Results from the trial of various plastic covers on a range of vegetable and flower crops showed that, in the case of asparagus, spears were heavier and more were produced per crown.
"Soil water management was better, which I believe contributed to the dramatically better growth," said consultant Peter Knight.
Despite fears that the protected environment would favour pest and disease outbreaks, levels were actually reduced.
Researcher Cathryn Lambourne of Stockbridge Technology Centre, where the trials were conducted, said: "We saw no Botrytis whatsoever and very little Fusarium."
Lambourne added that in 2007, when the trial was extended for another look at asparagus, ferns established about 10 days earlier under plastic than in the open ground and stayed greener for two to three weeks longer at the end of the season. Spears emerged about five days earlier.
"The standard plastic very often produced the best results of all the plastics," she said.
Knight said polytunnels would offer better environmental control than open-sided Spanish tunnels, because Spanish tunnels did not lend themselves to retaining heat. But he added that the costs would rise. He calculated that the outlay on an improved structure, the crowns planted at a high density, trickle irrigation and an Enviroscan soil moisture monitor would be £4.70/sq m for a Spanish tunnel and £6.88/sq m for a polytunnel.