How to Kill Weed Seeds in Hay

Hay brings a lot to the garden. A thick layer of organic mulch such as hay or straw smothers weeds and keeps them from taking over walkways and the spaces between plants. It also holds in moisture so that your plants don’t dry out too quickly, and when tilled in at the end of the season, mulch adds nutrients as it lightens the soil. To make sure your hay doesn’t also bring along its own crop of weeds, treat the hay in the sun using a process called solarization before using it in or near your garden.

Hay bale

Clear plastic

Duct tape

Place a bale of hay in a location near the garden where it will receive full sun every day.

Soak the hay bale completely either by applying a lot of water from one side and letting it soak in or by soaking one side, then rolling the bale and soaking it on the other side as well.

Wrap the wet bale in clear plastic. Roll the bale in the plastic so that all sides are covered.

Tape the ends and seams of the plastic down with duct tape so that the bale is totally sealed inside.

Leave the bale in place for at least two weeks to kill all the weed seeds it contains.

Place the hay bale in a convenient location near your garden. Leave the string on it.

Water the hay until the bale is well-soaked. Leave it alone and allow any seeds in the bale to sprout until there’s an inch or two of green growing on it.

Break the bale apart and spread the hay to kill any seeds that have sprouted.

Substitute straw for hay when possible. Straw is much less likely to contain weed seeds than hay, making it more suitable for many projects such as mulching. It’s still a good idea to use solarization on straw, since straw often contains some grains that will eventually sprout if it’s not treated.

A good soaking and plenty of time in the sun are key elements in ensuring that all weed seeds are killed, even those lurking down in the center of the bale.

In some cases you may be able to find weed-free hay. While such hay may actually contain some types of weed seeds, it has been checked and certified to be essentially free of noxious weeds and undesirable plants.

Hay that has already begun to decompose generally doesn’t need this treatment, since any weed seeds it contains will have already sprouted and died by the time the decomposition becomes obvious.

First-cutting hay typically has more weeds seeds to deal with than hay that is cut later in the season.

The hay bale can be soaked and wrapped anywhere and then moved to a sunny spot, but a heavy, wet bale is harder to move.

Don’t try to rush the process, since if you pull the bale out of the plastic too soon, you’re likely to end up with viable weed seeds left in the center of the hay. Leave it in place at least two weeks.