Abstract
This study was conducted in a newly established (2-year-old) almond orchard to investigate the effects of five different mulching materials (woven and nonwoven fabric, black and white polyethylene, almond shell) and flame treatments applied at two different frequencies (FL20 and FL30) on weed control and almond growth compared with those of conventional herbicide (glyphosate) application and weedy control. Thus, this study included nine different treatments. The impacts of these treatments on weed density and coverage were periodically monitored. Additionally, the biomass of the weeds was measured at the end of the season to evaluate the effects of the treatments. Because the almond orchard was not yet in the economic fruit-bearing stage, the effects of the treatments were examined in terms of parameters that characterize almond growth, such as plant height, trunk diameter, shoot length, and shoot thickness. The chlorophyll content and water potential values of the trees were also determined. The results of this two-season study indicated that synthetic mulches provided the best outcomes in terms of weed control and almond growth. No weed emergence was observed throughout the season in any of the synthetic mulch treatments. Although almond shells used as organic mulch were highly effective for blocking sunlight, they failed to prevent the growth of some vigorously growing perennials such as Cynodon dactylon and Sorghum halepense that emerged from gaps. Flame treatments demonstrated rapid and effective results; however, they were less successful against the aforementioned monocot perennial weeds and required frequent repetition because of the lack of residual effects. Glyphosate, an herbicide that is commonly used in conventional orcharding, was applied five times throughout the experiment and proved effective weed management compared with that of the weedy control. However, considering the increasing herbicide resistance, environmental and health issues, and growing interest in organic almond cultivation, synthetic mulch applications have emerged as good options. Despite the initially higher establishment costs, synthetic mulches effectively controlled weeds and reduced water stress, thereby promoting almond tree growth.
The almond [Prunus dulcis (Mill.) D.A. Webb] is a fruit species within the Prunus genus of the Rosaceae family with notable significance to human health. Native to Central and Western Asia, the almond has spread from these regions to China, India, Iran, Syria, and Mediterranean countries. Naturally found in regions with a Mediterranean climate, the almond is distributed between latitudes of 30 to 44° in the northern hemisphere and those of 20 to 40° in the southern hemisphere (Küden et al. 2014). More than half of the global almond production occurs in the Americas. According to statistics from 2022, 3.63 million tons of in-shell almonds were produced on 2.357 million ha globally. This production was distributed as follows: 52.5% in the Americas; 16.6% in Asia; 11.3% in Europe; 9.9% in Oceania; and 9.7% in Africa. The United States leads, by far, in global almond production, followed by Australia, Spain, and Türkiye (FAO 2022).
Fresh almonds are widely consumed as a fruit, whereas raw and roasted almond kernels are popular as snacks. Because of their gluten-free nature, almonds are commonly used as flour in the confectionery, chocolate, and bakery industries, and their oil is used in the cosmetics and pharmaceutical sectors. Furthermore, almonds contain various phenolic compounds (Garrido et al. 2008) and cyanogenic glycosides such as amygdalin (Lee et al. 2013). They are also notably rich in vitamin E (Çelik et al. 2019). Because of their nutritional components and chemicals, almonds are a significant healthy food source. Almonds are particularly beneficial for cardiovascular health and are known to lower cholesterol, aid in diabetes management, and support healthy weight loss (Berryman et al. 2015; Cohen and Johnston 2011; Jung et al. 2018).
