Effect of Different Sustainable Cultivation Methods on the Biometric Parameters and Yield of Mint

Abstract

Mint is one of the most valuable herbs, and has multiple benefits and uses. The effect of cultivation methods on the biometric parameters of mint plants was determined by comparing a hydroponic system involving moveable flood tables with plastic covers and a raised-bed growing system in an open field. The morphometric parameters of mint plants may vary depending on species and cultivation method. An interaction between both factors was observed for plant height, leaf length, leaf width, and the number of leaves. Spearmint and apple mint grown under field conditions were characterized by higher average values of plant height and width and a higher number of branches than hydroponically grown plants. In the hydroponic system, the number of branched stems decreased by up to 80% with increased plant height. Leaf width was correlated with the total number of leaves. An increase in the number of leaves per plant induced a decrease in leaf width (up to 61%). Field-grown mint is usually characterized by higher marketable and total yields than hydroponically grown mint. However, the marketable yield of apple mint was approximately 50% higher in the hydroponic system than in the field.

1. Introduction

The importance of biodiversity is increasingly recognized, and various plant species and botanical varieties are being introduced to enhance biodiversity.

The genus Mentha, a member of the family Lamiaceae, comprises 19 species and 13 naturally occurring hybrids worldwide [1]. Mint is considered to be native to the Mediterranean region and East Asia, and has been cultivated since ancient times due to its medicinal properties. According to a Greek myth, the name Mentha comes from a nymph named Minthe who was turned into a plant. Mint is one of the most valuable herbs, and has multiple benefits and uses. It exerts relaxing effects on the muscles, acts as a natural diuretic, cholagogue, and choleretic, aids digestion, relieves bloating and flatulence, has antibacterial properties, and exerts soothing, analgesic, anti-inflammatory, and diaphoretic effects. Mint herbage and leaves are used as raw materials [2]. Mint is known for its fresh aroma and cooling, menthol flavor. In Poland, mint is added mainly to desserts, including ice cream, and refreshing lemonade. In other countries, mint sauce is highly popular, and mint is also served with meats and many other dishes and beverages.

Mint can be grown both in the field and in containers. It thrives on fertile and permeable soils rich in humus and nutrients, with pH levels ranging from slightly acidic to neutral. The highest mint yields can be achieved in alluvial soils, loess, rendzina, and chernozem [3]. Deep plowing should be done in fall, and dragging and harrowing should be done in spring. All weeds, in particular perennial weeds with creeping rhizomes (such as couch grass), should be removed from the field [2]. However, there are some limitations to traditional soil-based cultivation, including soilborne pests and diseases that reduce productivity, especially of high-value crops.

Therefore, we should be fully aware of the negative impacts of agricultural intensification and the effects exerted by this process on agricultural landscape biodiversity. Ecological infrastructure solutions should be implemented in horticultural and agricultural farms to promote a sustainable, climate-neutral approach to crop production and preserve biodiversity. Sustainable farming practices contribute to efficient use of resources, optimal utilization of inputs, and protection of the natural environment and the farm surroundings. In recent years, different plant species, including mint, have been cultivated with the use of various methods to increase biodiversity and create new habitats.

Hydroponics is a soilless food production system [4,5,6]. Hydroponic cultivation contributes to maximizing yields (plant growth potential) and increasing water and nutrient use efficiency due to advanced growing technologies, such as integrated plant protection, fertigation, drip irrigation, and climate control [7,8]. The development of controlled-environment agriculture technologies, including hydroponics, offers a viable alternative enabling to increase water use efficiency and productivity in an environmentally sustainable manner [9]. Hydroponic cultivation of fresh organic mint reduces water use and eliminates the need for pesticides. Plants are protected with biological control agents; therefore, the produced herbs are healthy and free of chemicals.

