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Asparagus, Asparagus officinalis, is an herbaceous perennial plant in the family Asparagaceae which is grown for its young shoots, or spears, which are eaten as a vegetable. The asparagus plant is tall with scale like leaves emerging from the underground stem (rhizome) and has stout stems and feathery foliage. The flowers are bell shaped and occur alone or in pairs. They are green-white to yellow in color. After flowering, a round red berry is formed with 1 to 6 black seeds. Asparagus can live for 15 or more years and can attain a height between 100–150 cm (39.4–59.1 in). Asparagus originates from Europe, northern Africa, and western Asia.
Asparagus farming in Kenya
Photo credit: Husfarm
Asparagus exports from Kenya have been growing with time. Purple asparagus, green asparagus, and white asparagus, fresh or chilled is a much sought-after vegetables in Europe. It is also in high demand in high-end restaurants in Kenya. Kenyan farmers can take advantage of this and begin growing asparagus.
Asparagus has few pest or disease problems and can be grown without artificial pesticides, making it a relatively easy crop to grow organically. Good weed management, particularly during  establishment, is essential to promote healthy growth and satisfactory yield and quality.
Asparagus farming, like other vegetable production systems, involves developing a functional
system that provides adequate fertility while maintaining effective weed management. A well managed asparagus stand may stay productive for 15 years or more

Uses of Asparagus

Asparagus spears can be eaten raw or cooked. They are low in calories and very low in sodium. Asparagus is a particularly good source of vitamin B6, calcium, magnesium , zinc, vitamin A, vitamin C, vitamin E, vitamin K, thiamin, riboflavin, rutin, niacin, folic acid, iron, phosphorus, potassium, copper, manganese, selenium, chromium, dietary fiber, and protein.

Asparagus Growth Requirements

Asparagus responds well to a deep, well-draining soil with a light texture profile (sandy loam). Ideally, the soil should contain at least three per cent organic matter and have a pH between 6.0-6.8. Asparagus will not tolerate standing water at the root system so soil needs to drain properly and be free of hard-pans.
The soil should also be free of stone and gravel, as these can injure growing stems making them unmarketable. Asparagus will develop a large, proficient root system that can reach 1.5 metres deep at maturity. New plantings should not be placed in fields that have been previously used for asparagus cultivation – two soil born Fusarium diseases can cause serious production problems and as a perennial crop there is no opportunity for crop rotation in an established asparagus stand.

Basic requirements

Asparagus grows best in regions with hot days and cool nights and requires 90–150 days of cold temperature to break dormancy. It will grow in most soils as long as they are well-draining and will grow optimally at a pH between 6.5 and 7.0. Asparagus should be planted in a sunny location in the home garden and it is best to avoid low lying areas as a late frost will cause damage to newly emerged spears.

Growing from seed

Growing asparagus from seed is more time consuming and labor intensive than growing from divided crowns. If growing from seed, be aware that you will be unable to harvest the spears for longer than if grown from crowns (spears can generally be harvested in the third year after planting). Asparagus seeds should be sown indoors 12 to 14 weeks before planting outside. This should be timed so that the outdoor planting date is after all danger of frost for your area. Seeds should be sown in mix, planting 0.6 cm (0.25 in) deep. The seeds should be kept moist, but not wet and will germinate in approximately 3 weeks. Seedlings should be fully hardened off prior to planting outdoors, using a cold frame where possible.

Transplanting

The asparagus nursery bed should be prepared in advance by digging deeply to break up the soil and incorporating plenty of organic matter such as well-rotted manure and by blending a complete fertilizer into the soil just prior to transplanting.
Plant the seedlings 25 to 38 cm (10-25 in) apart in a trench approximately 10 cm (4 in) deep. The trench should be gradually filled in as the seedlings grow. Keep the soil moist by providing about 2.5 cm (1 in) of water weekly. Once the plants are established, irrigation is generally only required in very dry areas due to the deep root system that the plant produces. It is important to control weeds as not only will it prevent any competition with the asparagus plant but it will also ensure that the young spears can be seen and harvested at the correct time. The asparagus plants should be transplanted to their final planting site in the second year following the directions for planting crowns (below).

Planting crowns

When purchasing asparagus crowns, choose disease-free, 1 year old crowns from a reputable grower. All-male hybrids tend to be produce more spears than females and are a good choice for growing at home. Crowns should be planted once the soil has warmed to 10°C (50°F). Dig a trench 12 to 15 cm (5-6 in) deep and add super phosphate fertilizer to provide the crowns with nutrients immediately after planting.
Position the crowns in the trench 30 to 45 cm (12-17 in) apart. It does not matter how the crown are oriented in the trench as they will grow regardless. Backfill the trench to the original level. If planting more than one row, allow at least 1.5 m (5 ft) between rows. New spears should begin to emerge from the soil within a week of planting. Avoid harvesting the spears until the following year. Allowing the ferns to develop and provide energy for the crown to produce spears the following year.