As with any cultivated plant, almond production is limited by factors that not only reduce yield but also decrease quality. Among these factors, weeds are particularly significant because they compete with newly planted trees, restrict tree growth, and host various insect pests and diseases. Effective management of these weeds is an unavoidable necessity. The yield losses caused by weeds can be particularly high in newly established orchards (Shrestha et al. 2012). Chemical control methods are predominantly preferred for combating these weeds. However, there are environmental and health issues associated with the extensive use of herbicides, such as herbicide resistance (Beckie et al. 2014) and preharvest fruit drop (Gairhe et al. 2022), and the growing interest and demand for organic almond cultivation are increasing annually (Cárceles Rodríguez et al. 2023; Klonsky and Richter 2011). In this context, cultural, mechanical, and physical control methods such as mulching, cover crops, mowing, tillage, and thermal applications can be used for weed control in organic almond orchards. Research of the use of thermal control methods has increased in recent years. These methods include hot water (Hansson and Ascard 2002), hot water steam (Kolberg and Wiles 2002), microwaves (Sartorato et al. 2006), and laser applications (Mathiassen et al. 2006), with flaming being the most commonly used method in practice (Ascard 1994). Flaming is a thermal control method that involves the application of high heat for a short duration to damage weeds. The primary principle involves the expansion of cell sap in plant cells exposed to high heat, thus causing the cell walls to rupture and subsequently leading to plant wilting and death (Ascard 1995; Ellwanger et al. 1973). Propane and similar combustible gases are commonly used for this purpose. There are specially developed handheld and backpack tools for small-scale use, as well as tractor-integrated models suitable for large-scale applications (Kitiş 2010a).
Although tillage is one of the most commonly used methods in orchards, it can damage tree roots (Kitiş 2010b) and reduce the amount of organic matter and nitrogen in the soil (Cucci et al. 2016). However, overtillage is considered responsible for limiting tree growth and reducing fruit yield and size (Granatstein et al. 2010). In contrast, mulch applications that cover the soil surface effectively control weeds (Kitiş et al. 2017; Verdu and Mas 2007) and preserve soil moisture (Kitiş et al. 2017; Liao et al. 2021; Turk and Partridge 1947). Additionally, plastic mulches increase soil temperature (Kitiş et al. 2017; Mage 1982), which accelerates the growth of young trees. In addition to these benefits, mulch applications prevent soil erosion by water and wind (Liang et al. 2002; Wan and El-Swaify 1999) and increase the amount of useful nutrients and organic matter in the soil (Ashworth and Harrison 1983; Bhella 1988); furthermore, many organic mulches enhance the activity and number of soil microorganisms and earthworms (Buck et al. 2000; Tiquia et al. 2002). They also promote root system development in plants (Wien et al. 1993) and improve fruit quality (Estes et al. 1985), and some organic mulches help regulate soil acidity (Iles and Dosmann 1999).
In this study, the effectiveness of five different mulch applications, one organic (almond shell) and four synthetic [woven fabric, nonwoven fabric, black polyethylene (PE), and white PE], and flame applications applied at two different frequencies (FL20 and FL30) were compared with conventional herbicide application (glyphosate) and weedy control in terms of weed control, and the effects of all these applications on the development of almond trees were investigated.
Material and Methods
This study was conducted at a 30-ha almond orchard established in Antalya, Türkiye, during 2019 and 2020. The orchard, which was irrigated using a drip irrigation system, comprised 2-year-old almond trees planted at a density of 4 × 5 m. The almond saplings [Prunus dulcis (Mill.) D.A. Webb] were of the Ferragnes cultivar, which is a late-blooming cultivar obtained from the crossbreeding of Cristomorto × Ai. The Ferraduel cultivar was used as a pollinator. The soil pH of the experimental area was 7.9, with a lime content of 42.7%. The soil texture was clay-loam, containing 1.08% organic matter, 0.1% nitrogen, 9.43% phosphorus, and 34.8% potassium. Because chlorophyll measurements were performed during the study, the fertilization program of the orchard is provided in Table 1. Fertilization was performed using a drip irrigation system with equal amounts for each tree.
Pure fertilizer amounts provided during the study (g/tree).
