Asparagus production guide
Table of contents
- Site selection and preparation
- Soil fertility requirements - New plantings
- Soil fertility requirements - Established plantings
- Secondary and micronutrients
- Asparagus varieties
- Asparagus bed establishment
- Crown plantations
- Seedling transplants
Crown and root system of a six-year-old asparagus plant; each block represents one foot square.
Asparagus is a high value specialty crop and the earliest producing spring vegetable. It currently is priced as a gourmet item and will remain in this category until growing, harvesting, and processing costs can be reduced. Although production in the United States is concentrated in California, Washington, and Michigan, many other areas have great production potential climatically and geographically. The cost to establish an asparagus field is substantial, so the decision to grow asparagus is one that should be thoroughly investigated. This publication describes the cultural practices that must be considered to produce asparagus successfully.
Production is most successful in areas where freezing temperatures or drought terminates plant growth and provides a rest period. Without this rest period, reduced yields are likely. Asparagus tolerates great temperature variations: it grows in the Imperial Valley of Southern California, where temperatures can reach 115° F, and it grows in Minnesota, where temperatures can plunge to -40° F. Asparagus can be grown in a wide range of soils and under various climatic conditions, but it thrives in fertile well-drained soils in moist temperate regions that have long growing seasons and sufficient light for maximum photosynthesis.
In Minnesota, asparagus is susceptible to late spring frosts that may kill emerged spears, delaying subsequent spear development. Therefore, production fields should not be established in low areas or in other frost-susceptible locations.
Site selection and preparation
Unlike most other vegetables, asparagus is a perennial crop which can be productive for 15 years or more. Consequently, it is important to pay particular attention to site selection and preparation for this crop. In Minnesota, asparagus is grown on many different soils ranging from sandy coarse-textured soils to clay fine-textured soils. Highest yields are usually obtained on medium-textured sandy loam to loam soils. Asparagus plants have a deep root system that will penetrate at least six feet. Shallow soils or soils prone to a high water table should be avoided. Asparagus roots will not tolerate saturated soil conditions.
Soil fertility requirements - new plantings
Field preparation should take place the year prior to planting. Soil tests (0-12") can be used to determine needs for lime, phosphorus, and potassium. Asparagus will not tolerate extreme acid soil conditions and grows best at a pH of 6.5-7.0. The objective during the first 3 years after planting is to encourage maximum fern growth so that plants build extensive storage root systems.
Plow down soil amendments before furrow construction or bed shaping. All furrows should be 6 to 8 inches below the normal soil surface. Rates of fertilizer for asparagus are dependent on soil test values and relative organic matter levels. Refer to Tables 1, 2 and 3 for suggested rates of fertilizer. For nitrogen, (N) approximately 1/3 to 1/2 of the recommended rate should be broadcast after planting. The remainder of the N should be sidedressed at the first cultivation. Most of the phosphorus (P) and all of the potassium (K) should be broadcast and incorporated prior to furrow construction or bed shaping. Apply 25-30 lb/A P205 in the trench before crown setting. If soil test P is high, omit the broadcast application and apply only that recommended for the trench.
Table 1. Nitrogen recommendations for asparagus
|Asparagus||Organic matter level||How to apply|
|N to apply lb/A|
|New planting||120||100||80||½ broadcast, ½ sidedress during cultivation;|
|Established planting||80||60||40||Topdress after harvest|
Table 2. Phosphorus recommendations for asparagus
|Amount of Phosphate (P2O5) to apply (lb/A)|
Table 3. Potassium recommendations for asparagus
|Amount of Potash (K2O) to apply (lb/A)|
For the second and third year following crown setting, disk in during the spring prior to spear development (40-60 lbs. N/A and recommended rate of P and K according to a soil test). An additional 30-40 lbs. N/A should be applied as the soil warms up.
