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Asparagus is originally native to the eastern Mediterranean. The plant is a long-lived monocotyledonous, herbaceous perennial. It is cultivated for its succulent, fleshy shoots (spears) where summers are not excessively hot and winters are cold enough to allow for dormancy.
As asparagus spears age after harvest, shoot fibers can lignify and toughen, and their flavor can change from sweet to bitter. Spears should therefore be harvested when immature. To allow plants to develop strong, healthy tops, wait until the second year after planting to harvest spears. Limit harvest to two weeks in the second year and one month in the third year after planting. From the fourth year after planting, you can harvest in Minnesota from early May to late June. Yields should increase up to the seventh year after planting, then stabilize until plants are fifteen or more years old. Overharvesting one year will decrease the following year's yield by reducing plant vigor and draining sugar reserves in the crown.
Harvest spears 6 to 9 inches in length by cutting or snapping the spears. If using a sharp knife to cut the spears, tilt the knife 45 degrees to the soil surface and cut the spears at ground level or 2 inches below the soil surface. Cutting below the surface may damage spears that have not yet emerged, but leaves a woody base that may restrict water loss and slow the entry of decay organisms.
Because spears are harvested when actively growing, their respiration rates are very high. Reduce respiration by harvesting when temperatures are cool (early morning or evening), shading field boxes or bins, and cooling spears to 34 degrees F immediately after harvest, preferably by hydrocooling.
Pack asparagus vertically into shipping containers with ventilation slats and absorbent pads that go under the spear bases or perforated plastic bags stored vertically. Asparagus spears will bend, especially at high temperatures, which is especially noticeable if spears are packed horizontally. Vented packaging allows for dissipation of ethylene gas produced by the spears, which can cause toughening and bitterness.
Asparagus can be kept for 3 weeks at 36 degrees F. It is subject to chilling injury if kept at 32 degrees F for more than 10 days. Provide high humidity to prevent shrinkage by placing asparagus butts on wet pads, packaging spears in perforated plastic bags, or misting spears with cold water. Keeping spears in pans of cold water for too long may cause nutrient leaching or microbial infection.
Chilling injury may occur if spears are kept for more than 10 days at 32 degrees F. Symptoms of chilling injury are flaccidity and dull, gray-green tip to stem color. Chilling injury may not show up during chilling, but when the spears are rewarmed.
Other disorders include toughening and bract opening (feathering). Toughening is caused by exposure to ethylene gas and controlled by adequately venting packaging. Bract opening is most prominent in over-mature spears and promoted by high storage temperatures. Control feathering by maintaining storage temperatures of 32 to 36 degrees F.
The primary disease of asparagus is bacterial soft rot, caused by Erwinia or Pseudomonas spp. Spears with white butts are less perishable than all-green asparagus, but may be less desirable to consumers. Symptoms of this disease are soft, water-soaked, dark green, slimy pits on the spear, frequently on the tip or butt. Control soft rot by cooling spears immediately after harvest and maintaining low temperatures during storage. Soft rot can also be partially controlled by briefly exposing spears to 20% carbon dioxide before storage or keeping spears in 7% carbon dioxide during storage.
|Fusarium rot||Fusarium spp.||small, brown, oval lesions at stem base or tips, enlarging to soft rot and white or pink mold|
|Phytophthora rot||Phytophthora spp.||odorless decay with white mold|
|Blue mold rot||Penicillium spp.||white to blue mold at butt ends and bud scales|
For information covering related areas, consult the following Minnesota Extension Service publications. They are available from your county extension office or by writing to the University of Minnesota Extension Store, 20 Coffey Hall, 1420 Eckles Avenue, University of Minnesota, St. Paul, Minnesota 55108-6064.Commercial Postharvest Handling of Strawberries (Fragaria spp.)
Larry Anderson and Cindy Tong
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