By: Robert K. Lyons and Barron Rector
Mesquite is native to Texas, where three major species and four varieties of the plant occur (Table 1). The most widespread type is honey mesquite (Prosopis glandulosa var. glandulosa). When most people in the state talk about mesquite, they are probably referring to honey mesquite.
Historically, mesquite has had many uses, such as serving as material for fence posts, firewood, food, furniture, and flooring, and shade for livestock (Fig. 1). Today, many think of mesquite as only a pest. However, it is still used to make fine furniture and appears to have ecological benefits if the stand is not too dense.
Mesquite is common across the southwestern United States at elevations of up to 4,500 feet. Its northern limit—southwestern Kansas, southeastern Colorado, New Mexico, southern Utah, and southern Nevada—is defined by cold.
Mesquite grows well in many types of soil except deep sands and soils that remain wet for long periods. Its geographic range in Texas does not appear to have changed since European settlement, although its density within that range has increased significantly.
To manage mesquite successfully, landowners need to understand the plant’s growth and development processes as well as the management options that have proven to be the most successful, cost effective, and environmentally safe. Using this knowledge, landowners can time their control efforts to treat the most vulnerable growth stage and direct those efforts to the right part of the plant.
Growth and development
Mesquite plants have three growth stages:
- Seedling: From seed germination until the establishment of the first true leaf
- Juvenile: From the appearance of the first true leaf until the plant produces mature, woody xylem tissue. This stage is also often called the seedling stage.
- Mature: Follows the juvenile stage
Seed dispersal and survival
Many people believe that mesquite seeds live indefinitely in the soil, which enables the plant to reinfest areas for long periods after it has been controlled. It is true that mesquite seeds from stored herbarium samples have maintained germination levels of 60 percent after 50 years.
However, seed survival differs significantly in natural settings. For example, a study found that some seeds planted in soil did not emerge after 6 years, and about 79 percent of the seeds that did live emerged the first year after planting. Texas A&M AgriLife Research at Uvalde and Vernon reported that after 1 year of exposure on the soil surface, 95 to 99 percent of seeds could not germinate. These findings suggest that the seeds must germinate during the same year that they are produced.
For mesquite, maximum germination occurs when the temperature ranges from 80 to 85°F and when about 0.2 inch of soil covers the seed. Establishment is successful if the seeds germinate beneath a thin layer of soil or organic material or debris.
Greenhouse studies have reported that fewer seedlings are established when the seeds are planted deep. Seeds scattered on grassy surfaces also do not germinate well, which suggests that a well-managed grass stand may limit mesquite establishment. Seeds on the soil surface will germinate but usually die.
Any animal that swallows an intact mesquite bean can spread the seed. Cattle drives in the 1880s are thought to be partially responsible for increased mesquite density within stands of mesquite.
However, research near San Angelo in 1999 and 2000 suggests that the presence or absence of livestock does not affect the disappearance of mesquite seed. In this experiment, seeds disappeared from the ground within 3 to 5 weeks—even when livestock were absent—apparently from wildlife consumption.
To survive, mesquite seedlings need light and soil cover, as well as soil moisture for at least 3 to 5 days after germination. Soil temperatures must be at least 77°F for the seedlings to establish; they grow fastest at soil temperatures of 80 to 90°F; and their growth slows above 95°.
Seedlings are usually established about 10 days after germination, when the first true leaf, or cotyledon, is completely developed.
Insects and browsing animals can destroy a newly emerged seedling if they remove the plant top below the cotyledon. However, if they remove the top above the cotyledon, the seedling will branch. Older seedlings are more likely to survive top removal, and mesquite can withstand several years of successive top removal.
The plant’s ability to survive top removal is the reason that cutting off the top often does not control mesquite.
Although the trees can produce many seeds, and animals can disperse large numbers of them (Fig. 2), only a few seedlings survive. In a Texas A&M AgriLife Research study at two West Texas ranches, many planted mesquite seeds emerged but few seedlings survived. In this study, the average seedling emergence was 35 and 39 percent, but the seedling survival rate at 2 years after emergence was only 9 and 3 percent (Table 2).