This study included nine different treatments comprising five different mulches, two flaming treatments, one herbicide, and a weedy control. No weed control was performed in the weedy control plots. The experiment was established using a randomized block design with four replications (Fig. 1). The plot size for each treatment was adjusted to 24 m2 (12 m × 2 m) to accommodate three trees in a row. Air temperature and relative humidity were recorded hourly throughout the experiment using Hobo electronic measurement and recording devices (Onset Computer Corporation, Bourne, MA, USA) that were placed in the experimental area. The air temperature and humidity of the trial site were regularly recorded between Jul 2019 and Sep 2020. The highest monthly average temperature was 31.4 °C in Aug 2019, whereas the lowest monthly average temperature was 11.3 °C in Jan 2020. The highest monthly average humidity was 60.5% in Dec 2019, and the lowest monthly average humidity was 48% in Aug 2019 (Fig. 2).
Almond orchard experimental conditions.
Citation: HortScience 59, 9; 10.21273/HORTSCI18047-24
Mulch applications.
During this study, five different types of mulch materials were used: black 65-µm-thick spun bond nonwoven fabric made of polypropylene, black woven fabric of the same thickness made of polypropylene, 100-μm-thick PE that was black on both sides (PE-black), PE with a white outer surface and black inner surface (PE-white), and almond shells as organic mulch applied at a thickness of 10 cm. The mulches and almond shells were spread to a width of 2 m, with 1 m on each side of the tree rows. Each treatment was applied to three trees in each plot.
To determine the shading rate of the mulches, the light intensity values beneath and above each mulch cover (at two different points per replication) were measured at regular intervals during the experiment using a lux meter. The difference between these measurements was calculated to determine the shading rates of the mulches. Measurements were obtained during midday hours, when sunlight was strongest and most direct, and under cloudless conditions.
Flame applications.
Flaming treatments were conducted using a flamethrower machine with a nozzle diameter of 57 mm operating at a constant pressure of 1.5 bar. During the flaming applications, the flame nozzle was positioned 20 to 25 cm above the weeds at an angle of 30°. Liquefied petroleum gas cylinders, consisting of 70% butane and 30% propane, were used as the flame source. Applications were performed in dry weather conditions, ensuring that no dew, moisture, and wetness were present on the weed leaves. The area treated with flame in each plot was kept equal to the area treated with mulch, covering 24 m2 in each plot, with 1 m on each side of the row and encompassing three trees. However, the flame treatment was applied only to the points where weeds were present, rather than covering the entire plot. Two different frequencies of flaming were considered during the experiment. With the first frequency (FL20), flaming was applied when the weed coverage reached an average of 20%. With the second frequency (FL30), the tolerance for weed coverage was increased, and flaming was applied when the average coverage reached 30%. These threshold values were used as a basis, and flaming applications were repeated until the end of the season (November–December). Accordingly, the FL20 treatment was applied six times and the FL30 treatment was applied five times during the experimental period.
Herbicide application.
In the plots allocated for chemical control, herbicide application was conducted when the overall weed coverage reached 20%. For this purpose, glyphosate isopropylamine salt (480 g/L) herbicide was applied at doses of 3000 mL/ha for annual species and 6000 mL/ha for perennial species, depending on the predominant weed species present. The application was performed using a backpack-type motorized sprayer at a pressure of three atmospheres (3.03975 bar = 303.975 kPa) and water volume of 200 L/ha. During spraying, a fan beam herbicide nozzle with an angle of 80° was used at standard working pressure. Herbicide application was conducted five times throughout the study period.
The growth stages of the weeds were recorded during both herbicide and flaming applications. Additionally, climatic parameters such as air temperature (°C), relative humidity (%), soil temperature at a depth of 1 cm (°C), light intensity (lux), and wind speed (km/h) were recorded at the time of application. The average values of the climatic records obtained during the applications are provided in Table 2.
Climate data obtained during herbicide and flame applications.
Determination of the effect of applications on weeds.
Specimens of unidentified weeds in the field were collected, and their herbarium was prepared for morphological diagnosis (Davis 1965). Observations were made at regular monthly intervals to determine the effect of applications on weed density and coverage area. Weed counts were not conducted during the winter months (December, January, February). Thus, a total of 11 observations were conducted in the plots. Weed densities were determined within two 0.5-m2 (0.5 m × 1.0 m) quadrats established in each plot, whereas coverage areas were evaluated considering the entire plot area (24 m2). Weed densities and coverage areas were determined separately for each weed species. The total count obtained for each weed species was divided by the total area counted to determine weed density; similarly, the total area covered by each weed species was divided by the total evaluation count to obtain coverage area values. Densities were evaluated in terms of number·m−2, whereas coverage areas were expressed as percentages (Odum 1971).
