Asparagus requires a period of rest for successful production of spears. The crop is grown commercially only in those regions where the plants are forced into a rest period by cold or drought. In warmer sections of the United States where the temperature does not go much below freezing and where the winter season is very short, conventional asparagus production is not successful (42).
In semi-arid and arid regions, asparagus is grown successfully, even though freezes seldom occur and the temperature is high enough for growth to continue throughout the year. Under such conditions, the plants are forced into rest by withholding water. This method is followed in the Imperial Valley of California, where irrigation is used during most of the year, but water is withheld for three or four months in the fall and early winter (42).
The productive life of an asparagus planting is longer in regions that have long, cold winters than in regions that have short winters, provided equally good care is given in both climates. However, the length of the harvest season is usually short in regions having short growing seasons (42).
Asparagus grows and yields best in a deep, well-drained sandy loam soil, but will tolerate heavier soils as long as the soil has good internal drainage and the water table does not come within four feet of the surface. This would interfere with the extensive and deep root system.
It is important to eliminate all perennial weed problems at least one year before planting. This can be done effectively by treating the actively growing weeds with a non-selective herbicide. Another way to reduce weed populations and help build soil organic matter is to prepare the field at least one year in advance. This can be done by planting a cover crop such as clover, or an early-maturing soybean variety. The soybeans can then be harvested, or clover can be chopped and plowed down and seeded to winter wheat or rye at 3-4 bushels per acre in the fall before planting asparagus. The cover crop can be plowed under the next spring to increase soil organic matter content before planting asparagus.
Soil test to determine pH and fertilizer requirements. The ideal pH range for asparagus is between 6.7 and 7.0. Asparagus does not tolerate acid soils and will not grow well at a pH of less than 6.0. Also, a fungus disease that contributes to asparagus decline (Fusarium crown and root rot) survives better at a low pH. Liming the soil to bring the pH up to 7.0-7.5 would reduce the survivability of Fusarium, especially if asparagus has been grown there previously. Phosphorus and potassium should be provided so that the soil contains 250 lbs of available phosphorus and 300 lbs of available potassium per acre. Phosphorus does not move readily in the soil and cannot be incorporated easily into the soil after the asparagus is planted. Essentially, one must incorporate phosphorus before planting. Also, apply 70 lbs of actual nitrogen per acre.
Broadcast the fertilizer and plow it under when preparing the land for the planting furrows. Then, each year after harvest, broadcast 70 lbs actual nitrogen and other nutrients (if needed) per acre so they will be utilized by the new fern growth to store for the following year's crop. Lime can also be added at this time if needed. Soil test every year for the first four years to determine if fertility and pH adjustments are necessary. Then soil test every two years.
Select a field that has never been planted to asparagus to reduce the possibility of Fusarium, a soil fungus that can weaken and kill the plant. Fusarium oxysporum is found in all soils, but the organism will increase in soils where asparagus plants had previously become infected. Fusarium moniliforme is another soilborne disease. However, both asparagus and corn are hosts for this disease. Corn seems to stimulate high Fusarium populations, so the concentrations of the fungus will increase (4). Therefore, it is recommended that asparagus not be planted within three years of growing corn (4).
All soils contain some Fusarium species because it can live on alternate plant hosts such as weeds and survives for many years in the soil. Soil fumigation studies show that no chemical is effective in long-term reduction of Fusarium populations.
Planting asparagus into an old asparagus field may also cause an alleleopathic response or suppression of growth of the new plants by substances that are released from old root pieces. New hybrid varieties are somewhat tolerant to the fungus than older varieties, and the growth of the hybrids is vigorous enough to yield well in spite of the presence of the disease. Ideally, growers should wait at least four years from plowing under an old asparagus planting until replanting it to asparagus, to give the roots time to decompose and also to reduce the population of soilborne Fusarium. However, some growers have observed that waiting four years is insufficient (44).
In a virgin soil (free of Fusarium), the expected life of an asparagus field (any variety) is 15-20 years. Growers feel that peak production occurs in the 6th or 7th year, with the best production occurring during years 7-12. There is a decline of production of about 5% per year in the 10th year and every year thereafter. After the 15th year, the field may no longer be economically profitable. Established asparagus growers recover their investment after the 5th year and years 5-10 are their most profitable years (44).