Agronomy

Description

Asparagus plants trace back to areas in Russia, the Mediterranean and the British Isles. Part of the lily family, asparagus is a perennial that the early Romans first cultivated as a source of food and medicine. Today, American consumers increasingly demand asparagus, so offering a local supply of the early-spring vegetable may have potential. Asparagus serves as a good source of vitamin C, vitamin A and potassium.

Uniquely, asparagus is a monocot vegetable. It has male and female plants, which make asparagus dioecious. The plants grow from crowns, which are a network of rhizomes and lateral roots. From the crowns, spears emerge in the spring. Unharvested spears eventually mature into ferns. The ferns have a purpose to synthesize and store energy for the plant, so the plant can produce a crop during the next year.

Site Selection

When growing asparagus, site selection is particularly important because the perennial plants can grow a marketable product for 10 years or longer if the site conditions are conducive to the crop’s needs. The selected site shouldn’t have grown onions, leeks, chives or garlic in recent rotations. These crops can harbor diseases that are harmful to asparagus plantings. Also, avoid sites that have previously grown asparagus.

Asparagus production sites should have deep, well-drained soil. Light to medium-textured loams work well. Sandy soils may also be an option. Regardless of the soil type, the water table shouldn’t be within 3 feet of the soil surface. Roots of an asparagus plant can grow deep, and the plants don’t do well in soggy soil. To achieve good drainage, producers may construct raised beds for planting asparagus. For a raised bed, possible dimensions are 4 inches to 8 inches high and 3 feet to 4 feet wide.

Soil pH is another important factor for the planting site. Avoid extremely acidic soils. Ideally, a site will have soil pH levels that range from 6.5 to 7.0. Also, compacted soils don’t work well for asparagus production.

Desirable sites lack rocks and low spots, which could increase the likelihood that asparagus spears experience frost damage. Asparagus plants don’t compete well with weeds, so sites prone to having perennial weed pressure aren’t recommended. Sites should also offer full sun. If a site is susceptible to wind erosion during spring harvest time, then windbreaks can reduce damage to asparagus spears. Early asparagus emergence may be more likely on south-facing slopes than north-facing slopes.

Fertility

After selecting a site, field preparation begins a year before planting. Site preparation includes ensuring that the site’s pH and nutrient levels meet asparagus needs. If a site doesn’t meet these needs, then producers must make adjustments accordingly.

Pre-plant site preparation fertilizer requirements will vary according to soil test results. Apply phosphorus and potassium as a broadcast application as indicated by the soil test report, and incorporate into the soil. Asparagus producers typically apply 75 pounds per acre of nitrogen as a broadcast application at pre-planting and then incorporate the fertilizer. Once fertilizers are incorporated into the soil, form the furrows or beds.

Nitrogen is applied in the early spring in the second and third years after planting. Apply the needed nitrogen – plus, phosphorus and potassium if needed – before spears develop, and lightly incorporate the fertilizer with a disk. Following harvest, producers often apply nitrogen in established asparagus fields. On average, assume that 50 pounds per acre of nitrogen will be required post-harvest. The phosphate requirement for established asparagus fields may range from none to 75 pounds per acre. Established plantings may require as much as 100 pounds of potash per acre. Results from soil tests scheduled every five years will inform whether a field will require added lime, phosphate or potash.

Producers can often time fertilizer applications in established asparagus fields for post-harvest. The asparagus plant’s fleshy root system is good at storing nutrients that the plant can use early in the growing season. On sandy, coarse soils, however, plants may require an early-season nitrogen application.

Micronutrients of particular importance to asparagus crops are boron and sulfur. To resolve boron deficiencies, use a soil or foliar spray. Soils that are sandy or lack organic matter may require a sulfur application.

Variety Selection

A dioecious crop, asparagus have male and female plants. Some asparagus varieties include male and female plants, whereas many recently developed varieties are exclusively male plants. Female asparagus plants tend to produce larger spears, and unlike male plants, they produce berries, which drop and develop into asparagus seedlings. These seedlings can become a weed in established asparagus plantings. Producing the seeds also consumes plant energy.

