ASPARAGUS Asparagus officinalis

FAMILY CHARACTERISTICS
    The Liliaceae family, with approximately 240 genera and up to 3000 species, contains many important medicinal, edible, and ornamental plants. These monocots include many notable horticultural plants such as lilies, gladioli, hyacinths, tulips, hosta, liriope, ornamental asparagus ferns, and the garden asparagus. Most members of the Liliaceae family are herbaceous perennials which are propagated primarily by bulbs, corms, rhizomes, tubers, and seeds. The garden or common asparagus is the best known edible member of this family. Other species in this family include:
    Lilium spp.; Lily

CROP HISTORY AND DEVELOPMENT
    The origin of the modern asparagus is uncertain because many wild types can be found throughout Europe and Asia. However, the most likely location is around the Mediterranean Sea where cultivation has been practiced for over 2000 years, first by the Greeks and then by the Romans (around 200 B.C.). Asparagus' natural habitat is along the banks of rivers and near salt marshes which are common in this part of the world. Romans used asparagus extensively for food and medicinal purposes. They also developed the first cultural practices for this perennial crop.
    In North America, asparagus was grown by the early settlers from Europe. Commercial production began only in the mid to late 1800's. Some of the original varieties of asparagus were 'Common Green', 'Giant Dutch Purple', 'White German', 'Early Giant Argenteuil', and 'Late Giant Argenteuil'. After the turn of the century, two groups of asparagus varieties evolved based on spear color. The more commercially important of these have a dark green color upon exposure to sunlight and include 'Reading Giant', 'Palmetto', 'Argenteuil', and 'Martha Washington'. The other group includes those producing light green spears such as 'Conover's Colossal' and 'Mammoth White'. The 'Martha Washington' and 'Mary Washington' varieties were developed in the early 1900s to overcome the asparagus rust disease problem which threatened the asparagus industry. 'Mary Washington' is not as resistant to rust as the 'Martha Washington' variety but is earlier and more vigorous. Currently improved strains of 'Mary Washington' are available with increased rust resistance as well as F-hybrid cultivars and strains that are resistant to Fusarium wilt. These new hybrids could strengthen the asparagus industry since they have increased yields in conjunction with greater potential for non selective mechanized harvesting.

PLANT CHARACTERISTICS
    Overview. Asparagus (commonly referred to as garden asparagus [Asparagus officinalis var. altilis] in the literature) is a perennial dioecious monocot, grown for its herbaceous, newly emerged shoot (commonly referred to as a spear). The perennial component of the asparagus plant is a woody crown root with fleshy roots which give rise to fibrous roots below ground and shoots above ground. The shoots emerge from the ground as spears (actually they are aerial stems) and then develop into fronds (also called ferns) which are modified leaves. Being dioecious (having male and female flowers that appear on separate plants), each plant bears either conspicuous male flowers or less conspicuous female flowers which produce three-celled berries. Prior to flowering, however, there are no observable differences between male and female plants.
 

Root System. The crown consists of rhizomes (underground stems), fleshy roots, and fibrous roots. The rhizomes are somewhat woody with distinct nodes (with buds) and internodes. The buds give rise to the spears. The rhizomes are somewhat thickened and are sites of nutrient and starch accumulation. Fleshy roots arise adventitiously from the rhizome and serve as absorptive organs for nutrients and water and as storage organs for the photosynthate translocated from the shoot during the summer growth period. Fibrous roots originate each spring from the fleshy roots and assist in the absorption of nutrients and water. The rhizomes and fleshy roots are the primary sites of stored food reserves utilized in the growth of the spears for spring harvest. Asparagus is normally considered a deep rooted crop which can penetrate to a depth of 15 feet or more to mine nutrients and water.

Roots of asparagus

Asparagus Spear-Frond

Shoot (Spear-Frond). Buds produced on the crown during the summer growth period give rise to the spears the following spring. The spears are aerial stems which are smooth, with a prominent modified leaf. Spears not harvested expand into strong, erect, extensively branched stems called fronds. Because of their appearance, they are sometimes referred to as 'ferns'. Fronds have the shape and appearance of branches with leaves (cladophyll) with three to eight needle-shaped leaflets (cladodes) occurring in compact clusters, each approximately one inch long, tapering outward and circular in cross section. The average height of a mature asparagus plant is six to eight feet.