Research in northwest Texas found that the rates of seedling survival 2 years after emergence in cattle, coyote, and deer feces ranged from 6.5 to 13.5 percent (Table 3). These and other studies have suggested that although perennial grass competition does not prevent mesquite from establishing, the grass does appear to limit it.
A Texas AgriLife Research study at Spur demonstrated that reinfestation could occur even when no livestock are present (Table 4). This study began in 1941 with hand grubbing of all the mesquite in a 1-acre area. Every 5 to 7 years, all of the seedlings were removed by grubbing. Over a 25-year period, 689 plants, including 473 seedlings, were removed from the area.
The juvenile growth stage lasts about 2 to 3 years. In this period, the plant produces no flowers or seeds. The first year is critical to plant survival. The optimum soil temperature for growth at this stage is 85 to 90°F.
The plants may grow a few inches or up to 3 feet during the first growing season. In 2 to 3 years, the plants usually reach 5 to 6 feet tall.
After the top has been removed, a mesquite plant can have a juvenile top with a root system and lower stems that are several years old.
After 3 years, the plants are usually mature (able to produce flowers and seed). Mature honey mesquite occurs in two growth forms:
- Single- or few-stemmed plants (Fig. 3), which indicate that their growth was unaltered and vigorous during the juvenile stage or later
- Multi-stemmed plants (Fig. 4), which indicate that the top growth has been removed or interrupted during the juvenile stage or later
Research has found that the roots of mature mesquite can extend 45 feet deep belowground and 45 feet horizontally from the base of a plant.
Mesquite and soil fertility
Mesquite is a legume and fixes nitrogen. Although small mesquite trees—those less than 14 feet tall with trunk diameters of less than 3 inches—actively fix nitrogen, they do not appear to increase the net amount of nitrogen in the soil.
However, significantly higher nitrogen and carbon levels have been found under mature mesquite trees than in the open spaces between them. Therefore, it appears that dense stands of smaller mesquite may not add enough nitrogen and carbon to compensate for their shading effect and competition for water. These smaller mesquite trees should be thinned.
Mesquite and grass production
Brush competition is not considered a problem for grass when the brush canopy cover is below 11 percent. However, when that canopy cover nears 20 percent, the brush competes with grass for water and nutrients.
The number of plants needed to reach 20 percent canopy cover depends on the mesquite trees’ size. For example, an area where the canopy averages 5 feet in diameter would need to have about 450 trees per acre (Fig. 5a) to reach this 20 percent threshold. When the canopy is 15 feet in diameter, only about 50 trees per acre would be needed to reach this threshold, and at a 30-foot diameter, only 12 trees per acre would be needed (Fig. 5b).
In some cases, the presence of mesquite can actually benefit the grass. In South Texas, a single large mesquite provides shade, increases soil nutrients, and moderates extreme soil temperatures. Grass can survive and thrive under these trees, whereas the open spaces between the trees are too harsh for the grass to do well (Fig. 6).
However, as mesquite density increases, so does competition for light, moisture, and nutrients, limiting grass growth.
For managing mesquite, it has been suggested that the weakest link is the young plant. If young plants are managed to prevent or reduce reestablishment, the treatments cost less and the plants cannot reach maturity, when they can produce seed.
To reduce mesquite plant density successfully, landowners must target the belowground bud zone.
Mesquite produces buds above ground on its stems and trunk and belowground in a central bud zone to a depth of 12 to 18 inches (Fig. 7). The bud zone is the target for mesquite control. Unless these buds are killed, the mesquite will continue to resprout.
The bud zone can be eliminated as a source of regrowth by either physically removing it from the soil or by applying herbicide that moves uniformly into the bud zone of the plant. For example, when herbicide spray covers mesquite foliage only partially, as in one side of a fence line, the herbicide does not move uniformly into the bud zone, and some plants survive (Fig. 8).