The present study was conducted to compare the cultivation of peppermint (Mentha piperita L.), spearmint (Mentha spicata L.), and apple mint (Mentha suaveolens L.) under field and hydroponic conditions. The research hypothesis was that soil-based cultivation, compared with hydroponic cultivation, improves the biometric parameters of mint plants and increases their total and marketable yields. Biodiversity is increasingly important to consumers who prefer environmentally friendly companies that strive to maintain a balance between the environment and human needs. They want to know the origin of foods so as to make informed decisions and choose the best-quality products.

The aim of this study was to evaluate the effect of conventional soil-based cultivation in the field and hydroponic cultivation in plastic tunnels on the biometric parameters and yield of selected mint species.

2. Materials and Methods

Three mint species, peppermint (Mentha piperita L.), spearmint (Mentha spicata L.), and apple mint (Mentha suaveolens L.), were grown in an experiment conducted in 2022–2023 in Swedeponic Polska Sp. z o.o., a company producing hydroponic culinary herbs. The effect of cultivation method on the biometric parameters of mint plants was determined by comparing a hydroponic system involving moveable flood tables with plastic covers, and a raised-bed growing system in an open field. Hydroponic cultivation was carried out in plastic tunnels designed for year-round use, with a total area of 1.7 ha. The field had an area of 1 ha, and mint plants were grown in brown soil of quality class IVb according to the Polish soil classification system. The experiment had a 3 × 2 factorial design for each variable. The experiment was performed in triplicate. Each replicate consisted of 60 plants. The experimental site was located at a latitude of N 52°12′ and a longitude of E 20°56′, 94 m.a.s.l.

Mint seedlings were grown on moveable flood tables in a heated greenhouse, in line with the generally observed standards for mint cultivation, in accordance with the PL-EKO-08 certificate, EU Agriculture. Then, plants were propagated vegetatively through tip cuttings that were placed in paper pots, two cuttings per pot. Each year, cuttings were collected from mother plants and rooted in peat pellets that had been soaked with purified water 24 h before planting. Three days after roots had developed, the seedlings with paper pots were placed in 10 × 10 cm pots filled with peat substrate with the following composition: N—10 mg/L soil, P—109 mg/L soil, K—310 mg/L soil, Mg—142 mg/L soil, Ca—452 mg/L soil, S—185 mg/L soil, Cu—0.1 mg/L soil, Mo—0.1 mg/L soil. Two paper pots were placed in each pot.

Substrate composition (

Table 1. Composition and pH of the substrate used for seedling production.

Specification	Value
pH ()	5.8
N (mg/L soil)	10.0
P (mg/L soil)	109.0
K (mg/L soil)	310.0
Mg (mg/L soil)	142.0
Ca (mg/L soil)	452.0
S (mg/L soil)	185.0
Cu (mg/L soil)	0.1
Mo (mg/L soil)	0.1

) was analyzed at the Laboratory of Chemical Analyses, Institute of Horticulture—National Research Institute in Skierniewice, under Research Laboratory Accreditation Certificate AB 1688 issued on 25 June 2018.

After 10 days, mint plants were pruned to promote growth. A Priva greenhouse climate control system was used. During seedling production, temperature and relative humidity were maintained at 24 °C and 85%, respectively. Twenty days after propagation, seedlings intended for open-field cultivation were hardened off by placing them in an unheated plastic tunnel for three days. One day before transplanting the seedlings, raised beds (1 m wide) were formed. Immediately before transplanting them in the permanent location, the substrate in all pots was saturated with water to facilitate the removal of roots. Twenty seedlings were planted per plot, at a spacing of 40 × 20 cm. Strips of bare soil between raised beds, 50 cm wide, were used as pathways.

Mint plants were regularly watered, fertilized, and monitored. Weeds were removed by hand three times. Plants were watered with the use of a drip irrigation system that featured pipes, tubing, and emitters to supply water and nutrients directly into the root zone and minimize water losses. Field-grown mint was irrigated with the nutrient solution used at the production facility. The pH and electrical conductivity (EC) were maintained at 6.2–6.5 and 2.5, respectively. The frequency of watering depended on the weather conditions, and the plants were usually watered once a week. Only organic plant protection products were used, in accordance with Regulation (EU) 2018/848 of the European Parliament and of the Council of 30 May 2018 on organic production and labeling of organic products and repealing Council Regulation (EC) 834/2007 (OJ L 150 14.6.2018) [10].