Flower. Male (staminate) and female (pistillate) flowers arise on separate plants. The yellowish-green male flowers are born in groups of one to four in the axils of branches and are bell-shaped. The less conspicuous female flowers produce three-celled berries which are approximately 1/4 inch in diameter, initially green in color and becoming red at maturity, and containing approximately three to nine seeds. The flowers are fertilized with pollen from nearby male plants. Bees are the primary pollinators.

Male flowers

Female plant

    Seed. Black in color, approximately 1/8 inch in diameter, rounded with a flattened side at the point of attachment to the seed cavity.

PROPAGATION METHODS
    Overview.  Methods of propagation include nursery grown crowns, nursery grown transplants (approximately 12 weeks old), transplants grown from tissue cultural methods, and direct field seeding. Additionally, the growth habit of asparagus plants varies by the sex of the plant. Female plants produce larger spears than male plants, but the male plants produce larger total yields. In field plantings the ratio of female to male plants is about 50/50. It is not economical to select for gender, however, because it takes two years for sex expression to occur. The use of tissue culture to propagate plants from vigorous stock plants of the desired sex offers tremendous potential to the asparagus industry. Labor costs associated with both planting and harvesting nursery grown crowns has resulted in a shift to the use of hybrid seed and transplants (at the seedling stage) which allows mechanization of both planting and harvesting.

Nursery grown crowns. Traditionally asparagus planting has been established with one-year-old nursery grown crowns. With typical nursery production practices, when crowns are left to produce two- and three-year- old crowns, they become crowded in the bed. Extensive damage to these crowns often occurs during digging. Thus two- and three-year-old crowns may be slower to become established and frequently have lower yields in comparison to one-year-old crowns. However, if plants are widely spaced in crown production and care is exercised in digging the crowns, two- and three- year-old crowns can be used successfully. In general, however, one-year old crowns are recommended for establishing asparagus plantings. For small acreage, crowns are generally purchased through mail order companies. Producers establishing large acreage of asparagus generally grow their own crowns for planting. Advantages in producing one's own plants are the ability to have the desired cultivar in the quantity wanted, the ability to select seed from vigorous plants (when produce own seed), the ability to select only the largest and healthiest crowns from the seedbed (i.e. uniformity and quality control), and the ability to transplant immediately after digging, thus preventing injury from excessive drying and loss of food reserves due to improper storage. If storage of crowns is necessary, dry conditions and temperatures near 40°F are recommended. Nursery grown crowns for resale are dug in the fall after growth has ceased and are stored until sold.    The site chosen for establishment of a nursery bed should not have been previously planted in asparagus due to the potential of residual disease (primarily Fusarium root rot and asparagus rust). Depending on the row spacing, between 8,000 and 10,000 crowns are required to plant one acre. Approximately two pounds of seed are required to produce this many crowns. An acre of nursery will produce enough crowns to plant four to six acres, again depending on spacing. Seedbed row spacing commonly used are 24-30 inches apart with 10-12 seed/ft planted in the row. Seed are normally planted one to two inches deep in loamy or heavier soils and three inches deep in sandy soils. Germination is slow when soil temperature is below 70°F or when seed is planted in cold soil. Under normal conditions plant emergence takes four to six weeks.

Planting stage  Asparagus crown Asparagus crown production

    Field planting of crowns requires a furrow that will allow them to be placed six to ten inches deep (twelve inches maximum) in loose friable soils. A slightly shallower planting of three to five inches may be employed in regions with heavy soils due to the difficulty of spear emergence. Most plantings employ a spacing of one foot in the row and five feet between the rows for the production of green asparagus. White or blanched asparagus requires a wider (eight feet) row spacing to allow for sufficient soil on the ridge between rows to cover the spear to the desired depth for blanching. Traditionally, asparagus crowns were planted into 8-10 inch deep V-shaped furrows with the furrows being filled in as the young plant grew. This allows for proper coverage of the growing frond and permits proper development of spears in subsequent years.

