Not all plastic mulches are created equal when it comes to permeability.

Even if they’re considered VIF, or virtually impermeable film, mulch, the actual permeability may vary among products.

That’s because they may not have been tested under the same conditions using the same protocols, according to a news release. So it’s more like comparing apples with oranges.

The U.S. Department of Agriculture is trying to solve that by testing dozens of films in specially designed chambers at its North Central Agricultural Research Laboratory in Brookings, S.D.

Sharon Papiernik, a researcher with the USDA’s Agricultural Research Service, led a team that sandwiched each film between two chambers and injected fumigant into one chamber.

They then measured the amount of fumigant that passed into the other chamber as well as how much was left in the original chamber and assigned a score.

Altogether, they tested 200 mulch-fumigant combinations and came up with a resistance factor. Because each fumigant had different chemical properties, each behaved differently with each tarp.