Another vital point is that it is unrealistic to try to eradicate mesquite once and for all. As illustrated in the Texas A&M AgriLife Research study at Spur mentioned previously, various species of wildlife will continue to bring seeds to the property.
Another example of seed dispersal by wildlife is the mesquite seedlings that grow on Conservation Reserve Program (CRP) lands that were in cropping for decades and were not grazed by cattle since being converted to CRP. The seedlings on these lands could not have grown from seeds left in the soil long ago before it was cropland, and cattle have not brought new seeds into the site.
Therefore, a mesquite management plan must be designed for the long term, incorporating follow up treatments to maintain mesquite densities at the level that meets landowner goals.
Mesquite can be managed with either broadcast or individual plant treatments. For dense stands (more than 400 trees per acre), the preferred method is usually a broadcast method such as aerial spraying.
However, another treatment option might be mechanical control—chaining or root plowing. Applying herbicide to individual plants or mechanical grubbing is used most often when there are fewer than 400 plants per acre and the trees are small.
- To estimate the density of mesquite on a site, a landowner can use one of two techniques: Measure off at least four square areas of 66 feet by 66 feet (1/10 acre) in representative areas of the pasture. Count the mesquite plants in these areas, average the number of plants per square, and multiply by 10 to estimate the density per acre.
- Walk at least four lines about 363 feet long (about 121 big steps) toward a landmark in representative areas of the pasture. Turn around and slowly walk a straight line to the starting point, counting every mesquite rooted within 3 feet of your path. Calculate the average number along these lines and multiply the average by 20.
To prevent resprouting effectively, mechanical broadcast and individual plant treatment must remove the bud zone.
Two of the most effective broadcast mechanical methods are chaining and root plowing. Both have advantages and disadvantages and must be applied under the right conditions (for information on these methods and conditions, see the AgriLife Extension publication Brush Management Methods).
Grubbing also controls individual plants effectively.
Herbicides can be applied as broadcast or individual plant treatments:
- Apply broadcast herbicides to the foliage. Dense mesquite stands require broadcast treatments, either aerial or ground.
- Apply individual plant treatments to either the foliage or the tree base. These treatments are used for small plants and at low plant densities.
For optimum control, apply foliar treatments— broadcast or individual—when the plant is transporting carbohydrates from the leaves to the roots, which is usually 45 to 90 days after bud break. Basal treatments can be applied any time of the year.
Each year, a mesquite tree undergoes a specific sequence of stages during its annual growth cycle. Events during the growth cycle determine the most effective time to apply foliar herbicide treatments for control.
The cycle begins with bud break in the spring. Twigs and leaves begin to grow when the soil temperatures reach 65 to 75°F and continue growing for about 6 weeks.
The best time to begin using leaf-applied herbicides is about 45 days after bud break, when the soil temperature at 12 inches deep reaches at least 75°F and the twigs and leaves stop growing. At this point, the plant begins to replenish the carbohydrate reserves in its root system through manufacture by the now-mature leaves. In this period, which lasts about 45 days, the leaves are dark green and the twigs have stopped lengthening.
Although mature trees progress uniformly through the annual growth cycle, individual plants may not all respond the same way to short-term weather changes such as late spring or early summer rains that interrupt plant growth. Because plants respond differently to these interruptions, an area may have mesquite trees in various physiological stages. These differences may explain the erratic results from herbicide applications in some years.
The foliage is densest on the trees 30 to 60 days after the leaves become fully formed. If rainfall is light and sporadic during drought, a tree canopy will have leaves in several stages of growth. Many of those leaves cannot absorb and move enough herbicide for adequate control.
In wet years, mesquite can be challenging to control because each pulse of rain initiates new growth. A season with slightly less than average rainfall can be an excellent time for control because the plants are at a more similar physiological stage and most are moving carbohydrates downward.
Control measures are less effective on older trees with corky bark than on young plants with stems that are less than 3 inches in diameter. The differences may be caused by the older trees’ lack of vigor and limited amount of foliage, which reduce the amount of foliar-applied herbicide moved to the root system. For herbicides applied to the stem base, trees with younger, smooth-skinned bark absorb the chemical more readily than do older trees with corky bark.