Harvesting spears

Allow asparagus ferns to remain until the Spring before cutting or mowing them back to the ground to allow new spears to grow. The new spears are ready to harvest when they reach 17 to 22 cm (7-9 in) in length. The spears can simply be snapped by hand close to the ground. The spears can be harvested every 2 to 4 days for a period of 4 to 6 weeks in the second year (third year if grown from seed). Avoid over-harvesting as this will decrease the yield the following year. The harvest can be extended in the third year (fourth for seed grown plants) to 6-8 weeks.

Crop Establishment

Seed Germination:
Asparagus seed requires a minimum of 10˚C to germinate, however germination success and seedling development are enhanced at temperatures between 15-30˚C. The optimum temperature for asparagus seed germination is 24˚C. Germination may also be enhanced by soaking seedlings in water at 32˚C for three to four days prior to sowing. Soil should be at least 15˚C when sowing asparagus seed.

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Asparagus transplants can be started in a greenhouse in middle to late February and transplanted into the field after eight to 12 weeks or when soil has reached an adequate temperature (15˚C).

Asparagus crowns are started in nursery beds and left in the ground for one year; plants are dug up in the spring and then replanted as soon as possible. Selection of the best crowns is essential. Damaged, diseased or under sized (less than 25g) crowns should be discarded. Selection of the best asparagus crowns is essential, all damaged, diseased or under sized (less than 25g) crowns should be discarded, this may be up to 40 per cent of the crowns in the nursery beds. Crowns should be planted at a depth of 15-20 cm.

Seeding/Planting:

Permanent field spacing for plants should be at a distance of 1.25-1.75 metres between rows and a
spacing of 30-45 cm between plants within the row. Asparagus seed should be planted at a depth of between 2.5-4 cm. Crowns should be placed in a 15- 20 cm deep furrow and covered with 5 cm of soil. Soil should continue to be added through the season, usually during cultivation to control weeds.

Crop Management

The first two to three years of establishment are critical for the productivity and yield of the mature
asparagus stand. During these first couple years young asparagus plantings can suffer from weed competition, inadequate soil moisture and/or soil fertility. It is also highly detrimental to over harvest plants during the first years of establishment.

After the last harvest of the season the asparagus bed should be shallowly cultivated to control weeds and incorporate nitrogen fertilizer. Care must be taken to not damage the root system of the asparagus during these activities. The remaining stems will begin to unfurl their frond-like leaves which will photosynthesize and capture energy to recover from the harvest period and store reserves for winter dormancy and regrowth in the spring.

While stems are still green they are still connected to the root system of the plant and should not be removed. When the stems turn yellow they can be removed, however the stems tend to catch snow which can help insulate the plant during winter. The stems can be cut and incorporated in the spring before the plant breaks out of dormancy. Incorporation of dead stem tissue can add 10 tonnes of organic matter per hectare.

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Pest and Disease

Disease: Asparagus rust (Puccinia asparagi)
Characteristics: Red/orange lesion develops on leaves and stems. This disease can cause premature defoliation of leaves which results in reduced yields in subsequent years.
Control: Monitor for the appearance of the disease so fungicides can be applied before significant buildup of infection and spore production. All infected material should be removed from the field or incorporated into the soil.

Disease: Fusarium crown and root rot (Fusarium oxysporum f.sp. asparagi) and (Fusarium moniliforme)
Characteristics: Rotted and hollow root system (both feeder and storage roots), can be  accompanied by a red/ brown discoloration. Above ground parts can appear stunted and/or wilted. F. oxysporum can be found in most soils. F. moniliforme is capable of infecting both asparagus and corn.
Control: It is imperative that new asparagus planting be placed in fields that have not been used for asparagus or corn production within the last five years. Avoid damage to the asparagus roots during cultivation. Fungicides are generally ineffective against this disease; however soil fumigation prior to planting may reduce the population of Fusarium in the soil.

Disease: Botrytis blight (Botrytis cinerea)
Characteristics: Causes tan lesions with brownish borders, in more advanced cases fuzzy gray spores will be visible. Disease can completely kill newly emerging stems, and is most problematic when high moisture is present either during wet weather or when air is unable to move in the canopy.

Control: Botrytis is a common invader of wounded or weakened plant tissues and is also prevalent in storage. Reducing injuries to plants will reduce infection point. Remove and destroy infected tissues to reduce further spread. Trifloxystrobin registered for asparagus rust control will provide some control of Botrytis.

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Insects: Common asparagus beetle (Crioceris asparagi) and spotted asparagus beetle (Crioceris
duodecimpunctata)
Characteristics: The most significant damage caused by these two beetles is feeding damage on stems and leaves of asparagus. Defoliation of stems can occur in serious cases, which can cause impacts on the following years yield. Larvae of the spotted asparagus beetle feed within berries, which will decrease seed yield.

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