Too often I hear people use the words interchangeably. Even worse, I’ll hear some people say that the only difference is that there are weed seeds in one, but not in the other.
The only difference between hay and straw is the time that they are harvested.
Straw is the same stuff allowed to go dormant. Grasses will often turn yellow. Most straw comes from grasses that went dormant complete with a big seed heat.
Grass seed is called “grain”. The grain will be nipped off the top of the seed stalk by a combine leaving the yellow leaves and stalks about two feet tall. What is remaining will be cut and baled. Straw is usually used for bedding for animals. Sometimes the animals will nibble at it.
I want to make something clear.
Both hay and straw can contain seeds.
Both hay and straw can contain weeds.
Both hay and straw can contain weed seeds.
Hay makes a great mulch for plants that like a lot of nitrogen. Straw makes a great mulch for plants that prefer less nitrogen. “Strawberries” get their name from doing so well when mulched with straw.
Consider this a warning: Most hay and straw contain persistent herbicides. If you use hay or straw with persistent herbicides as a mulch, your growies will be stunted or die. Except for grasses (the persistent herbicides are typically broadleaf herbicides that are designed to kill anything but grass).
A long time ago I wrote an article called “Organic Lawn Care for the Cheap and Lazy”. In it I recommend using moldy hay as a fertilizer. In fact, I even claim that moldy hay might very well be the best fertilizer….. IN THE WORLD.
Most hay is ready to become compost if you want to go in that direction: just add water and a bit of rich soil. The carbon to nitrogen ratio is nearly perfect!
But …. when it comes to mulch, I would much rather use hay directly than compost it and then use it as a mulch.