At the end of the experiment, aboveground parts of the surviving weeds within the quadrats in each plot were harvested to determine their fresh weights. Later, the same samples were dried at 65 °C for 72 h, and their dry weights were determined using a sensitive balance.
Evaluating the effects of applications on almond trees.
Because the trial was established in a 2-year-old almond orchard, the almond trees were not in their economic yield stage, and fruit yield could not be assessed. However, regarding the development of the trees, parameters such as plant height, trunk diameter, shoot length, and shoot diameter were monitored with measurements were obtained at the beginning and the end of the trial for all trees in the plot. Plant heights were measured from ground level to the highest point of the plant. For trunk diameters, a point 40 cm above the ground level was used as the reference, and measurements were made from this point. Similarly, shoot length and shoot diameter thickness were measured from four shoots per tree, one each from the east, west, north, and south directions, selected from shoots in 2019. Shoots were marked during the initial measurement, and the same shoots were measured during the second measurement. Plant heights and shoot lengths were measured using a tape measure, whereas the trunk diameter and shoot diameter thickness were measured using a caliper. Because the heights and trunk diameters of the trees in the trial area were not uniform from planting, the values obtained were converted to percentage increases and subjected to a variance analysis.
In addition, the chlorophyll content in almond leaves was determined using a chlorophyll meter, with readings taken from three healthy leaves per tree in the north, south, east, and west directions of each of the three trees in the plot every month throughout the growing season. Measurements were obtained using the FlourPen FP 100 device (Photon Systems Instruments, Drasov, Czech Republic).
The leaf water potential of almond trees was also measured three times during the growing season. Measurements were obtained using the Pump-up Chamber model pressure chamber (PMS Instrument Company, Albany OR, USA). Healthy leaves from young shoots that were not directly exposed to sunlight and that showed no signs of disease or insect damage were used for this purpose. Measurements were conducted separately for each application and on a replicate basis. An evaluation of the measurements was based on the pressure gauge reading, with higher readings indicating greater water requirements for the plant.
Data analysis.
The experiment was designed as a single-factor study with four replications following a randomized block design to ensure the reliability and accuracy of the results. Weed densities, biomass values, chlorophyll content, and leaf water potential of almond trees were directly measured and analyzed to capture the essential physiological responses. Additionally, percentage values representing the weed cover area and changes in tree growth were subjected to an arcsine transformation to stabilize the variances and meet the assumptions of the statistical tests. Data collected for all parameters underwent an analysis of variance to detect significant differences among treatments. Subsequent multiple comparisons of the means were performed using Duncan’s multiple range test to identify specific differences between groups. All statistical analyses were conducted using the SPSS software package (IBM SPSS Statistics for Windows, version 22.0; IBM Corp., Armonk, NY, USA) to ensure rigorous and standardized data analysis procedures.
A principal component analysis was performed to evaluate the impact of the treatments on plant growth parameters that characterize plant development, such as plant height, stem thickness, shoot length, shoot diameter, chlorophyll content, and water potentials, to determine whether the measured variables adequately explained the total variance, to assess whether they could be reduced, and to investigate whether the measured variables could be grouped, as well as the relationships between them. The principal component analyses were conducted using the R statistical software package (R Core Team 2024) and its open-source “factoextra” and “ggfortify” packages.
Results and Discussion
Shading rates of mulches.