Transplants. The use of nursery grown seedlings as a means of establishing asparagus plantings is rapidly being accepted as an alternative propagation method due to the high cost of producing, harvesting, and replanting asparagus crowns in the field. Nine- to 12 week- old seedlings grown in seedbeds or bedding plant trays can be transplanted to the field. With proper care, survival rate of seedlings is approximately 95% if transplanted in the early spring (before high temperatures and moisture stress becomes a factor) or in the fall in areas where winter stress reduces seedling survival rate. The use of V-shaped furrows (as used for crowns) may not be ideal in all regions for transplants due to the potential for excessive moisture to collect in the bottom of the trench from rainfall or irrigation required for plant establishment. Thus transplants should be placed on the side of the trench, three to four inches up from the bottom to prevent this potential moisture problem. The use of a W-shaped furrow may also be used with the transplant being placed in the raised center (two inches high) to protect the plant from 'wet feet.'

Asparagus transplants

    Asparagus seed has high genetic variability with resulting seedlings varying greatly in vigor and

ultimately in productivity. However, the new technology of tissue culture has increased the advantages of using transplants compared to crowns or seeds. This technique utilizes mother plants which are chosen because of their high vigor, yield, and other desired characteristics such as high disease resistance. From the desired mother plant, lateral buds or stem tip tissue is grown aseptically using tissue culture techniques to produce stock plants. Buds are taken from the stock plant and grown into plantlets which are then allowed to develop vigorous shoots and roots before transplanting into a greenhouse soilless mix. These plants are then grown for three to four months under greenhouse conditions before being transferred to the field either in the spring or early fall. This method allows the rapid propagation of numerous genetically identical asparagus plants with high vigor and yield. The resulting plants can be planted directly in the field for spear production or they can be used as parent plants for the production of high quality hybrid seed. Many of the resulting new hybrid transplants and seed have the advantage of setting concentrated buds which produce four to five spears at one time. This offers the potential of making mechanical harvesting possible and cost effective.

Asparagus seedling

Direct seeding.  With the development of hybrid seed (described above), direct seeding of asparagus fields is rapidly replacing the use of crowns and transplants as the primary method of establishing asparagus fields in many regions. Hybrid offspring produce higher yields and show less variability in growth which makes them better adapted to non selective machine harvest. Direct seeding into the furrow at a depth of  one to two inches with two to three inches between seeds, followed by thinning to proper plant spacing is required for an acceptable stand. Seed should not be planted in cold soils. Soil temperature should not be below 70°F; 75-85°F has given maximum germination. An ounce of seed contains approximately 700-750 seeds, and two to three pounds of seed are required for direct seeding one acre.     Propagation summary.  The disadvantages of using seedlings or direct seeding as opposed to use of one-year-old nursery crowns include (1) establishment is poor on land with excessive slopes and (2) irrigation, weed, disease, and insect control is critical. Advantages include (1) mechanization of planting lowers initial establishment cost; (2) time to first harvest is decreased by one year l in many

Direct seeded asparagus

regions (including time to grow crowns); (3) better control in preventing the introduction of disease problems into a new field is achieved; and (4) use of hybrid seed and transplants allows for more uniform plantings and allows for possible mechanization of the harvesting procedure with non-selective harvesters.

CULTURAL PRACTICES
    Overview. Asparagus requires a loose friable soil, moderate temperature and a dormant period, moderate fertility, weed control, irrigation, and insect and disease management for maximum spear production. Special consideration must be given to each of these factors during establishment as well as during the production years since asparagus may occupy the site for 10 years or more. It must be remembered that the health of asparagus plants during one season determines yield (and profit) for the next season. The most frequent cause of reduced yields is neglect of fields after harvest.

