White (blanched) asparagus is produced in the absence of sunlight. It is commonly grown in Asia and Europe (29). However, fresh white asparagus is rare even in major U.S. markets and supports a price structure of two to three times that of green asparagus. White asparagus is easily grown, and there seems to be a demand for it. The traditional growing method was to mound up soil over the plant row to an 8 to 10 inch height just before spears start to grow in the spring. Once the tip of the soil mound started to crack above the emerging spear, the asparagus knife was thrust into the mound, cutting the spear before it was exposed to light. This growing method is very labor intensive.
Makus (29) has grown white asparagus by covering an asparagus row with black plastic supported by wire hoops (Figure 8). The covering is opened on one side for harvest, then placed into position again. The plastic tunnel structure is removed when the cutting season is over. Otherwise, culture and pest control are similar to green asparagus (29).
Makus suggests using opaque (black) plastic (which does not allow sunlight to pass through), heeling it into the soil on one side of the row, and laying the plastic over metal hoops spaced 5 to 6 feet apart in the row. The side not secured by soil must be weighted down in any of several ways. Utility grade 2" X 2" lumber can be stapled to the plastic. Rocks and masonry block also can be used. By using lumber longer than the hoop spacings, growers can roll up the plastic on the wood and place it on top of the hoops during harvest. Skip a length of plastic equivalent to the length of the lumber. For example, for every 16 linear feet of plastic, use an 8-foot board (29).
Hoops need to be at least 40 inches long before bending into a semi-circle and forcing each side at least 4 inches into the soil. Use high-tensile steel that is about 12 gauge in diameter. Running twine along the top of the hoops will allow increased hoop spacing and prevent the plastic tunnel from sagging in the middles. A 4-mil plastic will exclude light whereas a 1.5 mil will not (l mil = 0.001 inch). A 4-foot-width plastic is adequate to cover a 14-inch hoop height. Repair torn plastic with duct tape. The installation of hoops and plastic cover can be mechanized by using a specialized machine (29).
The use of row covers provides only about a 2-degree Fahrenheit temperature increase for protection at night. During the day, Makus observed temperatures as high as 135 degrees F without detrimental effects on spears. Covers will kill spring weeds and perennial grasses if left on at least six weeks. Covers may be useful in areas where sand abrasion on spears from wind is a problem. Opaque white plastic can be beneficial in lowering daytime temperatures. The covered white asparagus spears will tolerate short duration temperatures below freezing to about 26 degrees F. Spears do not elongate as fast under plastics, have a tighter tip between harvests, and are slightly thicker in diameter than corresponding uncovered green spears (29).
Sedlock (38) grows white asparagus by using clean, opaque, black plastic 55 gallon drums that are sawed in half longitudinally (Figure 9). The halves are then placed over the row and are butted up against each other (Figure 10). When harvesting, the half-drum can be lifted off the row very easily and then put back in place when finished.
When harvesting white asparagus, avoid exposing spears to light for periods longer than 15 minutes or they will turn green. After harvesting, place the spears into opaque bags. Store between 32 and 38 degrees F, just like green asparagus.