During the experiment, all mulching materials effectively blocked more than 99% of light from reaching the soil surface. In terms of blocking sunlight, all mulching materials were sufficient. Although the shading rates were similar to each other, the highest shading rate (99.99%) was observed with the almond shell mulch application. In contrast, the lowest shading rate (99.80%) was recorded with woven fabric, which is somewhat looser in structure because of its woven characteristics (Fig. 3). These results are quite similar to the findings of a previous study by Kitiş et al. (2017). According to this study, the shading rates were as follows: 72-μm-thick nonwoven fabric mulch that was all black, 100%; 58-μm-thick nonwoven fabric mulch, 99.9%; 38-μm-thick nonwoven fabric mulch, 96.6%; 100-μm-thick PE-black mulch, 99.5%; and 40-μm-thick PE-black mulch, 94.8%.
Effects of applications on weeds.
In the area where the experiment was conducted, 33 weed species that belonged to 13 families were identified (Table 3). The prominent species among these were Sorghum halepense, Cynodon dactylon, Convolvulus arvensis, Heliotropium europaeum, Euphorbia chamaesyce, Chrozophora tinctoria, Malva sylvestris, and Sonchus asper. However, in terms of both density and coverage, the following two dominant species stood out: S. halepense and C. dactylon. The weed species identified in the almond orchard were similar to those found during survey studies previously conducted in the Manisa and Muğla provinces of Turkiye, as well as in Italy (Fracchiolla et al. 2016; Sokat and Catikkas 2019).
Weed species in the experimental area.
With mulching applications other than almond shell mulch, no weed emergence was observed. Although there was some weed emergence in the almond shell mulch, it was not significant. Perennial species such as S. halepense and C. dactylon were particularly noted. Except for the almond shell mulch application, the herbicide application provided the best results after other mulch applications. Throughout the experiment, weed density with the flame treatments (FL20 and FL30) remained below that of the weedy control (Table 4). However, with the FL30 treatment, weed density did not decrease sufficiently, indicating that the 30% coverage threshold set for the application was too high, and that the flame treatment should be applied at earlier stages. When examining the effects of the treatments on weed species, a total of 13 species were observed in the weedy control plots, compared with 11 species in the herbicide treatment plots, 10 species in the FL30 treatment plots, five species in the FL20 treatment plots, and four species in the almond shell mulch treatment plots. The most prominent species in terms of density was C. dactylon. The average density of this species was 85.7 plants/m2 in the weedy control plots, 83.7 plants/m2 in the FL20 treatment plots, 91.6 plants/m2 in the FL30 treatment plots, 53.2 plants/m2 in the almond shell mulch treatment plots, and 14.7 plants/m2 in the herbicide treatment plots. The second most important species was S. halepense, with an average density of 89.7 plants/m2 in the weedy control plots, 11.9 plants/m2 in the FL20 treatment plots, 33.6 plants/m2 in the FL30 treatment plots, 6.9 plants/m2 in the almond shell mulch treatment plots, and 3.2 plants/m2 in the herbicide treatment plots. The average density of other species did not exceed 1.5 plants/m2, thus remaining at more minor levels.
Effects of applications on weed density (number·m−2) and coverage area (%).
In terms of the effect of the applications on the weed coverage area, the highest coverage area was seen in the weedy control plots (46.4%), followed by those in the FL30 plots (18.2%), FL20 plots (14.3%), almond shell mulch plots (10.8%) and herbicide plots (10.4%) (Table 4). Although, statistically, weedy control was completely separated from other applications, FL20, FL30, almond shell mulch and herbicide applications were included in the same group. The dominant species in terms of the coverage area was S. halepense. The average coverage area of this species was 25.1% in the weedy control, 9.8% in the FL30, 7.4% in the FL20, 3.7% in the almond shell mulch, and 3.4% in the herbicide treatment plots. The second most important species in terms of coverage area was C. dactylon. In the weedy control plots, its average coverage area was 10.0%; however, they were 6.4% in almond shell mulch plots, 6.1% in the FL30 plots, 4.3% in the FL20 plots, and 2.6% in the herbicide treatment plots. Apart from these two species, C. tinctoria, E. chamaesyce, and H. europaeum in the weedy control plots were able to exceed a coverage area of 1.5%.