Some well managed commercial asparagus plantings over 50 years old are still in operation, however, this practice is seldom economical. As plantings become older, spear size decreases gradually and disease populations build up over time. In addition, the increased vigor (yield and disease resistance) of new cultivars makes replacement of older plantings more attractive. Asparagus plantings are more commonly replaced after 10-20 years. Other crops should be grown on the land during a two- to four-year interval between asparagus plantings.    Soil type. Due to the deep penetrating nature of the asparagus root, sandy loams are preferred with muck soils acceptable. Light textured sandy soils tend to produce spears earlier than heavy clay soils. Heavy soils can be used if crusting of the soils is prevented. Sandy soils that are extremely porous should not be utilized because of poor water holding capacity. Asparagus does not grow as well in acid soils (low pH). Slightly acid to neutral soils are preferred (pH 6.3-6.8).

Deformed spears due  to compacted soil

    Temperature. Traditionally asparagus is a cool season crop with maximum spear production occurring when the average daily temperature is between 65 and 80°F. A rest period is necessary for maximum spear production the following season. This rest period may be induced by low temperatures in cool climates or by withholding irrigation in warm dry climates. Spear production is slow when the average daily temperature is 50°F or below; however, spears tend to branch quickly upon emergence when the temperature is 100°F or higher. When temperatures are between 75- 80°F, a spear approximately six inches in length is produced in two days. In regions such as the Southeastern states where prolonged growth periods are promoted (long summers and mild winters with moderate rainfall), yield is decreased due to a reduction in stored photosynthate and lack of a required dormancy.
    Fertility. Fertilizer application varies widely among regions and depends upon many factors such as soil and climate conditions. Soils high in organic matter are not fertilized extensively, while mineral soils receive up to 2,000 pounds/acre of a complete fertilizer, with an N-P-K ratio of 1-2-1 or 1-2-2 recommended. Preplant fertilizer application should be made based on soil analysis. Subsequent fertilizer applications should be based on tissue analysis. On young asparagus fronds, tissue samples should be taken from the top four to six inches of at least ten fronds. With mature fronds, the samples should be taken from 18-36 inches above the ground. Again, ten fronds should be taken from the planting. A range of selected essential nutrients for use as a guide in evaluating the nutritional status of asparagus tissue for proper growth follows:
 

    Generally fertilizer applications are split with half applied in early spring before extensive cutting of the spears and again during frond growth and development. Asparagus is deep-rooted (up to 20 feet or more in older plantings) and responds slowly to fertilizer applications under normal conditions. Asparagus is sensitive to calcium deficiencies and special care must be practiced initially in the establishment of an asparagus planting to incorporate appropriate amounts of calcium before planting (amount determined by appropriate soil test). In acid soils, generally two to four tons of dolomitic limestone per acre are required. In neutral to basic soils, lime or gypsum may be used as a calcium source (if needed), but care must be taken to maintain the pH approximately within the pH 6-7 range.
    Weed control.  Improper or inadequate control of weeds is one of the primary factors reducing growth and development (and thus yield) of asparagus. Weeds are a major concern in asparagus production since deep plowing or cultivation is not possible and because asparagus does not produce a dense canopy to shade out weeds until very late in the summer. Perennial or persistent weeds should be controlled prior to planting asparagus. In established commercial plantings, shallow cultivation and chemical herbicides are the primary weed control methods. However, herbicides are generally preferred to cultivation, since cultivation can damage spears and the root system, particularly during spear development and emergence. Asparagus plantings established with transplants or by direct seeding methods are especially sensitive to weed competition during early seedling development. This makes control of weeds during this period critical in order to obtain a proper stand. Selection and use of herbicides differs with soil type, climate, and weed pests.
    Irrigation.  Asparagus, being deep rooted, absorbs water deep in the soil profile and is not irrigated extensively in most regions of the United States where moderate rainfall occurs throughout the year. In arid or semiarid regions such as the Western and Southwestern United States, up to 20 inches or more of water is applied during the vegetative or frond growth period. Also, in warm arid climates, the practice of withholding irrigation to fields is sometimes used to induce the required dormancy period after the summer growth period.

