H.Y. Hanna, Richard L. Parish and Regina P. Bracy

Thousands of acres of vegetables and strawberries are planted every year in the southern United States on black polyethylene-mulched and drip-irrigated beds. Black mulch increases early spring crop yield by retaining heat and moisture, conserving fertilizer and retarding weed growth.

Because the long growing season in the South offers the potential for double-cropping of existing mulched and drip-irrigated beds, many growers would like to produce a second crop such as cucumbers, muskmelons and squash following tomatoes, peppers or strawberries. The practice of double-cropping reduces production costs by enabling succeeding crops to use the existing polyethylene mulch, drip tape and fertilizers applied to the first crop. In Florida, cost analysis of this practice indicated that savings were ample enough to justify double-cropping watermelon behind tomatoes.

The color of polyethylene mulch influences the microclimate around the root system. Black polyethylene mulch is preferred for growing winter and early spring crops because of its warming effect on the soil around the roots. However, heat accumulation under the black mulch during sunny days in mid to late summer following crop removal is thought to limit its use for a succeeding crop. To prevent heat buildup, some growers paint the black polyethylene with white latex paint, and some researchers recommend using a mulch system that changes color from black to white at the termination of the tomato crop and before planting a summer crop.

Red River Studies

Eight studies were conducted at the Red River Research Station to determine if black mulch used in spring tomato production would adversely affect growth and yield of subsequent cucumber and muskmelon crops. For valid comparison, tomatoes were planted on an equal number of black and white mulched beds in early April. The same beds were used for cucumber and muskmelon studies after the termination of the tomato crop in late June.

Following plant removal after the last harvest of tomatoes, plots were sprayed with glyphosate at 3 pounds active ingredient per acre to kill existing vegetation and expose the mulched plots to sunlight. Cucumbers were planted in July and August of 1994, 1995 and 1996 (six studies) and muskmelons were planted in July of 1996 and 1997 (two studies). Soil temperature under both black and white polyethylene mulch was recorded at a 4-inch depth around 4 p.m. at the center of each row for three weeks, and the average temperature was calculated.

Cucumber plants were trained vertically using existing tomato stakes for support (Figure 1) and fertilized by injecting nitrogen at 12.5 pounds per acre (37 pounds of ammonium nitrate) through the drip-irrigation system when the plants reached the third-leaf stage. Two more applications of equal amounts were made at three-week intervals following the first application. Muskmelons were planted on tomato beds after removing tomato stakes and fertilized by injecting 15 pounds per acre of nitrogen (45 pounds of ammonium nitrate) through the drip irrigation system at the third-leaf stage and again three weeks later.

Cucumbers were harvested three times each week (Figure 2) and fruit were graded according to U.S. Department of Agriculture (USDA) standards to U.S. Fancy, No. 1, No. 2 and culls. Marketable yield was the sum of all grades except culls. Muskmelons were harvested nine times in 1996 and seven times in 1997, evaluated for grade and then weighed. Fruit that were well formed, well netted and free from decay, damage and sunscald were graded as marketable. Fruit that were deformed, cracked, rotten, or weighed less than a pound were culled.

Study results indicated that soil temperatures were higher under black than under white mulch in all tests. Marketable yields of cucumbers planted in July and August of 1994, 1995 and 1996 were not affected significantly by color of the mulch. Mulch color had no significant effect on muskmelon marketable yield either (Table 1).

Squash Studies

Squash studies were conducted at the Hammond Research Station in 2000 and 2001. In these studies, squash was mulched with black and white polyethylene mulch and with black mulch painted white at several paint rates. The paint was applied to the first study and a crop of squash planted in April 2000. Additional tests were planted in August 2000 and May 2001 on plastic that was painted in August 2000. Results indicated that there was no consistent yield advantage to white or white-painted plastic over unpainted black plastic (Table 1).

Results of these studies indicate that concerns over reusing black plastic mulch for a second crop are unfounded. Growth and yield of cucumbers and muskmelons were similar when planted on black or white polyethylene mulch. Squash yields on black and white painted mulch were also similar.

(This article appeared in the winter 2003 issue of Louisiana Agriculture.)