Soil fertility requirements – established plantings
Once the plants are established, the primary objective is to maintain plant vigor. Asparagus has a very fleshy root system which is capable of storing a large quantity of nutrients. It has been estimated that the roots can store 150 lbs. N/A, 37 lbs. P/A and 170 lbs. K/A These stored nutrients, in part, can be used for the development of spears in the early spring. The actual amount of nutrients removed by a 2.5 T/A harvest is 23 lbs. N/A, 3 lbs. P/A and 20 lbs. K/A. Generally, it is not necessary to apply fertilizer for an asparagus crop until after harvest. Delaying fertilization until after harvest can reduce early weed growth. For sandy coarse-textured soils, 20-25 lbs. N/A in the spring may be beneficial for spear development. Tables 1, 2 and 3 present fertilizer recommendations for established plantings. This fertilizer should be topdressed after harvest to encourage fern growth.
Secondary and micronutrients
Asparagus response to application of secondary and micronutrients is not well documented in Minnesota. Most soils low in calcium (Ca) and magnesium (Mg) are acid and should be limed with dolomitic lime prior to planting. Sulfur may be limiting on sandy soils with low organic matter. In general, asparagus response to micronutrients is low. Nutrient ranges from healthy mature asparagus ferns are presented in Table 4. Suspected nutrient deficiencies should be confirmed with soil tests and/or tissue analysis.
Table 4. Nutrient concentration ranges in healthy asparagus ferns
|Stage of Growth||Part Sampled||N||P||K||Ca||Mg||Fe||B||Cu||Zn||Mn|
| % ppm |
|Mature fern||Fern from 18"36" up||2.4|
Irrigation should be used on asparagus fields containing sandy soils or shallow root restrictions in central Minnesota. Inadequate soil moisture during fern development can cause significant reduction in next spring's spear production. Dry soil conditions during spear growth can also affect quality and yield.
Adequate soil moisture is also necessary for newly planted crowns to establish good root development and fern growth.
Asparagus roots can penetrate up to 10 feet to obtain soil water if not restricted but their greatest water uptake occurs from the top 6 to 24 inches of rooting zone (See cover). Maintaining adequate soil moisture in this zone during the fern stage especially should be the goal of an irrigating producer. Asparagus plants will use .10 to .20 inches of soil water per day during fern growth depending on climatic conditions.
To maintain healthy fern development, soil moisture during this period should not be allowed to deplete more than 50 to 60% of the soil's water holding capacity in the active rooting zone or go beyond a soil tension of 70 centibars before another irrigation. Asparagus plants do not generally show visual signs of wilting when moisture-stressed, so extra care must be exercised to ensure there is adequate soil moisture throughout the growing season.
Several soil moisture monitoring methods are available to assist the grower in proper timing of irrigation water to maintain healthy plant growth. A discussion of the typical monitoring methods can be found in University of Minnesota Extension article, Irrigation Water Management Considerations for Sandy Soils in Minnesota.
Light, frequent irrigation applications should be avoided during fern growth to minimize foliage disease development. On the other hand, over irrigation should also be avoided as it may cause some of the applied nitrogen to be leached below the plant's root zone and possibly into the ground water.
Water scheduling is an essential management practice for irrigated asparagus production. Utilization of any of the available soil moisture monitoring tools requires only about 30 minutes a couple times a week to provide an operator with valuable information for scheduling the next irrigation.