Asparagus spear

Field of asparagus spears

Early shoot growth

Young field

Maturing field

Mature field

INSECTS
    Asparagus beetles.  The asparagus beetle (Crioceris asparagi L.) and the spotted asparagus beetle (Crioceris duodecimpunctata L.) are the primary damaging insects. Adult asparagus beetles over winter in old asparagus stalks or other similar debris and emerge during the spring and early summer to feed on the young spears and fronds. Eggs are laid on the young spears and fronds as well as on the mature plants (climate permitting), hatching in seven days into larvae which feed upon the foliage for two weeks and then enter the soil to pupate for two weeks, after which the new adults emerge. Two to three generations develop during one season depending upon the climate. The life cycle of the spotted asparagus beetle is similar to that of the asparagus beetle with emergence timed to fruit development. Control of both asparagus beetles is obtained by (1) removal of top growth each year after the summer growth period to prevent over wintering and, (2) the use of insecticides. More on asparagus beetles

    Other insects. Spotted cucumber beetle, cutworms, thrips, garden centipede, bulb mite and spider mite, aphids, imported cabbage worm and armyworm also attach to asparagus plants. Chemical control of these insects is normally employed.

DISEASES
    Several diseases cause severe losses for asparagus growers. The most important are asparagus rust and diseases caused by Fusarium species. Producers can reduce much of this loss by selecting resistant cultivars, selecting a site not previously used for asparagus production, using proper cultural practices to keep plants vigorous and healthy, and employing preventative measures (including sanitation) to guard against introduction and spread of disease organisms.
    Asparagus rust (Puccinia asparagi). Asparagus rust is a fungal disease which develops under moist conditions. It appears as small reddish-yellow spots on the main stems of the plant. Since the spores require free water to germinate, the severity of the disease varies from year to year depending upon climatic conditions. The reddish spots are pustules which contain spores that appear as rust colored powder. The spores are disseminated by both wind and rain. As the disease progresses, the needle-like cladodes drop from the plant leaving only primary and secondary branches. This injury reduces photosynthate (food reserves) available for further spear and frond production. Severely infested fields turn brown and appear dead. Initial infection symptoms generally appear on the fronds with no indication apparent on the harvested spears. However, because of a reduction in stored photosynthate (food reserves), spears in the subsequent year are fewer in number and are smaller and spindly. Control is normally obtained by planting rust resistant varieties such as 'Martha Washington' and 'Mary Washington', and the newer 'Eden' and 'Jersey Centennial' cultivars. However, high rust resistance has not been maintained in all stock plants or strains developed. Removal of wild asparagus plants and unused asparagus beds will reduce the severity of this disease. Cultural practices which encourage air movement and thus drying of the fronds will also slow infection.
    Fusarium root rot and decline (Fusatium oxysporum and F. moniliforme). Fusarium species are fungi which cause a complex array of asparagus disorders which include wilts, root rots, and decline. The soil borne species attack feeder roots and then enter the fleshy roots. Crowns infected with these organisms will often have reddish-brown lesions on fleshy storage roots and the interior of the crown is frequently discolored. In the growing plant, shoots wilt in hot weather, and often turn yellow, and the feeder roots become limp and hollow. Over time the disease weakens the plant and eventually kills it. Spears of severely infected plants exhibit a brown discoloration which makes them unmarketable. Control is difficult due to the persistence of Fusarium oxysporum which builds up in the soil with time. Avoidance of land previously planted in asparagus and use of Fusarium-resistant strains are the primary methods of control. Because seed can transmit this disease, they must be treated with fungicides prior to planting. Also, only disease free crowns should be planted. Resistance to Fusarium is improved when asparagus plants are maintained near optimum growing conditions. Environmental and cultural stresses (e.g. drought, flooding, over- and underfertllization, insect damage, over harvesting) increase the susceptibility of asparagus to Fusarium damage.
    Other diseases. Asparagus has numerous leaf and stem diseases and root rots that are of local concern. Cercospora leaf spot is of concern in tropical and subtropical climates where warm temperatures already make asparagus production marginal. Other leaf and stem disorders include Phoma asparagi Saccardo which produces elongated gray stem lesions surrounded by a reddish brown border and Colletotfichum sp. which produces pale colored cankers covered with black pustules. Other root rots include Phytophthora megasperma, Zopfia rhizophila, and Penicillium martensii. P. martensii is evident on crowns by extensive blue-green color of fruiting bodies at the time they are dug and removed from storage.