Multi-stemmed plants are more difficult to control than are plants with fewer stems. An aerial study reported 75 percent plant mortality for mesquite with one or two stems compared to only 40 percent mortality for plants with six or more stems (Fig. 9).
A possible explanation for the reduced mortality is that herbicides move into the mesquite bud zone more easily in a plant with fewer stems than in one with multiple stems.
Stem sprays are also less effective on multi stemmed regrowth than on plants with one or a few basal stems. Because stem-applied herbicides do not move laterally, the herbicide does not reach the buds that are not directly below the treated stems.
Plant mortality is also lower for taller plants. An aerial study found a decline in plant mortality of about 13 percentage points for plants over 6.5 feet tall compared to the mortality rates of shorter plants (Fig. 10).
Three other factors influence plant mortality:
- Herbicide leaf coverage: Tree mortality increases as herbicide covers more leaves. Surfactants or crop oils are added to the herbicide spray to improve coverage by breaking the surface tension between the spray droplets and leaf surfaces.
- Spray volume is also significant and can be too little (poor coverage) or too much (the spray washes off the leaves).
- Spray droplet size: A Texas A&M AgriLife Extension study conducted with aerial-applied herbicide demonstrated that mesquite plant mortality declines sharply as average spray droplet-size increases above 500 µ (Fig. 11). Tripling the spray volume to 10 gallons per acre did not compensate for the large droplets.
By fall, mesquite trees become dormant. Insects and freezes defoliate most trees by November.
Several options for herbicide broadcast and individual plant treatments are described in the AgriLife Extension publication Chemical Weed and Brush Control Suggestions for Rangeland. This publication lists information on herbicide rates, spray volume, application time, and special considerations for specific treatments. It also provides an expected plant-kill rating for each practice, which is useful for selecting treatments and knowing what to expect.
The majority of the herbicide is absorbed and moved within 4 days of application, with most occurring on the day of spraying. Any spray additive that removes leaves too quickly after spraying will reduce control.
Do not apply sprays to wet foliage. However, once the herbicide is on the leaf, rain is unlikely to wash it off. In experiments in which simulated rainfall was applied to leaves within 15 minutes of spraying, both clopyralid and triclopyr were still highly toxic to the honey mesquite leaves. A safer rainfast interval is probably 2 to 4 hours of drying time.
For aerial or ground broadcast sprays, diesel oil and water emulsions can still serve as herbicide carriers. However, applicators today use water plus a surfactant, crop oil, or methylated seed oil as the herbicide carrier. These carriers cause fewer mixing and handling problems than diesel oil and water emulsions. Individual plant leaf sprays also require crop oils, surfactants, or methylated seed oils.
Foliar sprays work best when:
- Soil temperatures at 12 to 18 inches deep are more than 75°F.
- The leaves are dark green and have no significant damage from drought, hail, or insects.
- Recent rains have not initiated any new leaves.
- All mesquite bean pods are fully elongated.
- All flowers are yellow, not white.
Several basal-type individual plant treatments are available. These treatments are best for mesquite with one or a few basal stems. Basal treatments involve either using a cut-stump treatment or spraying herbicide completely around each stem from about 12 inches high to ground level.
Multi-stemmed plants are more difficult to treat with basal methods, and control levels are typically lower for multi- than for one- or two-stem plants. For multi-stemmed trees with more than three stems, foliar treatments are best.
Detailed information and instructions on mesquite individual plant treatment are in Texas A&M AgriLife Extension publications Chemical Weed and Brush Control Suggestions for Rangeland, Brush Busters: How to Beat Mesquite, and Brush Busters: How to Avoid Lumps When Using Cut Stumps.
Adult mesquites are fire resistant because the bud zone is underground (Fig. 7), where it is protected, and the plants can resprout after the top has been removed. Reports of whole-plant mortality from fire are rare, occurring only in large trees with rat nests at the base.