Male plants tend to yield spears with smaller diameters, but they generally produce more spears. Cultivars that only produce male asparagus plants tend to have more productivity and uniformity. Other characteristics of all-male varieties include longer plant life, earlier emergence and less lodging.

Other varietal characteristics to consider are spear color, bract color, spear thickness, spear length and maturity date. Growers should identify varieties resistant to disease and insect pressures and that appeal to buyers. The table below lists asparagus varieties that often perform well in Missouri.

Common Missouri Asparagus Varieties

Planting

Most often, growers establish new asparagus plantings by placing transplants or crowns in furrows supplemented with compost or composted manure. A direct-seed asparagus field would experience too many weed problems to do well.

If an operation chooses to use transplants, then it could start seeds in a greenhouse during the fall before field planting. At planting, the transplants would be 3 to 4 months old. Another option would be to choose transplants that are 10 weeks old to 14 weeks old. As an alternative to transplanting in spring, growers may consider planting seedlings in the fall. Containers that have 2-inch long sides and 3-inch depths are suitable for starting asparagus plants. The greenhouse temperature should range from 60°F to 80°F. Seeds germinate best if the temperature is 75°F. Seedlings tend to emerge more quickly if seeds are soaked prior to planting. A limitation of transplanting is that seedlings may struggle if heavy rainfall occurs post-planting.

If an operation chooses to grow its own crowns, then select a sandy site that hasn’t previously grown asparagus. Sandy soil will enable producers to more easily dig the crowns. Amend the site with phosphorus and potassium as recommended in a soil test report, and incorporate the fertilizer well. Before planting, apply 30 pounds per acre of nitrogen. Later, a top-dress nitrogen application of 50 pounds per acre will be necessary. At planting, space rows at 2.5- to 3-foot intervals, and within a row, drop eight or nine seeds per foot. Planting depth should be roughly 1 inch to 2 inches. After planting, seedlings emerge in two weeks to three weeks.

For operations that raise their own crowns, crowns are removed from the nursery after one year’s growth and planted in production fields. If an operation must dig crowns before replanting, then store the crowns at 40°F. Avoid stacking them in large piles, which can lead to overheating. Additionally, freezing temperatures can harm crowns, and the crowns shouldn’t dry too much. Producers may alternatively purchase crowns from a supplier. If so, then choose disease-free crowns. At planting, the furrow should measure roughly 6 inches deep. When placing the crowns in the furrow, spread the roots, and position growth buds upward. Initially, add enough soil so that crowns have 2 inches of soil covering them. Then, add more soil gradually. Slowly filling the furrow with soil may help to manage weeds. In a single row, plant similarly sized crowns. If small and large crowns are placed in the same row, then the large ones may outcompete the small ones.

Planting dates depend on region. Producers in southern Missouri may plant in late March or early April. For central and northern Missouri, time planting for early April or mid-April. In terms of spacing, maintain a foot between plants in a row, and rows may be 5 feet to 6 feet apart. To encourage moisture to dry quickly on foliage and limit disease incidence, consider forming rows to run from north to south.

Water Management

As an asparagus field is establishing itself — particularly during the first two years — adequate soil moisture is critical. Young crowns require water to develop a strong root system and produce healthy ferns. Adequate moisture also ensures that plants are strong and can more readily compete with insects, diseases and weeds.

Because asparagus plants have deep root systems, mature plants tend to tolerate dry conditions better than other crops with shallow roots. However, mature plantings may require supplemental moisture as they produce ferns. The fern supplies food and energy to a plant’s crown and roots. Fern health in one year directly affects asparagus yield during the next year. Water needs will vary by climate, but as a general rule, assume that the daily requirement for asparagus plants producing ferns is 0.1 inch to 0.2 inches of soil water. Avoid applying too much moisture as it may cause nitrogen leaching. In the rooting zone, asparagus plants tend to absorb water most from the 6 inches to 24 inches nearest to the soil surface, so moisture availability in this zone would be important.