Asparagus varieties should be both high yielding and disease resistant. Asparagus is a dioecious (dye-EE-shus) plant, meaning that there are both male and female plants. Generally, females produce larger spears than males, but the males produce greater numbers of smaller diameter spears. Only female plants produce berries. Breeding work is in progress worldwide to produce high yielding all male asparagus lines. The main benefit from an all-male hybrid is that it doesn't produce seed, which can later germinate and create a significant weed problem in the form of several volunteer asparagus seedlings. Asparagus spears produced from all male hybrids are usually very uniform. For many years, the most common varieties have been from the Washington series (Mary, Martha, Waltham), developed by the U.S. Department of Agriculture which are dioecious. However, several of the all-male hybrids developed in New Jersey (Rutgers University) offer proven higher yields and increased rust resistance and tolerance to fusarium crown rot and are often the preferred choice. 'Jersey Giant', 'Jersey Knight', and 'Jersey Prince' have done well in Minnesota. Other all-male hybrid varieties released from the Jersey series with excellent resistance to fusarium include 'Jersey Jewel', 'Jersey King' (green spears with purple bracts), 'Jersey General', and 'Jersey Titan' (green spears with purple bracts). These have not been tested in Minnesota but have been reported to do well in other states, including Michigan, and Canada. A newer all male hybrid released from the University of Guelph called 'Guelph Millenium' has performed very well in Canada but has not been evaluated in Minnesota. An open-pollinated variety that has been grown for years in Minnesota is Viking KB-3. Although this variety is a proven survivor in Minnesota conditions, many of the Jersey hybrids will offer better results. However, it should be noted that in northern Minnesota the Jersey hybrids have had winter kill at temperatures of -30°F with no snow cover. Purple Passion is a variety that produces attractive purple spears for an added twist. This unique variety could provide a niche market opportunity. Varieties from California have been bred for warm climates and do not possess the longevity or hardiness needed in Minnesota. One of the key attributes associated with California varieties has been their strong production potential during very warm conditions and delaying the onset of fern development. However in Minnesota, yield decline has often been observed in these varieties shortly after the establishment years.
Asparagus bed establishment
Commercial asparagus plantations can be established either by traditional crown planting or by transplanting seedlings. Direct seeding into a permanent location is discouraged because of the difficulty of establishing a stand. If you are planting a large acreage, asparagus crown nurseries offer the opportunity to produce many crowns per acre easily. Generally, ten production acres can be established from the crowns produced in a one-acre nursery. One pound of asparagus seed will produce enough crowns to plant one acre. Asparagus seed with a high germination percentage should be seeded on level ground about one inch deep and spaced about two inches apart within rows. Row width should accommodate machinery to facilitate mechanical digging. A modified potato digger has been used successfully to dig crowns. Generally, single rows spaced about 24 inches apart will allow enough space for large crown production. This spacing scheme requires 130,000 seeds per acre for the planting. An 80-percent recovery of crowns will net approximately enough plant material to plant ten production acres with four feet between rows and one foot between plants within rows. Usually one ounce of asparagus seed contains 500 to 700 seeds.
To grow high quality crowns, obtain seed with a high germination percentage. Plant the seed in sandy soils so crowns can be easily dug and will be relatively free of soil. Apply and incorporate phosphorus and potassium fertilizers prior to seeding the nursery at the rates suggested in Table 2. Apply approximately 50 pounds per acre of nitrogen after the first shoot ferns out, and topdress an additional 50 pounds per acre in midsummer.
The slow rate of germination is a problem with direct seeding. Optimum temperatures for germination range from 77 to 86° F. Although lower soil temperatures slow germination, it is advisable to plant asparagus seed as soon as the soil is workable in the spring. Since the growing season needed to produce large crowns is limited in Minnesota, early spring seeding will allow germination to occur as soon as the soil environment becomes favorable. To prevent infection by soilborne pathogens, asparagus seed should always be treated with fungicides.
Weed control in direct-seeded asparagus presents a second challenge. A few satisfactory preemergence herbicides are labeled for direct-seeded asparagus (see Midwest ). Adjust the rate according to the texture of the soil type. Inevitably, mechanical cultivation is necessary in the nursery. Any cultivation should always be shallow to prevent damage to asparagus roots, which are very near the soil surface. Although mature asparagus is quite drought-tolerant, seed beds are shallow rooted and require constant water management Irrigation should be available on demand.