Research indicates that seedlings are susceptible to fire until they are about 1½ years old. A study in North Texas investigated mesquite seedling mortality with prescribed fires in relation to seedling age and grass fuel amount (Fig. 12).
For the mid-grass site, more trees died in the summer burn than in winter. However, for the tallgrass site, mortality was similar for winter and summer burns. The findings suggest that if an area has less grass to burn, a summer burn would be more effective.
Managing for a mesquite savannah
Texas A&M AgriLife Research at Vernon has investigated the potential of using fire and herbicide to establish mesquite savannahs, which are grassy areas with relatively few, large trees.
The researchers used low-intensity fires to kill the lower canopy foliage and stems but left enough top leaf to prevent the trees from resprouting from the base. The reduction in lower canopy foliage provided several benefits:
- Decreased the competition with understory grasses for water
- Improved the vegetation for livestock management • Left mesquite trees to provide shade for the livestock
- Increased the availability of understory forage
Mesquite savannahs may also improve soil fertility, plant diversity, and wildlife habitat.
Widespread, low-intensity fires may help manage mesquite seed production because flower and bean development are inhibited during the growing season after a fire.
Fire can be used to develop a savannah under certain conditions:
- Season: Winter
- Relative humidity: More than 30 percent
- Air temperature: Less than 68°F
- Wind speed: Between 8 and 20 mph
Three or four fires may be required over a 10- to 15-year period to create the savannah.
Fuel that dries readily, is consumed rapidly by fire when dry, and has a high ratio of surface area to volume is called fine fuel. Examples of fine fuel are grass, twigs, pine needles, and tree moss.
When creating a savannah, use fine fuel loads of 800 to 2,500 pounds per acre. For afternoon headfires (intense fires that burn with the prevailing wind), use fuel loads of 800 to 1,500 pounds per acre.
Fuel loads of more than 2,500 pounds per acre top-kill mesquite, causing resprouting. Headfires of less than 800 pounds per acre have no effect on mesquite.
Mesquite trees that survive a treatment of clopyralid herbicide alone tend to develop a ragged appearance called stem flagging. Research has indicated that an initial treatment with a low rate of clopyralid to produce stem flagging combined with subsequent low-intensity fires can develop a mesquite savannah faster than can fire alone.
General management recommendations and considerations
- Establish long-term goals and management plans.
- Remember that eradicating mesquite is not possible but managing it is.
- Concentrate your control efforts on smaller, younger mesquite (those with stems of less than 3 inches in diameter are controlled more effectively).
- Also create a plan to manage the mature trees, even if you must implement it a little at a time.
- Multi-stem regrowth is difficult to control when it is less than 4 feet tall.
- Treatments that top-kill mesquite—such as chaining, shredding, roller chopping, or high intensity fires—will shift the mesquite plant into a multi-stemmed regrowth form. Plan to retreat it later.
- You may need to retreat every 3 to 7 years to prevent new plants from producing seed.
- Defer grazing of pastures where many seeds are produced.
- Fence off areas with high seed production.
- Confine livestock for 3 to 6 days after having access to mesquite beans to allow passage and reduce dispersal.
- Leave enough grass to discourage seedling establishment.
For more information
Behind the Dozer: A Comprehensive Investigation of Follow-up Brush Control Options. 2004. Agriculture and Natural Resources Field Day, April 29. Texas A&M University Research and Extension Center, San Angelo
Bovey, R. W., Mesquite: History, growth, biology, uses, and management. 2016. AgriLife Research and Extension Service Series. Texas A&M University Press, College Station
These publications are by the Texas A&M AgriLife Extension Service and are available at http://www. agrilifebookstore.org/:
- Brush Busters: How to Avoid Lumps When Using Cut Stumps
- Brush Busters: How to Beat Mesquite
- Brush Busters: How to Estimate Costs for Mesquite
- Brush Management Methods
- Chemical Weed and Brush Control Suggestions for Rangeland
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