To prevent foliage disease, avoid irrigating in light, frequent applications. Trickle irrigation systems are good options as they expose the asparagus plants to little moisture and minimize evaporation losses. If irrigating asparagus, then stop during fall months to encourage asparagus plantings to move into dormancy.

Cultural Management

Asparagus growers in cool climates may choose to mulch asparagus during the winter to protect the crowns. If following this practice, then remove the mulch in early April of the following spring by raking it to the middle of rows. Removing the mulch will make way for the emergence of asparagus spears.

Asparagus plants produce fern-like growth after harvest. Ideally, producers will leave ferns in place until the New Year. During the winter, the fern residue can catch snow, which would serve as a form of insulation from cold temperatures. The fern should be completely brown when removed. Removing the residue at the appropriate time helps to curtail disease risk and reduce cover for pests. To manage fern residues, operators may burn fields, or a brush-hog mower can cut the residues near the soil surface. If operations cut residues, then a shallow disking can help with incorporating the fern residues into the soil. Shallow disking is important; otherwise, the equipment may damage crowns or developing spears.

To produce white asparagus, producers cover asparagus rows with an 8- to 10-inch soil layer before spears emerge. Using straw or black plastic row covers are other options. The goal is to block the sun from reaching spears. Doing so prevents chlorophyll production. White asparagus, which has a unique flavor and texture, often captures a price premium.

Weed Control

Asparagus, particularly a young stand, doesn’t compete well with weeds. During the establishment period, producers should aggressively focus on weed control. The goal is to have eradicated perennial weeds before planting. In the year before planting asparagus, growing cover crops, such as buckwheat, rye or wheat, is also a strategy to reduce weed pressure. Herbicides are another weed management tool to consider.

Cover crops can serve as a weed management strategy after planting, too. By planting rye or wheat between rows in the spring, growers can control weeds. To prevent weed pressure in asparagus rows after planting, producers may apply 4 inches to 6 inches of grass clippings, wood chips, straw, hay or compost to serve as an organic mulch. Herbicide control is another option. For example, an early-spring glyphosate application that occurs before spears grow can kill weeds, and another application can follow the harvest. Other herbicides are available for application during other points of the growing season.

Although mechanical cultivation may help to control weeds as asparagus establishes itself, cultivation in an established asparagus field could cause damage to crowns and roots. Other possible side effects of cultivation include introducing soil-borne diseases to crowns and roots and exacerbating volunteer asparagus seedling growth. If a producer chooses to use tillage to address weed pressure, then shallow tillage is key, particularly because crowns can move toward the soil surface over time.

Insects and Diseases

Insects that may target asparagus crops include asparagus beetles, aphids, cutworms, Japanese beetles, tarnish bugs, grasshoppers and leaf miners. Asparagus beetles can present a particular problem. They can cause damage by chewing asparagus spear tips, which later scar; laying eggs that can’t easily be removed from spears; and staining plants with a dark substance.

Aphids cause damage to asparagus ferns. Their feeding on ferns may lead to fern stunting or deformities. The sticky honeydew substance that aphids produce can entice ants or trigger fungal growth. Cutworms may damage spears and ferns. Often, spears with full or partial cuts near the soil surface have had cutworm pressure. Grasshopper damage can ultimately result in defoliation.

Depending on the pest and the acreage, control options include removing the pests by hand and applying a botanical or synthetic insecticide. Beneficial insects may target some pests. For example, lady beetles and parasitic wasps can help manage aphids.

From a disease perspective, Fusarium pathogens can lead to root and crown rot and trigger a loss of asparagus stand productivity. Symptoms that indicate infection include stunted growth, yellowing stalks and wilt. Minimize crown rot risk by avoiding overharvesting, choosing a site with the appropriate pH and drainage and controlling pest pressure. Fusarium wilt is another pathogen-related risk. Asparagus rust and Cercospora leaf spot can target the plant’s foliage and ultimately cause a decline in yields. Stemphylium purple spot, characterized by elliptical purple spots that are sunken, can lead to asparagus being unmarketable.