Asparagus crowns should be dug in early April or before the buds have begun to grow. Old plant tops should be mowed and removed from the field if they interfere with crown digging. A potato digger, peanut digger, or common moldboard plow can be used to lift the asparagus crowns from the nursery row. Avoid injury to the crowns during digging and handling. If dug crowns need to be stored prior to replanting, keep them cool (about 38° F) and dry. High humidity will cause rapid decay. Crowns can become overheated if they are stored in a deep pile. Crowns in storage should be stacked only a foot or so deep. Avoid freezing temperatures in storage, since severe injury or even complete loss is probable.
Figure 1. A high-quality asparagus crown.
For small plantings, it is easiest to buy one-year-old crowns from a reliable grower. Only one-year old crowns are recommended which transplant easier, produce as vigorous plants as two-year-old crowns, and are less expensive Crowns should be large, with many storage roots and buds (see Figure 1). Each bud will eventually produce a spear. Storage roots contain high levels of sugar that nurture the developing spears. The larger the crown, the more vigorous the resulting asparagus plant will be.
Figure 2. Side view of typical furrow construction. X = crown or transplant placement; F = fertilizer placement (two-three inches below the bottom of the furrow).
Crowns usually are hand planted with buds up, spaced 12 inches apart within rows in furrows four to five feet apart (9000 to 11,000 crowns per acre). Six to eight inches is the optimum depth for crown planting (see Figure 2). Shallower planting depths cause production of spindly, thin spears, whereas deeply planted crowns produce fewer spears of larger diameter and emergence is delayed. In addition, as crowns grow in mass, they 'migrate' upward making the crown more susceptible to frost damage during first spear emergence. Planting crowns closer than 12 inches results in reduced spear size and quality. Spacing crowns farther than 18 inches apart may result in larger spears but fewer spears per acre.
Figure 3. Planting crowns: (1) set crowns upright in wide furrows, six to eight inches deep, with roots spread. (2) cover with two inches of soil, (3 and 4) gradually fill the furrow as the plants grow.
After placement in the furrows, cover the crowns with two to three inches of soil (see Figure 3). Gradually fill in the furrow as shoots emerge. By the end of the season, the furrows should be entirely filled in, although the developing asparagus fern should never be buried.
Weeds cause the greatest problem in establishing an asparagus bed from crowns. All perennial weeds should be eliminated before planting any asparagus. An appropriate herbicide, applied immediately after the crowns are covered should control weeds until the asparagus is large enough to be cultivated easily and safely.
Transplanting seedlings into the field is an acceptable alternative to crown planting if monitored closely. Seedlings are produced in greenhouses and are usually transplanted into permanent commercial fields when they are about 10 to 14 weeks old. The young seedlings can be mechanically transplanted, which reduces planting costs. Studies indicate that survival rates are comparable to those of crown plantings. The plant spacing is the same as in crown planting (see Figure 3). A transplant solution of 10-52-17 or 9-45-15 should be used at planting time. Follow the manufacturer's recommendation for mixing. Each transplant should receive at least 4 oz. of transplant solution. For best results, irrigation should be applied if rainfall is insufficient to maintain adequate soil moisture. Seedlings should be thoroughly hardened off before field planting. Place the plants in a moderately shady location and keep them moist. After about three days, transplant them to the field. In many cases, the asparagus fern will totally yellow and die; this is normal.
Transplanting may take place either in the spring (early May) or in the fall (early to mid-September). While spring transplanting is more common, fall transplanting has proved successful in Minnesota, and provides flexibility in the scheduling of both labor demands and greenhouse space. Regardless of the timing, the transplants should be as large and vigorous as the transplant equipment will allow, and without becoming root bound in the original containers.
Weed control is a challenge in the transplanting of asparagus. Cultivation will be necessary to fill in the furrows as the fern grows and as herbicides lose their residual activity.
The question of whether to use transplants or crowns is still unanswered. They are comparable in price, but crowns are one-year old plant material, whereas transplants are only 10 to 12 weeks old. Since the growing season is short in Minnesota and transplants do not grow to a large size in their first season, transplants may come into harvest up to a year later than crowns.