Under tropical conditions, French beans are mainly grown during the dry season in the Sahel zone (Senegal and Burkina Faso). In Kenya, production zones are at different altitudes with different climates and so beans can be produced all the year round. Although it grows on many soil types, French bean does best in soils that are fairly heavy but not too much so as the plants are susceptible to root asphyxia. The vegetative cycle is very short, especially in Sahel areas, with harvesting beginning 45 to 60 days after sowing. The cycle is longer in highland zones in Kenya, lasting for a minimum of 60 days. For this, plant development must be enhanced, with rigorous attention paid to fertilisation and irrigation. In spite of its short vegetative cycle, green bean is subject to attacks by pests and diseases that can affect both produce quality and financial returns.
PHOTO HARICOTCrop rotations are necessary to reduce fungal attacks after germination and emergence and to enhance field productivity. The best preceding crops are cereals. Leafy vegetables and Cucurbitaceae should be avoided. Soils with various textures (silty-sandy, clayey-sandy) can be chosen. Cultivation is possible on sealing soils (silty) but a few precautions are required during the sowing to emergence period. Bean has a short vegetative period and so must develop rapidly. This is why meticulous, rigorous attention must be paid to soil preparation. Soil suitable for growing French beans must have the following features:
The basal dressing must be placed when the soil is prepared so that it is located in the root development zone. Organic fertilisation is not recommended as it can increase susceptibility to pests and diseases in case of poor decomposition. Only well-rotted organic material turned in when the land is prepared can be applied.
Sowing must be performed so as to allow reasonable but not excessive vegetative development of the plants. Too high a density causes the etiolation of the plants, thus increasing fragility and sanitary risks. This is why the following factors must be taken into account in sowing densities:
Registered, treated seed is recommended as this protects the seedlings against the first pests (bean fly) and is free of diseases of the Fusarium and common blight type. The quantity of seed required is some 30 kg per hectare for densities of about 200 000 to 250 000 plants per ha. When trickle irrigation is used, higher densities of up to 300 000 plants per ha are sown. Interrow spacing is generally 30 to 40 cm and the plants are set out at 5 to 7 cm intervals along the rows. In rainy or very humid periods, greater spacing along the row allows the plants to dry more quickly after rainfall. Seeds are sown at a depth of 2 to 5 cm on soil that has been left to drain well (if irrigation has been applied prior to sowing). Care should be taken not to sow at too shallow a depth in light filtering soil (sandy). In contrast, a depth of 2 to 3 cm is enough in heavy and/or sealing soil. Although irrigation is necessary and recommended in sandy soil, watering is not recommended between sowing and emergence in sealing soil. As a general rule, pre-sowing irrigation of silty-clayey soil is sufficient for germination and homogeneous, regular emergence in the field.
The choice of form of fertiliser is important as the French bean cycle is short. The fertilisers used must contain elements that are taken up easily and quickly. The main elements (N, P, K) must be applied rationally, party before the crop is sown (basal dressing) and the rest as top dressing during the vegetative cycle.
French bean fertilisationSingle and complete fertilisers are applied in many production zones. Nitrogen fertiliser (N) is in urea, ammonia or nitric form, phosphorus (P) is applied as ammonium phosphate or triple superphosphate and the most common forms of potassium are potassium sulphate or nitrate. Chloride forms should be avoided as chlorine is toxic for many market garden crops and for French beans in particular. Shallow hoeing should be performed when top dressing is applied or fertilisation can be followed by watering. In some perimeters with more sophisticated installations, fertilisation—especially top dressing—can be combined with trickle irrigation.
In addition to the main elements, secondary elements are generally combined with the basal dressing; these are magnesium in the sulphate form and calcium in the form of slag. The trace elements essential for the crop (molybdenum, zinc, copper and manganese) are applied if necessary by foliar spraying or via the trickle irrigation system.
As for most counter-season crops in tropical countries and especially in the Sahel zone, it is essential to irrigate fields of French beans for a crop that is satisfactory in terms of both quality and quantity. In some highland zones (in Kenya), rainfall may cover part of the water requirements. Fields are traditionally watered by gravity irrigation. Some perimeters have installed more sophisticated techniques such as trickle irrigation and sprinkler irrigation.
This type of irrigation requires good land levelling so that the water flows freely along the rows. Defects in levelling cause local accumulation of water with a risk of plants wilting as a result of root asphyxiation and the spread of certain fungal diseases. Very large amounts of water are required and the quantity is difficult to manage, especially during the rainy season when the total of irrigation water and precipitation may sometimes be excessive).
Very well suited to this kind of crop, trickle irrigation requires rigour, in particular as regards filter equipment as water with a particle load can foul irrigation piping. This method uses much less water than gravity irrigation and the management of quantity is more flexible and rational. In addition, the technique can be used to apply top dressing with the applications split and distributed throughout the cultivation period. Much care must be taken in the choice and quality of the fertilisers used for fertigation and the risk of blocking of the drippers can affect the overall quality of irrigation of all the rows and the field.
This technique also enables better management of volumes and application of water throughout the vegetative cycle. It is better to water in the morning to avoid risk of leaf burn and also to limit risk of fungal infection during sultry weather. Sprinklers with large jets or large drops should not be used as they can damage foliage and also splash the pods with earth and reduce the commercial yield of the field. This watering technique has the advantage of limiting the populations of certain pests—in particular spider mites and thrips.
French beans are sensitive to water stress, especially at the mergence stage and during flowering/pod growth. However, excessive irrigation should not be applied at the post-emergence stage in order to avoid fungal attacks at the root collar of seedlings. A slight, rational water shortage after emergence enhances plant rooting as the roots explore a deep level of the soil. Irrigation frequency and dose are determined in the light of meteorological conditions, taking daily evaporation into account in particular. Installing tensiometers in the field can be a help in taking decisions concerning irrigation depths and frequencies. Knowledge and experience of the soil and the crop should be taken into account in irrigation management. It is preferable to perform both furrow and sprinkler irrigation in the morning, allowing the foliage to dry during the day. Sprinkler irrigation should not be carried out immediately after leaf spraying with fungicide or insecticide. Irrigated just before picking should be avoided. Harvesting conditions will be more comfortable (no wet areas) and the produce of better quality (no risk of mud or wet earth on the pods).
Click "Continue" to continue shopping or "See your basket" to complete the order.
