Spinach is a nutritious, hardy cool weather crop, grown for use as a cooked green vegetable or as salad. There are two main types of spinach, that is, smooth leaf and savoy (crinkled leaf).
It is always important to test the soil nutrient level before planting as the pH should be maintained at 6.5 to 6.8. Soils with low pH will result in slow growth and chlorotic leaves. Because of sensitivity to magnesium deficiency, older spinach leaves may tend to show yellow colour similar to that caused by nitrogen deficiency or downy mildew. Do not automatically apply more nitrogen to try to develop the desired deep green colour. Spinach will accumulate excess nitrates if nitrogen is used in an attempt to induce green colour and this is dangerous to health.
Spinach is a cool weather vegetable and can survive the first frost of temperate climates. It germinates and grows optimally at temperatures between 4-16°C. However, it can withstand temperatures as low as -7C. Spinach grows best in a well-draining loamy soil with an optimum pH of between 6.4 and 7. It is sensitive to acidic soil and if the pH is too high. Although it prefers full sun, spinach can still produce significantly in partial shade. In hot weather, seeds germinate slowly or may fail to germinate completely. Heat also causes the plants to bolt (go to seed) quickly, ruining the flavour of the crop.
Planting and cultural practices
Propagation of spinach is solely by seeds. It is first raised in a nursery before transplanting into the main field.
Prepare the nursery bed, a metre-wide against the required length. Drill seeds across the bed at a depth of about 1cm, cover seeds lightly with soil and the nursery bed with a thin layer of dry grass (mulch). Water the nursery bed and seeds germinate within 5-7 days. Seedlings should be watered regularly.
Seedlings are ready for transplanting after 4-5 weeks, when with 3-4 leaves. This, however, depends on the ecological factors in the region, like temperature. Transplanting should be done on a cloudy day or late in the evening when the sun is cool. Note: Wet seedlings an hour before uprooting them from the nursery. This prevents root damages. To prevent transplanting shock, water can be sprayed to the transplants.
Select a site with full sun to light shade and well-drained soil. Plough the land 2-3 weeks in advance at a depth of at least 20cm and harrow the field. Prepare raised beds, recommended for proper drainage and root establishment depending on the amount of rainfall available.
However, in dry areas like Makueni, you can make a flat or sunken bed because of rain limitation. The beds should be 15cm high, a metre wide and about 100 metres long. Incorporate soil with manure and DAP. Water the beds, unless the soil is wet enough.
Plant the seedlings in the beds. This should be done at the same depth they were previously growing while in the nursery. They should be spaced at 30cm to 45cm between rows and 15cm between the plants. This gives room for the leaves to reach full size.
When seedlings sprout to about 10 to 12cm, they should be thinned 8cm apart. This helps in reducing competition as well as ensuring sufficient air circulation within the crops.
Weeds compete with the crop for growth factors like nutrients as well as they harbour pests. They should, therefore, be controlled. Since spinach roots are shallow and easily damaged, care must be taken when weeding.
This helps to keep the soil moist and to suppress weeds. Decomposed mulch releases nutrients into the soil, which are absorbed by the crop. It also helps prevent soil erosion.
Watering should be done regularly because spinach requires high levels of moisture.
Spinach requires sufficient nutrients for proper growth and development. It is, therefore, important to ensure that these nutrients are made available and in the needed quantities. For instance; for basal fertilisers like DAP, put a table spoon per hole and mix with soil at transplanting.
For CAN, put a tea spoonful and is banded around the plant as top-dressing at three weeks after transplant. NPK, are also be applied after the third harvesting to generate more leaves.
Spinach should not be planted on the same plot over and over, because this causes pests and diseases to build up in the soil. Rotations should be done with other crops like beans, peas, lettuce, tomatoes and potatoes.
Pests and control
Cutworms and wireworms: Cutworms cut off the stems of young seedlings at ground level while wireworms feed on the foliage and roots. Drench soil with PENTAGON 50EC 20ml/20l or PROFILE 440EC 60ml/20l or alternative available in agrovets.
Aphids: These are small soft bodied insects found on the underside of leaves and/or stems of plant; usually green or yellow in colour. Infestation causes yellowing and distortion of leaves, formation of necrotic spots and stunted shoots. Aphids secrete a sticky, sugary substance (honeydew) which encourages the growth of sooty mold on the plants. Spray KINGCODE ELITE 50EC 10ml/20l or LEXUS 247SC 8ml/20l or LOYALTY 700WDG 5g/20l or available alternative.
Spinach crown mite: These mites live deep in the crown of the spinach plant. Their feeding causes deformation of leaves and small holes form in newly expanding leaves. Damage can be to newly emerged seedlings or to older plants. Spray ALONZE 50EC 5ml/20l or BAZOOKA 18EC 10ml/20L or available alternative.
Slugs and snails: They leave relatively large holes in spinach leaves. Slimy trails are evident. Drench soil with PROFILE 440EC 6oml/20l
Leaf miners: These leave meandering tan trails or mines on the leaves as they feed. Heavily infested leaves curl and become distorted. Spray ESCORT 19EC 10ml/20l or any available pesticide recommended by agro vet.
Flea beetles: These feed mostly on young foliage and the damage consists of a number of small holes, leaving the leaf looking as though it had been hit by a blast. The leaves sometimes have bleached and pitted areas. Spray LEXUS 247SC 8ml/20l or KINGCODE ELITE 50EC 10ml/20l or PRESENTO 200SP 5g/20l
Cabbage loopers: These caterpillars are pale green with white lines running down either side of their body. Feeding leads to formation of large or small holes on leaves and the damage is often extensive. Spray KINGCODE ELITE 50EC 10ml/20l or BACIGUARD 16WDG or LEXUS 247SC 8ml/20l
Armyworms: Feeding causes singular, or closely grouped circular to irregularly shaped holes on foliage. Heavy feeding by young larvae leads to skeletonised leaves, and egg clusters may be present on the leaves, which are covered in a whitish scale giving them a cottony or fuzzy appearance. Spray KINGCODE ELITE 50EC 10ml/20l or LEXUS 247SC 8ml/20l or PENTAGON 50EC 10ml/20l or BACIGUARD 16WDG 15g/20l
Damping off and root rot: Symptoms consist of poor seed germination, pre-emergence death of seedlings, post-emergence death of newly emerged seedlings, stunted plants, yellowed lower leaves, general poor growth, wilting, and eventual collapse and death of older plants. Roots of the infected plants appear water-soaked or brown to black in colour.
The upper taproot may be girdled by a necrotic lesion, and the tip of the taproot may be necrotic. In severe cases of infection, nearly all roots become girdled or rot off. While all stages of spinach can be infected by root rot organisms, newly emerging plants and young seedlings are very susceptible. Seek advice from agrovet for effective fungicide fungal diseases.
Leaf spot: Infection leads to development of small dark spots on the leaves. These enlarge forming brown lesions, which become brittle and eventually crack at the centre. The lesions can also develop on petioles and stems. Spray CHARIOT 500s 20ml/20l or RANSOM 600WP 15g/20l or BRADLEY 500SC 10ml/20l, however you will be advised on best alternative available
Downy mildew: Initially dull to bright yellow spots form on cotyledons and leaves of all stages. These spots enlarge and become browning and dry. Close inspection of the underside of the leaf often reveals the purple growth of the fungus. In severe cases of infection, leaves appear curled and distorted and may take on a blighted effect as a result of numerous infection sites.
Remember, every environment has specific challenges that require extra attention.
Otieno works in the Department of Crops, Horticulture and soils. Egerton University.