HARVESTING
    Harvesting traditionally begins after two full years of growth. Harvesting is normally limited to two to six weeks during the first cutting season (the third year after planting). The length of harvest is dependent on the vigor of the plants which in turn is controlled by the growing conditions, particularly climate and fertility. Plants grown in colder climates with shorter growing seasons are typically harvested for two to four weeks while in warmer climates the initial harvest period may last up to six weeks. The duration of harvest may be gradually increased up to 14 weeks in subsequent years. In cooler regions with short growing seasons the harvest time may be limited to 8-10 weeks. Harvesting is normally done every one to two days; however, the warmer the temperature the more frequent harvesting must be done. The duration of harvest must be monitored with care. Most spears are harvested within the first four weeks. Thereafter the number and size of the spears produced are reduced. Size and condition of the spears is a better indicator of when to cease harvesting than is a predetermined number of weeks. Once spears become smaller than the diameter of a pencil harvesting is usually terminated. Over harvesting is frequently a reason for poor yields the following year because it drains the plant of food reserves and it is unable to produce adequate fern growth to accumulate photosynthate. It also generally weakens the plant and makes it more susceptible to attack by diseases.
    Individual spears should be straight and buds compact. 'Feathered' spears (ones with expanding buds) are overmature and are of little market value, but they must be harvested (particularly in the early part of the harvest season) to ensure continued spear production. Crooked spears are an indication of insect or mechanical damage such as being cut with a harvesting knife. While marketable, their value is reduced.

Spears may be hand harvested with a specially designed knife or snapped at ground level by hand. For fresh market it is advantageous to harvest the spear with a knife below the ground level (which normally means with white showing on the cut end) as this part of the spear is more woody than the succulent green top and reduces moisture loss, and results in retention of higher marketing quality. Cutting the spears also increases tonnage per acre and reduces the sites for infection by rust spores. Snapping the spear at ground level reduces labor cost and is normally used where immediate processing or consumption of the spear is intended. Snapping is commonly used in pick-your-own operations since all of the spear is tender and edible (keeping the customer happy) and there is less chance of damaging emerging spears and roots by inexperienced harvesters.

    Mechanical harvesting, using cutting bars, is economical primarily for processing asparagus since it is non-selective. The resulting spears can be graded to produce various types of processed products, e.g. whole spears, cut pieces, and chopped asparagus.

POST HARVEST

Asparagus deteriorates very quickly after harvesting, particularly during the first 24-48 hours if not properly handled. The two main factors contributing to this deterioration are high temperature and water loss by spears. At high temperature asparagus becomes fibrous (tough) very quickly. Fiber development begins at the base of the spear and progresses upward to the tip. When a spear is bent, it will snap (break) at the point where the tender and fibrous parts meet. Besides becoming tough, asparagus loses flavor, vitamin and sugar content very quickly at warm temperatures. Spears should be cooled as quickly as possible and then stored in high relative humidity (95%) at approximately 36°F. If hydrocooling is used to remove field heat, then the asparagus should be dried before placing in storage. In addition to temperature, ethylene will promote spear deterioration. Controlled atmospheric conditions (cool, humid conditions with ventilation to remove ethylene, reduced oxygen and increased carbon dioxide concentrations) can be used to extend the shelf life of asparagus by reducing respiration, thus slowing enzymatic reactions that cause fiber development and other decreasing effects. Green asparagus may be stored for approximately three days without controlled atmospheric (CA) conditions, but blanched spears can only be held for about one day. With CA, green asparagus can be held for approximately two weeks; however, quality continues to decline even under these conditions. In retail markets, asparagus is usually displayed with the stem end in water and/or packed in crushed ice. The primary advantage of this procedure is to keep the spear turgid.

Bundled asparagus Boxed asparagus