Introduction to Green Beans:
Green beans are an excellent source of vitamin C, K, and A, as well as manganese, fiber, potassium, folate, iron, magnesium, vitamin B1, copper, calcium, phosphorus, protein, omega-3 fatty acids, and niacin. Fresh green beans are considered one of the most important vegetables, and in 1977, ranked fifth in terms of cultivated area and seventh in nutritional value among 22 major vegetables.
Botanical Characteristics of Green Beans:
Green bean is an annual plant from the Leguminosae family, scientifically known as Phaseolus vulgaris (L.). It has a main taproot and numerous lateral roots. Small brown irregular nodules responsible for nitrogen fixation are present on the roots. The leaves are compound, consisting of three leaflets attached alternately to the stem with long petioles. The leaflets may be covered with fine hairs. The flowers grow either in clusters or singly from lateral buds near the leaves. The corolla is white or purple, and the flowers are self-pollinated. After pollination, fruits in the form of pods develop. Green bean pods are typically long, narrow, and sometimes curved, containing 4 to 12 seeds each. The immature pods are consumed as green beans, commonly referred to as "Snap Beans." Pods may be stringy or stringless; stringless varieties, especially when harvested young, are more desirable and are called "Stringless Beans."
Planting Green Beans:
Green beans are short-day, warm-loving plants that thrive in warm days and cool nights. The optimal daytime temperature for growth is between 20°C and 28°C, and nighttime temperature between 15°C and 20°C. Temperatures above 30°C are not ideal, and above 35°C can prevent seed formation. Similarly, temperatures below 15°C are unsuitable, with 10°C considered the physiological zero and 5°C the minimum tolerable temperature. The soil must be deep, soft, and fertile. Loamy to clay-loam soils are best suited for green bean cultivation. Green beans are sensitive to salinity, tolerating up to 2 dS/m, though yield reductions can start at salinity levels as low as 0.8 dS/m. They can grow in soils with a pH up to 8, but the optimal pH range is 6–7.
The planting date is crucial to avoid exposing sensitive growth stages, especially flowering, to summer heat and drought or to autumn frosts. Planting dates vary widely across bean-growing regions in the country, ranging from early April (in Lorestan province) to late July in central regions.
Green Bean Cultivation:
The water requirement depends on soil texture, structure, climatic conditions, and other factors. In medium-textured soils, more frequent watering is needed compared to heavier soils due to higher water infiltration and evaporation rates.
Irrigation of Green Beans:
In furrow irrigation, the timing between the first and second irrigation differs from subsequent watering. The first irrigation after planting occurs 15 to 25 days later, depending on soil type and climate, allowing seeds to germinate and reach the two-leaf stage. The second irrigation happens between 10 to 80 days after planting. Subsequent irrigations are typically every 4 days on lighter soils and every 5 to 8 days on heavier soils. The critical period for water stress in green beans is from the beginning of flowering until pod formation. Water shortage during this stage can cause severe yield loss. The final irrigation should coincide with about 25% pod maturity. Green beans are highly sensitive to irrigation, so water must reach the root zone properly.
To maximize yield, a balanced and sufficient nutrient supply tailored to the expected yield and specific variety is essential. Fertilizer recommendations are based on soil and tissue analysis and anticipated crop performance.
Harvesting Green Beans:
It takes approximately 60 to 70 days from planting to harvest. Tall varieties have a longer fruiting period, continuing until cold weather sets in. Green beans are usually harvested manually. Bush types are typically harvested in 2–3 stages, while pole types require 5–6 harvests. Yields for bush varieties range from 8 to 13 tons per hectare, and for pole varieties from 20 to 28 tons per hectare. After harvest, green bean quality deteriorates quickly, especially due to heat. For high quality, pods should be consumed fresh within 24 hours of harvest. Otherwise, they should be stored at 0°C with 80–90% humidity. Late harvesting causes pods to lose tenderness and become fibrous.
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