An assessment of weed biomass was based on the measurement of fresh and dry weights of weeds within the fixed quadrats of each plot. As evidenced by the highest density and cover area values, the weedy control plots exhibited the highest biomass, followed by those of the almond shell mulch, FL30, and FL20 plots. Because of the systemic and partially residual effect of glyphosate, the biomass of surviving weed species in these plots was significantly reduced (Table 5). The high weed biomass in the weedy control plots was expected. However, although weed density and cover area values were lower in the almond shell mulch treatment plots compared with those in the flame treatment plots, the biomass values were higher. This discrepancy can be attributed to the presence of perennial monocot species that, despite being fewer in number in almond shell mulch, were allowed to thrive because of the lack of intervention. In contrast, the flame treatments, which did not cover the entire plot surface, resulted in a larger number of weeds across the plot, but their repeated application kept the weed habitus small. When analyzed by species, S. halepense and C. dactylon comprised the majority of the total weed biomass. The proportions of these two species within the total live weight were 98.0% in the weedy control, 99.5% in almond shell mulch, 98.7% in FL20, and 99.4% in FL30 plots.
Effects of applications on fresh and dry weights of weeds (g·m−2).
An overall assessment of the treatments revealed that annual weeds were controlled more effectively than perennial weeds. In particular, perennial monocots were less affected by flame treatments because of their protected growth points and robust regeneration capabilities. This observation is consistent with the findings of previous studies of flame weeding. For instance, Ascard (1995) classified weed species into four groups based on their susceptibility to flame weeding, with monocot perennials with protected growth points being the most challenging group to control using this method. Similarly, studies by Arslan and Tursun (2021), Domingues et al. (2008), Kitis and Ekinci (2014), Kitis and Gök (2013), Knezevic and Ulloa (2007), Sivesind et al. (2009), and Ulloa et al. (2012) have demonstrated that perennial and monocot species are generally more resistant to flame weeding than annual and dicot species. Despite not achieving the same level of effectiveness as mulch and herbicide applications, both flame treatments demonstrated promising results in terms of weed density, cover area, and biomass reduction compared with those of the weedy control. This limitation can be attributed to the transient nature of flame treatments, which lacked a residual effect. Furthermore, the efficacy of flame weeding is influenced by the target weed species. When dealing with annual and dicot weeds, flame weeding tends to be more successful. However, when perennial monocots dominate the weed flora, as in this orchard, the effectiveness of flame weeding is limited. Consequently, the timing of flame applications and the number of repetitions depend on the acceptable weed threshold level established for the orchard.
Synthetic mulches (woven polypropylene, nonwoven polypropylene, or PE) collectively provided the most effective weed control throughout the experiment because no weed species penetrated the barriers. Almond shell mulch also exhibited commendable performance and significantly curtailed weed growth, except for the two aforementioned weed species. However, almond shell mulch requires occasional replenishment because of wind dispersal and/or decomposition over time. In line with the findings of this study, numerous investigations conducted in orchards have consistently reported that mulch applications exhibit superior weed control efficacy compared with other alternative methods or herbicides (Abouziena et al. 2008; Granatstein and Mullinix 2008; Granatstein et al. 2014; Kitiş et al. 2017; Rowley et al. 2011; Ustuner and Ustuner 2011).
During this study, glyphosate applications, which were repeated five times throughout the trial, effectively controlled weeds. Glyphosate was included as a comparison treatment because of its widespread use as a weed control method in conventional agriculture. However, the effect of glyphosate on Conyza canadensis was relatively low (20%). A possible reasons for this could be the development of glyphosate resistance in C. canadensis, which is the most commonly used herbicide in orchards. Indeed, studies have shown that glyphosate resistance has emerged in some Conyza spp. populations in the Marmara, Aegean, and Mediterranean regions of Turkiye (Doğan et al. 2016; Inci et al. 2019; Serim et al. 2022). However, glyphosate efficacy was more limited against Inula viscosa, C. dactylon, and S. halepense, or these perennial species were able to continue to pose problems in plots because of their rapid regeneration. Glyphosate application was not as effective against all weeds as synthetic mulches, which was further supported by previous studies (Abouziena et al. 2008; Kitiş et al. 2017; Ustuner and Ustuner 2011). Considering the increasing herbicide resistance issues, increasing application costs, and environmental concerns, mulch applications have emerged as a more effective and crucial alternative for controlling weeds that limit almond production.
