal for pecan tree root health, it might be possible to fix the problem before planting trees. It depends on the problem and how much money you want to spend on the pre-plant fix. Changing the soil texture of a field (typically by hauling in large volumes of sand) is sometimes suggested, but I think this is probably not an economically feasible option in most cases. Using an excavator, backhoe, or slip plow to mix soil layers with different textures in stratified soil is a common preplant field modification if the textural transition is not too terribly deep in the profile. While this too can be quite expensive, it is certainly effective in helping with drainage issues that are damaging to tree roots.
Selecting trees with rootstock genetics that are well adapted to the orchard site’s soils is also critical. Unfortunately, at this time, we have less information about rootstock genetics than some other fruit and nut tree crops, but that’s changing quickly with a couple of recent USDA-funded grants. With one of these projects (now completed), we tested the adaptability of different ungrafted seedling pecan rootstocks to alkaline saline-sodic soil conditions in Arizona. Some of the rootstock varieties that have been commonly used in the west (like ‘Riverside’ and ‘Bradley’/’ Ideal’) showed tolerance to these harsh soils, but the real surprise standout was ‘Elliott,’ which had the greatest growth and survival rates under those conditions (Smith et al., 2021). We hope to learn even more about rootstocks and root health under saline/sodic conditions and other challenging soil conditions in the upcoming years with a new project.
Regarding root health, another crucial thing to consider close to the beginning is the care of the baby trees by the nursery crew that digs the trees, by everyone who handles the trees between digging and planting, and finally, by the planting crew. Everyone along this chain must understand what it takes to ensure that the roots remain cool and moist the whole way. A dried out root system will lead to trees that struggle to survive during that first year and can mean a prolonged start for the orchard. If you are working with a reputable nursery, they should be able to assure you that the trees were handled properly until they arrive at your field. If you are not able to plant bareroot trees right away, it will be essential that you either move your trees into refrigerated storage until they can be planted or heal the tree in. At planting, it is critical that the bareroot trees be taken to the field in relatively small batches and that the roots are kept covered to protect them from drying out in the wind and sun while they are awaiting planting. The trees should be “mudded in” when they go into the planting hole, meaning water is added to the hole as it is filled with soil. This does two things to help root health: it prevents the roots from drying out right away in the dry soil and removes air pockets around the root system that could cause longer-term drying/damage to the roots.
Another critical root health aspect to consider at planting is circling roots which can cause issues over the long term. Circling roots can be a major issue with potted trees, especially if a nursery uses smooth-walled, circular pots and if the trees were grown in the pots too long, becoming “potbound” or “rootbound.” If the roots are growing in a circle around the walls of a pot or along the bottom of the pot, they can prevent the root system from exploring out away from the tree and, ultimately, can even “strangle” or “girdle” the root system. If you are planting trees with circling roots, it will be essential to prune and/or straighten them so that they don’t destroy the tree’s root system over time.
This can also be a problem with bareroot trees. Sometimes if bareroot trees are planted into a too narrow hole, the lateral roots can be pushed into a circling pattern that causes the same issue. Growers can solve this problem by pruning the roots to fit the hole, bending the roots down, or augering a wider diameter hole.
With bareroot and potted trees, we can create a circling root problem with an auger. If the site’s soils contain significant damp or wet clay, the spinning auger can cause a kind of impermeable “glaze” to form on the walls of the auger hole. This glaze will act like a clay pot and could cause the roots to circle the interior of the auger hole like they would circle an actual pot. The solution is to avoid planting in fields with heavy clay soils or scrape each auger hole’s walls using a sharp metal tool like a hoe or shovel.
Especially during establishment, getting the irrigation right is the most important part of maintaining root health. This starts at planting. Do not delay applying the first irrigation after the trees have been planted. Once that has happened, you will walk that fine line between preventing the soils from becoming too dry or too wet to maintain a healthy root system. The soil texture and the irrigation system complicate the details of this. In any case, monitoring the soil moisture near some of your baby trees will go a long way in helping you know whether you are successfully achieving this delicate balance. With clay soils, it is very easy to drown the roots of immature trees. Don’t use surface soil moisture as the indicator. Dig down several inches to see what the soil moisture is down where the roots are, or bury some soil moisture sensors down several inches where the roots are.
If you are using drip irrigation (or even microsprinkler) irrigation, make sure that the emitters are delivering water to the baby root system, which does not reach out very far away from the tree. Be aware that the emitters on surface drip systems “wander” around a bit as the temperature changes during the day (as the plastic expands and contracts). Because of this, it might be a good idea to plug in a drip emitter (or two) on the end of a “spaghetti tube” that is staked in a permanent spot near each newly planted tree to ensure that there is always at least some water being delivered right at the tree.
Common Diseases of Pecan Roots
Earlier I mentioned that mycorrhizal fungi can “infect” pecan roots. These fungi are good guys and are always welcome in a pecan orchard. However, some other organisms out there cause disease in roots. We’ll talk about the three common ones. First, cotton root rot is a disease of broadleaf plants, including pecan, caused by a fungus called Phymatotrichopsis omnivora. This fungus inhabits alkaline pH soils in the southwestern U.S. and northwestern Mexico and quickly destroys the roots, killing the plant. The main way to manage this disease is by avoidance—choose sites that don’t have the fungus. Avoiding those sites requires planting a susceptible crop like alfalfa on a future orchard site before planting trees. Dr. Stein from Texas A&M presented some interesting research several years ago that suggested that certain pecan rootstocks, such as ‘Riverside’ and ‘Apache,’ may have increased tolerance to cotton root rot compared with others, but this is no guarantee. In some states, a flutriafol fungicide formulation is permitted for use against cotton root rot in pecans. Research in Arizona and grower observations indicate that this may be an effective, albeit pricey, way to manage cotton root rot (before doing this, check with your Department of Agriculture about whether this is allowed in your state or not).
Second, there is a disease called Crown Gall caused by the soil bacterium Agrobacterium tumefaciens. This is not strictly only a root disease, but that is one of the most common places for it to occur. Pecans are not nearly as susceptible to this disease as other fruit and nut trees, including their close relatives English walnuts, but occasionally it can be a problem. There are not really any good chemical controls for this, but again, it should be managed by avoiding infection through contaminated pruning equipment or by purchasing trees with knots or galls on the roots.
Finally, a microscopic worm called the pecan root rot nematode (Meloidogyne partityla) can cause serious damage to pecan roots. This organism is best managed by avoiding it in the first place. Avoid it by purchasing healthy trees from a reputable nursery with trees that are not already infected with root-knot nematode. Avoid introducing it with contaminated implements, soil, or irrigation water. If your trees are already infected, it will become even more important than usual to manage the trees well, avoiding all physiological stresses from nutrient disorders, salinity, drought, etc.
These three common diseases can cause significant problems for your trees and your orchard. And while our eyes are one of our best tools for learning about our trees and monitoring for issues, we don’t have many options—currently—for seeing what’s happening underground with the root system. Understanding and utilizing these best practices can help us support the roots, the trees, and ultimately our orchard, even when blind to what is below the surface.