Do I need to be concerned about produce that is grown in or on a substrate that includes straw or hay?

Since either straw or hay can be sourced from wheat, it is understandable why this question comes up. The GIG stance is that a substrate is not a cause for concern.  

What exactly are straw and hay? Straw is an agricultural by-product that consists of the dry stalks of cereal plants. Hay is generally defined as the entire plant that has been dried. Both can come from various types of plants, including wheat. Either may be used as part of the substrate for growing plants. 

The two questions to answer are: 

1) When straw or hay come from wheator another gluten-containing plant, do they contain gluten? and 

2) If they do, does this gluten make it into the final plant that is grown on the substrate containing straw or hay? 

Since straw is the stalk of the plant, and gluten is found in the seed, straw technically does not contain gluten, although the possibility of cross-contact cannot be entirely ruled out. Hay is the whole dried plant, but it is generally harvested and dried before the seed develops, so again, it is unlikely to contain gluten. 

Even if a plant sprouts from a substrate that could potentially contain gluten – from straw or hay – the mature plant itself (i.e., the edible plant) would, for the most part, be physically removed and separated from the substrate/growing surface. The concern would be possible cross-contact due to gluten on surfaces the produce touches 

All produce should be washed before eating. Thorough cleaning can remove any potential residues of gluten that could even potentially result from cross-contact. Two specific types of produce we have received questions about in terms of straw or hay substrate are strawberries and mushrooms. We investigated and found these answers: 

Strawberries
According to the North American Strawberry Growers Association, hay is not used in strawberry production. Straw may be used as a winter cover in northern climates to protect against cold and keep the berries clean. It is very unlikely that there would be any gluten associated with the fruit at harvest time. 

Mushrooms
Hay or straw can be incorporated into a growing medium for mushrooms along with other substances that, together, form a compost. The compost is put into a bed on top of which goes the “mycelium” that functions like a root system that produces the spores responsible for a mushroom’s reproductive process. On top of the mycelium is a layer of peat moss that creates a physical barrier. The root system may come in contact with the hay or straw in the compost, but the mushrooms do not. The “dirt” we see on mushrooms is peat moss, which is pasteurized and is not a gluten-containing substance.