Three methods of planting asparagus are by direct seeding, seedling transplants, or crowns.
Asparagus seed can be planted in the field to produce crowns. Crowns are transplanted to their permanent row spacings the following year, but the practice is not recommended. Asparagus seed is slow to germinate and hybrids are costly. A considerable amount of labor, including irrigation and weed control, is required to establish the plants. They should be grown in a sandy loam soil to facilitate digging the crowns the following year for transplanting. Thus, one year of spear production is lost due to transplanting crowns.
Due to the high cost of hybrid asparagus seed, some growers will germinate seed in flats in a greenhouse and grow these seedlings for 10-12 weeks before planting in the field. These seedlings are called "transplants." They can be transplanted to the field in the spring or in the fall if greenhouse space for growing seedlings is limited during the spring (7).
Buy seed with a high germination rate or separate out the small asparagus seed. Small seed produces seedlings with much less vigor, but amounts to only about 1-2% of the seed population.
Be sure that the seed has been treated to aid in controlling asparagus root rots.
Before seeding, treat the seed with household bleach, if not done by the seed grower, to aid in controlling asparagus root rot. If the seed has not been treated, check with your seed supplier or state Extension Plant Pathologist located at your state land-grant university to provide seed treatment information.
Plant seed in a sterilized soil mix containing 1/2 sterilized soil, 1/4 peat, and 1/4 sand, or use a commercially prepared peat-lite mix. The "Cornell Mix" (1/2 peat, 1/2 vermiculite) works well for asparagus seeds. The planting depth of asparagus seeds is critical, according to Benson (2). Sow one seed per cell, placing it 1/2 to 5/8 inch below the top of the soil to keep the seed from pushing itself out. The germinating asparagus seed has a very large radicle (root). If the seed is not sufficiently covered, the radicle will push the seed out of the tray.
Seed placement is also critical because the roots of asparagus plants grow only below the seed. In an inverted-pyramid-shaped cell, the volume of the cell becomes less toward the bottom. Therefore, the rooting volume is less. The size of the cell should measure at least 1 inch square on the top and 2 to 3 inches deep with a slight taper or no taper from top to bottom. A 64-cell plastic tray that measures 18 X 18 inches with the cell dimensions 2 X 2 X 3 inches deep can be used. The cell tapers from 2 inches at the top of the cell to 1 1/4 inches at the bottom. A hard plastic or non-porous plastic tray should be used. The cells should have holes at the bottom and sides, with a slightly raised center.
Foam trays do not work well because the asparagus roots are very invasive and grow through the foam; nor do the roots air-prune. The transplants must be cut out of the trays, thereby ruining them. Using trays with smaller lengths or volumes will result in poorer-quality transplants.
Seeds can be germinated in a warm room for five to seven days at 85 degrees F. Following germination, put the flats into a greenhouse at 75 to 85 degrees F day temperatures with high relative humidity, and 70 degrees F at night. Allow two to three weeks for emergence. When the seedlings reach 1 to 2 inches, drop the temperature to 75 degrees F during the day and 60 to 65 degrees F at night. Reduce the relative humidity to produce a strong plant.
Feed young plants with 50 parts-per-million of a 20-20-20 water soluble fertilizer solution during the first three to four weeks after germination if the ferns appear a pale green or yellow color. The most desirable ratio of nitrate nitrogen to ammonium nitrogen is 75% nitrate N to 25% ammonium N. Therefore, select a commercial water-soluble fertilizer that does not contain more than 50% ammonium nitrogen. Begin full-strength fertilization after the first three- to four-week period with 75 parts per million of a 20-20-20 fertilizer solution when needed. Be sure to "harden off" the plants by gradually withholding water and fertilizer, beginning one week before planting in the field.
Apply irrigation water in the greenhouse by using a gentle spray. Water applied in a forceful stream can knock the ferns over, predisposing them to fungus diseases because they are unable to dry out properly.
If a peat-lite mix is used, iron deficiency frequently shows up as slightly yellow plant tips. Asparagus is susceptible to iron chlorosis. The deficiency is easily corrected with iron sulfate or iron chelate. Every 10 to 14 days thereafter, apply additional iron to prevent recurring chlorosis. Applying too much iron may result in manganese deficiency, so use only the recommended amount. Iron deficiency should not pose a problem if a fertilizer containing micronutrients is used.
Grow transplants on wire benches or off the ground so the air movement below the trays will "prune" the roots. Air-pruning results in better distribution of the roots in the cell and reduces the number of roots growing out of the bottom of the cell. If the top of the plant is pulled off, leave the roots in the cell, because a secondary bud will send up a new shoot.
Be alert for aphids and thrips, which stunt the growth of the young seedlings. Treat with an approved insecticide.