Effects of the applications on the development of almond trees.
Compared with the weedy control, all applications increased the height of almond trees. The greatest increase in tree height was observed with the PE-white mulch application, followed by those with woven fabric, FL30, FL20, PE-black, nonwoven fabric, and almond shell mulch applications. Similarly, all applications increased the trunk diameter of almond trees more than that of the weedy control. The greatest increase in trunk diameter was observed with the PE-white and nonwoven fabric mulch applications, followed by those with the herbicide, woven fabric, FL20, PE-black, almond shell mulch, and FL30 applications. All applications increased the shoot length of almond trees compared with that of the weedy control. The greatest increase was observed with the PE-white application, followed by that with the PE-black application. The greatest increase in shoot thickness was again observed with the PE-white mulch application, but there was no statistically significant difference among treatments for this parameter (Table 6). The PE-white mulch emerged as the superior treatment and significantly promoted tree height, trunk diameter, shoot length, and shoot thickness compared with those of other mulching applications and the weedy control. This outstanding performance was attributed to the effective weed control of PE-white mulch and the reflective properties of its white upper surface, which enhanced light availability for plant growth. Thus, according to Decoteau et al. (1988), plant growth on white plastic mulch resulted in increased early branching compared with that on black plastic mulch. A vineyard study investigated the impact of mulch color on light reflection and grape yield. White mulch significantly increased the light reflected from the ground to the vine branches, particularly during the early development period. Compared with black mulch treatment, white mulch treatment resulted in higher vineyard yields. However, no significant differences in fruit ripening and chemical composition were observed (Hostetler et al. 2007).
Effects of applications on the development of almond trees.
The effects of the applications on the chlorophyll content of almond trees were examined, and the average of the monthly chlorophyll measurements are provided in Table 7. The applications with the highest chlorophyll contents were FL30, with 6710.0 µmol·m−2·s−1, and nonwoven fabric, with 6669.1 µmol·m−2·s−1. Statistically, these two applications were in the same group. The contents of these applications were followed by those of woven fabric (6572.3 µmol·m−2·s−1), PE-white (6548.1 µmol·m−2·s−1), PE-black (6505.6 µmol·m−2·s−1), FL20 (6331.4 µmol·m−2·s−1), herbicide (6295.6 µmol·m−2·s−1), weedy control (6239.3 µmol·m−2·s−1), and almond shell mulch (6215.4 µmol·m−2·s−1) in descending order of the average chlorophyll content. Wang et al. (2015) investigated the effect of plastic film and straw mulch applications on the amount of chlorophyll in peach cultivation compared with that of the control without mulch. The highest chlorophyll content was detected with the plastic film application, followed by that with straw mulch. Bhagat et al. (2016) stated that the highest chlorophyll content in potatoes was obtained from mulch and nitrogen fertilizer applications. According to Ni et al. (2016), mulching improved plant growth by increasing the root activity, soluble sugar, and chlorophyll a content, as well as by providing suitable moisture conditions and nutrients in the root zone.
Effects of the applications on the chlorophyll content and water potential of almond trees.
