Asparagus is a cool-season vegetable that belongs to the family Liliaceae. Botanical name of asparagus is Asparagus officinalis. It is an herbaceous perennial plant. Tender shoots (spears) of asparagus plants are used as a leafy vegetable. Asparagus is believed to be originated in Europe, Africa and Asia. Young shoots of Asparagus are very rich in minerals like iron, phosphorus, potassium, copper, manganese, selenium, calcium, magnesium and zinc. It is also rich in vitamins and major vitamin present in asparagus are vitamin B6, vitamin A, vitamin C, vitamin E, vitamin K, thiamin, riboflavin, rutin, niacin, and folic acid. Asparagus shoots are low in calories, low in sodium, rich in protein and a rich source of dietary fiber. Asparagus shoots are also rich in an amino acid called asparagine. Dried asparagus roots are used as a medicine because of its diuretic properties. Productive life of a well-managed commercial asparagus plantation is about 10 to 15 years.
Cultivars of Asparagus
There are three types of cultivated asparagus: white asparagus; green asparagus and purple asparagus. Among these, green asparagus is largely consumed as a vegetable worldwide. Though white asparagus is less bitter than green asparagus, it is popular only in European countries like the Netherlands, France and Germany. Purple asparagus is commercially produced in Italy and it has high sugar and low fiber levels.
Soils for Asparagus Cultivation
Asparagus can be grown in many soil types, even in saline soils but deep loam or sandy soils with good surface water and air drainage are best. Good production is also possible in heavier soils. However, soil fertility may become a limiting factor for commercial cultivation of asparagus. Therefore a grower must ensure that soil is prepared well before planting the crowns in the main fields.
Soil pH
Asparagus plants grow well if the soil pH is within a range of 6.5 to 7.5. If soil pH is below 6.0 it should be raised to 6.5 by applying lime according to soil test report recommendations. Asparagus will thrive in soils having a salt content too high for many other crops, but it will not tolerate extreme acidity. Although asparagus will tolerate less than optimum soil conditions, yields are likely to be reduced and the life of the planting will be shortened in these soils.
Soil Drainage
In asparagus production, it is important that the plants develop an extensive storage root system. Therefore, good soil drainage is essential. Asparagus roots can develop to a depth of 10 ft in well-drained soils. It will do well in most soils if the water table does not come within three feet of the surface during the growing season.
Soil Preparation
Soil should be made fertile and free of troublesome weeds before the crowns are planted. Since it is more difficult to improve soil after the crowns are planted, soil improving practices must be started at least a year before planting. Asparagus thrives best in soils well supplied with organic matter. Applications of animal manure or turning under a green manure crop are desirable practices prior to planting asparagus. Green manure crops also improve soil structure and enhance soil fertility. Phosphorous, potassium, and lime amendments—based on a soil test—may also be incorporated prior to planting.
Propagation
Asparagus can be propagated both by seeds and by crowns. For small plantations and home gardens, it is advised to purchase one-year-old crowns from a reliable grower or nurseryman. For large commercial plantations, an asparagus producer can raise crowns by sowing seeds in nursery beds.
Raising Crowns from Seeds
Nursery bed preparation
In order to grow crowns for a commercial plantation, high quality seeds should be purchased from a reliable nursery and then planted in well prepared nursery beds that have never grown asparagus before. Sandy soils are recommended for nursery bed preparation so that crowns can be easily dug from the nursery beds. Based on soil testing report, NPK (Nitrogen, Phosphorous and Potassium) fertilizers should be incorporated into the soil prior to sowing the seeds.
Seed Sowing Time
Normally in tropics, seed are planted in early April
Spacing
Seeds are sown in rows which are kept 2 to 3 feet apart. A planting depth of one to two inches is needed.
Seed Rate
400-500 grams of seeds is required to produce crowns for planting one acre of asparagus.
Germination
Asparagus is slow to germinate requiring two to three weeks for the seedlings to emerge. One-year old seedlings are used for transplanting.
Handling Crowns
While lifting the asparagus crowns from the nursery rows, care should be taken not to injure the crowns during digging. Crowns should be planted in the main field as soon as possible after digging.
