Oyster mushrooms are vigorous growers that love agricultural byproducts. Here in the Northeast, straw is easy to come by and pretty affordable so it's our go-to substrate. But what type of straw is best?
First, you want to use straw instead of hay. Hay is used as animal feed so it contains nutrient-rich seed heads which can become an overload of nitrogen in your grow leading to contamination. Those seed heads can also become weeds in your garden if you're installing a bed.
Since mushrooms have a tendency to accumulate toxins, it's important to find straw that hasn't been sprayed with pesticides, herbicides, or fungicides. If you can find organic, great - though it's possible there are farmers in your area who aren't using chemicals who may not be certified.
Ideal straw types are oat and wheat. You can certainly use other types like rye, barley or rice straw but they may be a bit more finicky to work with. When preparing your straw substrate, you may want to chop it to for smaller particle size. We've found best success with pieces 1-3" long but larger can work too if compacted some. With particle size you're trying to achieve a balance between leaving space for some air flow (you don't want anaerobic conditions) but not so much distance that it's a strain for the mycelium to reach between pieces. You can play around with pieces of different sizes to find what works best with your material.