Composting hay can reduce the number of weed seeds to a minimum but that has to be done the right way in order for the compost to reach a high enough temperature to kill the seeds. I’d be wary of composted hay unless you’re sure of how it was composted.
Straw on the other hand, is much better for use as a garden mulch. Since wheat and other grain crops are so competitive in a field, they suppress the growth of many weeds. Farmers also will control weeds one way or another to ensure the highest yields they can get of valuable grain. That results in straw with no or very little weed contamination.
You never know what plant combination you’ll get in a random bale of hay. More often than not they contain weeds that you can inadvertently introduce to your property. I’ve seen such tenacious perennial weeds like thistle come into a garden as a result of their seeds hiding inside a bale of hay.
Hay is a crop that is grown and harvested as a feed crop for cattle, horses and other farm animals. Straw on the other hand is a byproduct of a grain crop; in our area it’s usually usually wheat straw that we see.
Why would that make a difference to us in the garden? The problem lies with hay. Hay often is made up of a combination of different plants growing in a field or meadow. Farmers will cut and bale the plants in a field like that to feed to dairy cows that are in their resting stage, called dry cows. That kind of hay is of low quality and is less nutritious than say alfalfa hay, but that is fine for dry cows because they don’t require dense nutrition when they’re not producing milk.
Granted, there are exceptions to the rule. You can find weed-free hay, such as 100% alfalfa or timothy but these can be expensive. Sometimes straw can be highly contaminated with weeds if it was grown in less than optimum conditions.
In a garden, however, getting the two confused can lead to problems in the future. Hay and straw are often both used as weed control mulch in the garden but the results you get can be quite different.
Wrap the hay in a clear plastic sheet or tarp. Tape it shut with packing tape.
Wet the hay thoroughly with a garden hose. Once wet, the hay becomes heavy, so place it in an area that it won’t need moved for at least two weeks.
Hay is a type of dried grass that’s occasionally used in landscapes and gardens as mulch. Sometimes confused with straw, hay is baled and used as livestock forage because it contains beneficial nutrients. Straw is made from dry cereal grain plant stalks, such as wheat or oats, and has minimal nutritional value. It conserves moisture, maintains the temperature of soil, prevents weed growth, adds nutrients to the soil and is inexpensive compared to other mulches. Most hay contains grass and weed seeds, but there is a way to prevent the seeds from sprouting in your garden or landscape when used as mulch.
Place the sealed hay in a location that receives direct sunlight. The high temperatures inside the plastic kill the seeds and the hay is ready for use as mulch. The wet hay can be heavy and difficult to carry. Open the plastic and allow the hay to dry in the sun before moving it or transporting it in a wheelbarrow.