Mint grown in heated plastic tunnels was placed on moveable flood tables at a spacing of 0.4 × 0.2 m. Plants were irrigated by applying water and a nutrient solution directly onto the tables with the use of pumps. The pH and EC of the nutrient solution were monitored and maintained at the optimal levels (pH—6.2 to 6.5, EC—2.5) using the Priva greenhouse climate control system. The preset parameters and the composition of the nutrient solution are trade secrets of the Spisa Group. Nutrient deficiencies were corrected with the use of Spisa biofilters hosting nitrifying bacteria, Nitrobacter and Nitrosomonas. Biofiltration is a natural, sustainable technological solution. Bacterial colonies grow over the entire surface of biofilters, and water is constantly pumped to ensure adequate oxygen saturation. The optimal temperature for hydroponic biofilters is 27 °C. Bacteria were cultured on Prembt and Novarbo growing media. The excess water (up to 90%) and nutrient solution that have not been used by plants on the tables are recirculated and reused in a closed system, which supports rational water and nutrient solution management. The working principle of the system is as follows: the trays are flooded with a nutrient solution, and the substrate in pots becomes soaked with the solution; 20 m³ of the solution is needed to flood tables with a surface area of 1000 m² with a 2 cm layer of the liquid; approximately 10–30% of the solution is retained in pots, and the excess solution passes through the growing medium, is drained back into a reservoir tank, and reused.

Since mint was grown in monoculture, the major insect pests were Myzus persicae and Frankliniella occidentalis. Due to close monitoring and removal of infested plants at each stage of the production process, chemical plant protection products were not needed. Neoseiulus cucumeris (500 mites/m²), Chrysoperla carnea (10 insects/m²), and Aphidius colemani (1 insect/m²) were applied once a week as biocontrol agents to promote environmentally friendly cultivation.

In each mint species, total and marketable yields were determined at full vegetative maturity and are expressed as plant weight. The following biometric parameters were determined at harvest: plant height (measured from the base of the stem/soil surface to the highest point), plant width (measured at the widest part), number of branches, number of leaves, leaf width (measured at the widest part), and leaf length (measured along the main nerve). The results are expressed as means of replicates. Measurements were performed with a caliper (accurate to ±1 mm) and a ruler (accurate to ±1 mm).

The results were analyzed statistically using Statistica PL ver. 13.3 software (TIBCO, Paolo Alto, CA, USA). The mean values of the analyzed parameters were determined with the use of descriptive statistics. Significant differences in the mean values of the examined parameters were determined by Student’s t-test for independent samples and analysis of variance (ANOVA) at a significance level of α = 0.05. In the latter case, homogeneous groups were identified by Tukey’s test. The strength of relationships between the analyzed parameters was evaluated by calculating Pearson’s coefficients of correlation: if values were high, the corresponding regression equations were derived. Only relationships with a sufficiently high coefficient of determination (minimum 0.6) are presented.

3. Results and Discussion

According to the Association for Sustainable Agriculture and Food in Poland, sustainable agriculture is defined as efficient production of safe and high-quality food in a way that protects and improves the quality of the natural environment as well as the social and economic status of farmers, farm employees, and members of local communities. The main concept of sustainable agriculture/horticulture is not only to maximize profits but also to minimize environmental impacts, respect the environment and biodiversity, and meet the needs of present and future generations. The principles of sustainable agriculture apply to the entire production process on the farm, which is closely linked to the surrounding ecosystem. Such a strategy generates tangible benefits for the natural environment, contributing to improving the quality of air and groundwater, and soil fertility, reducing greenhouse gas emissions and energy consumption from non-renewable sources, and increasing biodiversity in agroecosystems and agricultural landscape.

