Black polyethylene sheets are strong absorbers of sunlight, and get very hot. Black sheets have longer useful lifetimes than clear sheets. Uncovered ground appears dead (first photograph, June 2005), but even after being covered for months, sprouts are evident upon close examination: Not all crowns are killed (second photograph, July 2005). The "potato sprout look" is an example of etiolation.
When exposed to sunlight, the sprouts begin to turn green (first photograph, April 2006). They develop into bunches of very dense foliage (second photograph, July 2006), which then begin to send out vines and take on a more normal growth pattern.
The effectiveness of black and clear sheets is compared elsewhere.