Transplants are planted at depths as suggested in Figure 3 in trenches known as "W-shaped" furrows. A machine consisting of a furrow opener and a W-furrow shaper has been developed by Dr. Stephen Garrison of Rutgers University. Grooves on each side of the plateau of the W-furrow trap water and eroding soil, and protect the plants from being covered (13). This can be very helpful on lighter soils, where the furrow walls tend to fall in after heavy rainfall. In heavier soils, an ordinary furrow can be opened to a depth of 4 to 5 inches with a lister plow and the 12-week old transplants will survive well.
The use of a water-soluble starter solution such as a 10-52-17 helps the plants get off to a quick start. Dissolve 3 lbs in 50 gallons of water and apply around the transplants after planting in the furrow.
Sometimes, the original ferns of the newly-set transplants may turn white about a week after transplanting. This is normal. These ferns will die and new buds on the crown will send up new spears that will produce healthy green ferns throughout the season.
As the plants continue to grow, the soil can be gradually filled in to support the taller fern growth. By the end of the first growing season, the furrows should be at ground level. Timing the filling in of furrows can be used to control newly emerged weeds. Also, 1 to 2 inches of soil can be used to form a ridge of soil over the row after the fern dies in the fall. This helps water to drain away from the crown area.
A one-year-old crown is the crown and fleshy root system of a one-year-old plant grown from seed (Figure 4). Starting asparagus plantings with one-year-old crowns is still the easiest and most popular method used by Eastern and Midwestern U.S. growers. Research has shown that using one-year-old crowns is more economical and higher yielding than using two- and three-year-old crowns (17). Two-year-old crowns appear to suffer greater transplant shock. Weak crowns have been shown to yield less than plants established from vigorous crowns. Often, two-year-old crowns are weak crowns grown an additional year to obtain additional size (48). Studies done in Virginia indicate that improved plant survival and increased yield during the early harvest seasons can be obtained with crowns rather than transplants, even though supplemental irrigation is not used (41).
Space the transplants or crowns 12-18 inches between plants in the row and 5 to 6 feet between rows. This practice requires the following quantities of transplants:
| Spacing Between Plants in Rows | Spacing Between Rows | Plants/Acre |
|---|---|---|
| 12 inches | 5 feet | 8,712 |
| 12 inches | 6 feet | 7,260 |
| 14 inches | 5 feet | 6,970 |
| 14 inches | 6 feet | 5,808 |
| 16 inches | 5 feet | 6,550 |
| 16 inches | 6 feet | 5,459 |
| 18 inches | 5 feet | 5,808 |
| 18 inches | 6 feet | 4,840 |
Note the plant population for the new hybrid varieties is considerably less than what is recommended for open-pollinated varieties. The wide between-row spacing is needed because the fern growth of the hybrid varieties is vigorous and will usually fill the between-row space after one growing season if one-year-old crowns are planted. It also allows for better air circulation to promote faster fern drying from rain and morning dews. This delays the onset of foliar fungus diseases.
Studies at Michigan State University have shown over a nine-year harvest period that plants grown at a 6 inch in-row spacing gave higher yields in the first five years of production as compared with a 12, 18, or 24 inch in-row spacing. This higher early-year production justified the additional expense of the initial planting. However, studies will be continued to determine the long-term effect of the spacing on yield and spear size (21).
Tests at Bixby, Oklahoma, indicate planting 9,000 crowns per acre will outyield 6,000 crowns per acre by a small amount for one or two seasons. By the third harvest season, yields were equal with 6,000 and 9,000 crowns per acre. A large percentage of small diameter spears will be produced if crowns are set too close together (34).
The distance between rows may be determined by the harvesting and field equipment to be used. Running the rows north-south may promote faster drying of rain and dew from the fern and should help delay the onset of fungus disease problems.
After planting several rows of asparagus, it may be helpful to leave a "drive row," which is not planted to asparagus, but could be left in sod. This will facilitate better insecticide and fungicide application. An air-blast sprayer may be used to blow the pesticide through several rows instead of traveling over the rows and damaging the ferns. Be sure to calculate the number of asparagus rows that the sprayer will effectively blow through and cover the ferns thoroughly. Then put in a drive row after the last row that contained good plant coverage. One can then spray into the block of rows from both sides, insuring good spray coverage.
Using crowns is desirable for several reasons. Many growers, especially those just starting out, may not have a greenhouse. All hybrid seed is expensive, and nurturing transplants is difficult. Also, a grower can recover approximately three-fourths of the crown cost the year after planting.
If one-year-old certified disease-free crowns are bought from a reputable supplier, growers can plant them in the spring and, with good growing conditions, can harvest about 500 lbs per acre the following year. If greenhouse-grown transplants are used, growers cannot harvest until the second year after transplanting (third year) because the root system must store enough food reserves for the next year's crop.
Good crowns should number 6 to 8 per pound, or about 100 crowns per bushel. The large amount of stored food reserves in these crowns will send up vigorous fern growth that will reach 5 to 6 feet during the planting year. The New Jersey hybrids remain green longer into the season than the older open-pollinated varieties.