Such an approach was also adopted in the current experiment. The cultivation process was certified to ensure that food and agricultural commodities are produced with the use of organic methods in a clean and safe environment without synthetic fertilizers, plant protection products, growth hormones, antibiotics, or genetically modified organisms.

Due to the high economic importance of mint, its morphometric parameters and chemical properties have been analyzed by numerous researchers who, however, focused mainly on the chemical composition of the fruits. The specific characteristics of mint have contributed to the increased interest in this herbal material in the pharmaceutical, cosmetic, and food industries [11]. In addition, the cultivation of mint increases biodiversity in agricultural and horticultural farms. Mint also provides phytosanitary benefits, which is another important consideration [12].

3.1. Biometric Parameters of Mint Plants

The morphological, chemical, and molecular diversity of mint species has been de-scribed by many authors [13,14,15]. Differences in the biometric parameters of mint plants grown in soil and in the hydroponic system are presented in

Table 2. p-values in ANOVA of the morphological parameters and yield of various mint species.

Parameter	Factor
	Species (A)	Cultivation Method (B)	Interaction (A × B)
Plant height	0.002	<0.001	0.004
Plant width	<0.001	<0.001	0.781
Leaf length	0.003	<0.001	<0.001
Leaf width	<0.001	<0.001	<0.001
Number of branches	<0.001	0.008	0.088
Number of leaves	<0.001	<0.001	0.010
Marketable yield	0.429	0.729	<0.001
Total yield	<0.001	<0.001	0.062

and Figure 1. The morphometric parameters of mint plants may vary depending on species and cultivation method. Genetic variability in mint species was described by Roshanibakhsh et al. [16]. Genetic variation is a multi-scalar variable, which is important for conservation programs [17]. Three levels of biodiversity have been identified: genetic diversity (changes in genes and genotypes), species variation (species abundance), and ecosystem variation (association between species and their habitat). Thorough knowledge of the genetic diversity and relatedness of species identified in high-value habitats is key to developing conservation strategies for small and isolated communities [18]. The study of genetic and phytochemical diversity of plant species is critical in breeding programs to preserve genetic resources and obtain high-quality essential oils [19]. An interaction between both factors was observed for plant height, leaf length, leaf width, and the number of leaves. The interaction effect exerted on plant width and the number of branched stems was not significant.

Soil-grown plants were larger than their hydroponically grown counterparts. They were taller and wider. They also produced more branches with a higher number of leaves, and the latter trait was associated with their smaller size.

In all analyzed mint species, soil-grown plants were similar in size and habit (height of 41 to 46 cm, width of 28 to 36 cm). The number of branched stems was highest (27 on average) in M. spicata. In the remaining species, the number of branches ranged from 12 to 15. Both M. spicata and M. piperita produced around 75 leaves, whereas M. suaveolens produced around 52 leaves, but all mint species had leaves of similar length (3.3 to 4.0 cm) and width (3.0 to 3.4 cm).

In the hydroponic system, M. piperita plants were tallest (approx. 38 cm), and M. spicata plants were shortest (approx. 26 cm). Average plant width was highest in M. suaveolens (approx. 27 cm) and lowest in M. piperita (approx. 19 cm). The number of branches varied in a wide range, from around 19 in M. spicata to around 5 in M. piperita. Both M. spicata and M. piperita produced a high number of leaves (approx. 75), whereas the number of leaves was significantly lower in M. suaveolens (approx. 52). The leaves of M. suaveolens and M. spicata were similar in length (approx. 7.7 cm), and the leaves of M. piperita were much smaller (approx. 4.5 cm). The analyzed mint species differed in terms of leaf width, which was highest in M. suaveolens (approx. 6.7 cm) and lowest in M. piperita (approx. 3.5 cm).