Water potential measurements were conducted throughout the trial period, and the average values are presented in Table 7. The water requirement was less in trees with mulch application. Within the applications, nonwoven fabric treatment exhibited the lowest water demand, with an average water potential of 11.9 bar, followed closely by that of almond shell mulch treatment, with an average water potential of 12.2 bar. The control group, which represented unmulched trees, displayed the highest water demand, with an average water potential of 14.8 bar (Table 7). The water potential of trees without mulch was generally high. In other words, plants in open areas needed more water. Gonzales et al. (1993) reported that mulched areas require less frequent irrigation, thus saving water and labor. Kitiş et al. (2017) conducted a study in a mandarin orchard and reported that the application with the highest moisture content at a depth of 10 cm in the soil was the 0.72-mm-thick nonwoven fabric, and that it provided better results than those of PE mulch. A study conducted by Karamürsel et al. (2017) examined the effectiveness of mulch materials to preserve soil moisture and promote water conservation in apple orchards and revealed that mulch materials effectively retained soil moisture, thus keeping the effective root zone moist for extended periods. This, in turn, resulted in reduced irrigation water requirements and significant water savings.
Figure 4 presents the results of the principal component (PC) analysis, which was performed to visualize the projection of high-dimensional data onto a lower-dimensional space. The analysis revealed that the data can be effectively represented by two PCs. The first PC explained 41.8% of the variance, whereas the second PC accounted for 22.9% of the variance. Together, these two PCs explained 64.7% of the total variance. The loadings of the PCs provided insights into the relationships between the original variables and the PCs. The results indicated that the increase in tree height, increase in stem thickness, and increase in shoot length had a strong positive influence on the first PC, whereas chlorophyll content had a weaker positive effect and water potential had a negative effect. However, the chlorophyll content had a strong negative influence on the second PC, whereas water potential had a moderate positive effect. These findings suggested that the chlorophyll content and water potential can be measured together to assess the effects of different weed control methods, whereas the increase in tree height, increase in stem thickness, and increase in shoot length formed a separate group that was influenced differently by the treatments. The first PC appeared to be associated with physical parameters such as plant height, stem thickness, and shoot length, whereas the second PC was related to physiological parameters such as the chlorophyll content and water potential. The results suggested that physical parameters alone are insufficient to explain plant growth. However, measuring these five parameters can provide a comprehensive assessment of plant development in almond trees subjected to weed control and explained 64.7% of the variation in plant growth.
Principal component analysis (PCA). Chl = chlorophyll content; ISL = increase in the shoot length (%); IST increase in the stem thickness; ITH = increase in the tree height (%); WP = water potential.
Citation: HortScience 59, 9; 10.21273/HORTSCI18047-24
Conclusion
This study investigated the effects of five different mulching materials and flame applications with two different frequencies on weed control and almond development in a newly established (2-year-old) almond orchard compared with those of a conventional herbicide (glyphosate) application and a weedy control. The results demonstrated that synthetic mulch applications provided the most effective weed control in terms of weed density, coverage area, and biomass, followed by those of herbicide, almond shell mulch, and flame applications. Although almond shell mulch effectively prevented sunlight from reaching the soil surface, it was unable to control perennial species such as C. dactylon and S. halepense. Flame applications were highly effective in the short-term, but they lacked residual efficacy, resulting in less success than glyphosate despite repeated applications. The study also found that the 30% coverage threshold for flame applications was too high, and that flame applications should be conducted earlier during the weed growth cycle for effective control. The herbicide application, which was repeated five times throughout the season, provided good results compared with those of the weedy control. However, increasing herbicide resistance, environmental and health concerns, and the growing interest in organic almond production suggest that synthetic mulches could be a viable alternative in almond orchards.
All treatments were more successful than the weedy control in terms of their impact on the growth of almond trees. In particular, the PE-white treatment surpassed other treatments in terms of tree height, trunk thickness, shoot length, and shoot thickness. Additionally, mulch applications reduced the water needs of the trees. The best result for this parameter was obtained from the nonwoven fabric treatment, indicating that mulches similar to nonwoven fabric, which allow rainwater penetration, are suitable, especially in orchards where irrigation is not available.
In summary, all treatments tested during the study were more successful than the control in terms of weed control and the impact on the growth of almond trees. Among all treatments, synthetic mulches stood out slightly more. Although the initial installation cost of such mulch materials is somewhat high, considering the long-term advantages they provide, they appear to be one of the best alternatives for both weed control and tree growth in orchards.
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