Setting the Crowns
Before planting the crowns in the main field, the individual crowns that are grown together are separated and prepared well by removing unhealthy leaves and roots.
Planting Asparagus Crowns in the Main Field
Field Preparation
Main fields are prepared by ploughing and levelling and by incorporating organic manures into the top soil. Since asparagus is a perennial crop and the plants occupy the soil for several years it is essential that the soil should be enriched by incorporating vast amounts of organic manures in the form of farm yard manure or compost or green manure crops into the top soil. After soil is set, furrows which are four to six inches deep are prepared for planting the crowns.
Planting Crowns
Crowns are placed in the bottom of furrows carefully and then the crowns are covered with two to three inches of soil. A light irrigation is recommended soon after planting.
Spacing
Rows are spaced five to six feet apart, and plants are spaced 12 to 15 inches apart in the row.
Plant Population per Unit Area
Recommended plant population is 6,000 to 9,000 crowns per acre for the given spacing in and between rows.
Planting Time
In the tropics, asparagus crowns are normally planted at the onset of winter or during the winter months. In such cases, first shoots start appearing during spring season.
Fertilizer Application
Fertilizer Application during the First Year
Based on the soil test analysis, required amounts of NPK fertilizers should be supplied to the soil as basal dressing before planting the crowns in the main field.
Fertilizer Application of Established Plants
Recommended NPK fertilizer dose for acreage of established asparagus plantation in tropics is an annual dose of 50-60 Kg of nitrogen (N), 25 Kg of phosphorous (P), and 50 Kg of potassium (K) in two applications, once in spring and again after harvesting.
Irrigation
Asparagus is a drought-resistant crop. Since asparagus has an extensive and deep root system, frequent irrigation is not necessary. Irrigation should be done as and when necessary.
Insect Pests
Major insect pests that are found affecting an asparagus plantation are, asparagus beetle, spotted asparagus beetle, and asparagus aphid.
Control Measures
- Field sanitation by burning all the trash and debris found at the growing site; these are the places where the beetles are likely to hibernate
- Application of biopesticides such as Rotenone or rotenone-pyrethrum mixtures
- By using natural predators of beetles such as chalcid wasps and lady beetle larvae
- By using insecticidal soap solution and rotenone-pyrethrum sprays
Diseases
Major fungal diseases that are found affecting an asparagus plantation are asparagus rust, needle blight, crown rot, root rot and wilting of plants. Major symptom of rust disease is defoliation of leaves and in needle blight disease, leaves get purple-brown spots and are slowly defoliated. Needle blight is caused by Cercospora asparagi. Wilting of the plants is caused by the fungal species Fusarium and asparagus rust is caused by Puccinia asparagi.
Control Measures
Asparagus rust can be controlled by growing rust-resistant varieties. Other fungal diseases may be controlled by using organic fungicidal solutions.
Weeds and Weed Control
Asparagus plantation is found affected by both annual and perennial weeds. Weeds can effectively be controlled by adopting a combination of cultural, mechanical, biological and chemical weed control techniques. In cultural weed control techniques, a combination of tillage, mulching, soil solarization and cover cropping may be adopted at least a year before planting crowns in the field to ensure a weed-free site for plantation.
Harvesting
Asparagus should not be harvested during the first two years of planting. In the year of setting crowns, the plants should be allowed to get established in the field and during the second year plants should be allowed to grow vigorously and develop a strong storage root system. From third year of planting onwards, tender shoots, which are also called ‘spears’ may be harvested. Harvest is done when there are three to four flushes spear emergence. In an established plantation, harvest season may be extended to eight to nine weeks. Approximately, two-thirds of the crop is harvested during the first half of the harvest season itself. Asparagus spears are harvested when they are about 9-10 inches in length. Spears are either handpicked by snapping them just above the ground or cut by a special knife just below the ground.
Shelf Life
Shelf life of freshly harvested asparagus spears is up to one week to ten days at room temperatures.
Economic Life of an Asparagus Plantation
A well-established asparagus plantation starts yielding sizeable returns after about three years. Under good cultural management conditions, a plantation may remain economical for about 12 years.
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