Nutrients play a key role in maximizing crop yields and improving quality attributes. In modern agriculture, adequate proportions of essential nutrients and balanced plant nutrition are a prerequisite for increasing yields, quality, and profits in a sustainable manner [20]. In a field experiment conducted by Lothe et al. [20], the application of various mineral nutrients had a significant effect on plant height and the leaf/stem ratio in Mentha arvensis L. The morphometric parameters of hydroponically grown mint plants determined in this study were more satisfactory than those noted by Loera-Muro et al. [6] in M. spicata grown in a closed hydroponic system, and similar to those reported by Knaus et al. [5] in the aquaponic production of M. spicata.

3.2. Mint Yields

Differences in the marketable and total yields of mint plants grown in soil and in the hydroponic system are presented in Table 2 and Figure 2. Marketable yield was not affected by mint species or cultivation method, but an interaction between these two factors was observed. In turn, total yield was influenced by both mint species and cultivation method, and there was no interaction between these two factors.

Field-grown mint is usually characterized by higher marketable and total yields than hydroponically grown mint. However, the marketable yield of M. suaveolens was approximately 50% higher in the hydroponic system than in the field. This species was characterized by the highest (approx. 45 g, hydroponics) and lowest (approx. 31 g, field) average values of marketable yield. In M. spicata, no significant differences in marketable yield were observed between plants cultivated by two different methods. The average value of total yield was lowest in hydroponically grown M. piperita and highest in soil-grown M. suaveolens, and significant differences in total yield were noted between the two cultivation methods in all species. It should be noted that the differences in the total and marketable yields of mint between the two cultivation systems are due to the high demands placed on the plants used as raw materials. Even the slightest discoloration and other defects are considered disqualifying. Field-grown plants are exposed to more adverse conditions than their hydroponically grown counterparts.

According to Telci et al. [21], the commercial production of peppermint (M. piperita L.) depends on its genetic structure and environmental conditions that affect yields. They demonstrated that fresh herbage yields were higher in locations with a warmer climate. A similar response was noted in M. suaveolens in the current study. These results corroborate the findings of Marino et al. [12] and Polanski et al. [22], who observed that adequate irrigation plays an important role in mint cultivation, since drought stress leads to a significant reduction in all growth parameters, and in the yield and percentage of essential oils. Therefore, adequate water supply is the key to high mint yields.

In the work of Surendran et al. [9], the productivity of M. spicata was higher in a hydroponic system than in conventional soil farming. The growth rate of mint plants was faster because they were supplied with adequate amounts of essential nutrients. In that study, the yield of hydroponically grown M. spicata was 61% higher than in soil-grown plants. In the hydroponic system, water and nutrients are delivered directly to the plant’s roots, which promotes plant growth, stem elongation, and leaf development, thus increasing yields. Hydroponics uses less space and minimizes the risk of weed and soilborne pest invasions. Previous research has shown that in a hydroponic system, plants grow faster and can reach their maximum yield potential. The generated yields are higher than in the field because the roots have easy access to oxygen and can efficiently absorb water and nutrients from high-quality nutrient-rich solutions. Optimal pH levels are also maintained [23,24,25].

3.3. Correlations between the Parameters of Mint Plants Cultivated Using Two Methods

According to Mansoori [26], total fresh mint yield is an important indicator required by growers to estimate the economic value of its cultivation. In that study, the main effects of plant density and harvest time significantly affected fresh biomass yield. The results of a linear correlation analysis of the morphological traits and yields of mint plants cultivated using two methods are presented in

Table 3. Coefficients of Pearson’s correlation between the parameters of mint plants cultivated using two methods.