Harvest and Storage

Asparagus plantings require time to reach full maturity. Year after year, operations can increase the intensity of harvest until the plants have reached maturity. Growers should avoid overharvesting. Removing too many spears or harvesting over too long a period may weaken asparagus crowns and increase disease risk. In the second year after planting asparagus crowns, producers can harvest spears during a short two-week period. During the following year, the harvest time can increase to five weeks to eight weeks. Ultimately, Missouri commercial asparagus growers can anticipate average yields of 2,000 to 2,500 pounds per acre. Typically, asparagus harvest in Missouri ranges from April 10 to May 25 in the Bootheel, April 14 to May 30 in southern Missouri and April 20 to June 5 in northern Missouri.

When air temperatures are below 70°F, asparagus spears are ready to harvest when they reach a 7- to 9-inch height. Large spears are those with a diameter that measures more than 3/8 inch. In cases when spear diameter is small, consider closing the harvest. At harvest, spears should have a tight head and not appear like a fern. Warm weather causes the fern-like growth when spears are shorter, so as temperatures increase, asparagus growers may need to harvest asparagus spears at shorter heights. As a guide, when temperatures are warmer than 70°F, harvest spears when they measure 5 to 7 inches tall. Harvest frequency may also increase during warm weather — as frequently as every day compared with every two days or three days when the weather is cooler. When harvesting, also remove any damaged spears.

To harvest spears, operations rely on hand labor. They often experience best results when workers snap spears cleanly from the plant. An alternative is cutting spears with a knife. Workers may make cuts above or below the ground, but cutting can introduce disease to plants. Cutting spears below the soil surface may also cause harm to spears that haven’t emerged or the crowns. A benefit to cutting spears is that the asparagus tends to maintain a woody base on each spear, so the spears may lose less water post-harvest.

Harvest asparagus in the morning. The spears are cooler, there is less field heat to remove, spears are easier to snap, and spears tend to weigh more.

Post-harvest, operations use hydro-cooling to remove field heat and keep asparagus at a good quality. Hydrocooling should reduce the temperature to 40°F. Producers typically package asparagus spears in bunches that weigh as much as 2 pounds or 2.5 pounds. A smaller bunch may weigh 1 pound. Typically, rubber bands are used to pack asparagus spears in bunches. When packing asparagus, the spears should stand upright. Place a damp pad at the base of the spears, or use a shallow tray holding water. For as long as two weeks, operators can store asparagus at 32°F to 36°F. Relative humidity levels should range from 95 percent to 100 percent.

After harvest, workers should handle asparagus gently as the spears can bruise. Also prevent direct contact between asparagus spears and ice as the ice can cause chilling damage. Ethylene gas can cause asparagus spears to become tough, so don’t store asparagus in an environment where ethylene is present. Maintaining a 7 percent carbon dioxide level in the storage area can deter decay and help with curtailing spears from becoming tough. 

Sources

Atlantic Provinces Agriculture Services Co-ordinating Committee. Asparagus. Advisory Committee on Vegetable Crops.

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Kaiser, Cheryl and Matt Ernst. 2016. Asparagus. University of Kentucky Cooperative Extension Service. University of Kentucky. Lexington, KY 40506.

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Nair, Ajay and Brandon Carpenter. 2016. Commercial Asparagus Production. Iowa State University Extension and Outreach. Ames, IA 50011.

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Suslow, Trevor. 1996. Asparagus Recommendations for Maintaining Postharvest Quality. University of California Postharvest Center. Davis, CA 95616.

Trinklein, David H., Lewis Jett and Denny Schrock. 2012. Growing Asparagus in Missouri. University of Missouri Extension. Columbia, MO 65201.