Cultivation Method	Parameter	Pearson’s Correlation Coefficient
		Hp	Wp	Ll	Wl	nb	nl	Ym
Hydroponics	Wp	−0.414	1
	Ll	−0.538	0.768	1
	Wl	0.089	0.741	0.546	1
	nb	−0.822	0.500	0.602	0.032	1
	nl	−0.394	−0.480	−0.207	−0.849	0.364	1
	Ym	−0.051	0.782	0.607	0.887	0.118	−0.739	1
	Yt	−0.040	0.766	0.603	0.879	0.088	−0.753	0.988
Pots	Wp	−0.101	1
	Ll	0.151	0.271	1
	Wl	0.330	0.019	0.667	1
	nb	−0.529	0.390	0.083	−0.118	1
	nl	−0.130	−0.373	−0.565	−0.647	0.267	1
	Ym	−0.286	−0.377	−0.512	−0.542	0.157	0.575	1
	Yt	0.312	0.327	0.237	0.312	−0.416	−0.553	−0.370

H_p—plant height, W_p—plant width, L_l—leaf length, W_l—leaf width, n_b—number of branches, n_l—number of leaves, Y_m—marketable yield, Y_t—total yield. Values in bold denote significant correlations at p < 0.05.

. The absolute value of the correlation coefficient was highest (0.988) for the correlation between marketable yield and total yield in hydroponically grown plants, and lowest (0.077) for the correlation between plant width and leaf width in soil-grown plants. The number of significant relationships was considerably higher in the hydroponic system than in soil cultivation. Significant correlations were noted in 20 cases out of 28 comparisons in hydroponics, and in only 10 cases in the field. Only five pairs of the compared traits followed a similar trend in both cultivation methods, which indicates that the significance of their relationships might be affected not only by the cultivation method but also by the species.

Regression equations with a sufficiently high coefficient of determination (minimum 0.6) were derived only for the correlations between the parameters of hydroponically grown plants. Such a coefficient of determination was noted in six relationships (Figure 3), and all of them could be described by linear functions. The highest coefficient of determination (R² = 0.98) was noted for the correlation between total yield and marketable yield. The marketable yield was 2% lower than the total yield on average. It was found that the marketable and total yields of mint grown in the hydroponic system could be predicted based on, leaf width, among others, and these relationships were directly proportional. Changes in leaf width in the range of 3.5 cm to 7.2 cm increased total yield by around 55% (from approx. 31 g to approx. 48 g) and marketable yield by around 48% (from approx. 31 g to approx. 46 g). The marketable yield of the analyzed mint species could also be predicted based on plant width. Changes in plant width in the range of around 16 cm to around 28 cm increased marketable yield by approximately 62%. In the hydroponic system, the number of branched stems decreased with increased plant height. Changes in plant height in the range of around 22 cm to around 42 cm induced an 80% decrease in the number of branches (from approx. 20 to approx. 4). Leaf width was correlated with the total number of leaves. An increase in the number of leaves per plant from around 20 to around 72 was accompanied by a 61% decrease in leaf width (from approx. 6.6 cm to approx. 2.6 cm). Similar observations were made by Mansoori [26] and Wajid et al. [27], who found that plant productivity was determined by leaf size.

4. Conclusions

The morphometric parameters of mint plants may vary depending on species and cultivation method. An interaction between these factors was observed for plant height, leaf length, leaf width, and the number of leaves.

The marketable yield was on average only 2% lower than the total yield. Such a small difference results from optimal growing conditions (plant material, cultivation practices, phytosanitary conditions), and is highly beneficial. Spearmint and apple mint grown under field conditions were characterized by higher average values of plant height (41–46 cm) and width (28–36 cm) and a higher number of branches than hydroponically grown plants.

In the hydroponic system, the number of branched stems decreased by up to 80% with increased plant height.

An increase in the number of leaves per plant induced a decrease in leaf width (up to 61%).

Field-grown mint is usually characterized by higher marketable and total yields than hydroponically grown mint. However, the marketable yield of M. suaveolens was approximately 50% higher in the hydroponic system than in the field.

In the present experiment, soil-grown mint plants were characterized by more desirable biometric parameters and higher yields. However, both cultivation methods analyzed in this study can be recommended to obtain raw herbal materials in accordance with the principles of organic production.