Could Hay Mulch Poison Your Garden?

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There are few garden subjects I am more passionate about than mulch. It prevents weeds and regulates moisture in the soil, among its other benefits.

But which mulch should you use? Perhaps the most common is shredded leaves, but what if you’re like me and you don’t have access to deciduous trees? Then what are your options?

I’ve used wood chip mulch, hay mulch, straw mulch, and pine needle mulch, and I talk about the pros and cons of each type of mulch here.

But after talking to Jill Winger of the Prairie Homestead, I realized using hay as mulch isn’t without its risks. In this episode of the Beginner’s Garden Podcast, Jill talks about how deep mulching with hay poisoned her garden. Click below to hear our conversation or read on to find out what happened, how she identified the problem, and what advice she gives to other gardeners considering using hay to mulch their gardens.

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Using Hay in a Deep Mulch Method

After struggling for years with weeding her garden, Jill discovered a book by Ruth Stout called Gardening Without Work. In this book, Ruth shared a gardening method called Deep Mulch.

In Deep Mulch Gardening, the gardener mounds several feet of straw or hay over the soil and then separates the mulch to plant in rows. This thick layer of mulch prevents weeds seeds from germinating, and the straw or hay breaks down and contributes to the nutrients in the soil. 

Hay as Deep Mulch

Some people question whether hay introduces seeds to the garden, but Ruth contended that if the hay is deep enough, this will not occur. It worked wonderfully for Jill for a couple of years, and she touted the amazing benefits of deep mulch in her garden.

But then, in the the third year, Jill’s garden didn’t grow as it had in the past. Her tomato plants became stunted and curled, while the rest of her garden ceased to thrive. 

Identifying Hay as the Source of the Problem

Jill set out in vigorous research to find out what happened to her garden.

After eliminating several possibilities, she finally determined the hay mulch was the source of the problem. She learned that the hay she used had been sprayed with an herbicide, and because of using such a high quantity of hay, the herbicide eventually built up in her soil and began poisoning her vegetable garden.

Tomato Leaf Curling - hay mulch

Other Causes of Tomato Leaf Curling

When it comes to soil poisoning, tomatoes are usually the first vegetables to signal a problem. As Jill researched the possible causes of her issues, she found many different causes of tomato leaf curling that do not result from herbicide-tainted hay.

If you find your tomato leaves are curling, you don’t have to assume that your mulch was contaminated. Here are some other possible causes to explore.

  • Too much moisture or not enough moisture. The first thing to rule out is to check the moisture of the soil. The leaves can curl if it’s either too wet or too dry and sometimes they can appear yellow.
  • Tomato or Cucumber mosaic virus. These diseases can also cause curling but you will also notice spots or patterning on the leaves.
  • Broad mites. This pest can invade the garden. If you look closely, you can see their eggs.
  • Too much salt. Some cattle manure, mostly from CAFO farms (concentrated animal feeding operations), can have a high salt concentration and can cause problems in your garden. If you want to use this manure, allow it to sit uncovered in the elements, and some of the salt will leech out of it.

To compare your leaf curling to Jill’s experience, click here to view photos of her tomato issues. Herbicide-tainted hay causes a unique distortion that’s hard to miss.

Hay Mulch in Garden

Should You Use Hay as Mulch in Your Garden?

Be very careful what you put on your garden. Using organic hay would be a better option, or at the least hay that you can be certain has not been sprayed with herbicide, specifically one with aminopyralid.

Jill also believes her problem was a combination of the amount of hay and the repeated use of the deep mulch method, which is why many gardeners use hay without the sudden problems that Jill had.

UPDATE: In the 2019 garden year, I (Jill M) chose to hill my potatoes with hay since it had prevented early blight in my past gardens. I had extra hay so I decided to mulch some of my tomatoes with it for the same reason. Two weeks after application, I started noticing this about those tomato plants:

tomato leaf curl due to herbicide poisoning

hay herbicide poisoning on tomato leaf curling

My other tomato plants — not mulched with hay — showed none of this leaf curling and twisting. I quickly removed all the hay from the tomatoes and potatoes, and I’ve learned my lesson. Herbicide-tainted hay can poison a garden more quickly than I realized. 

Before you add hay to your garden, consider Jill’s experience both of our experiences. Ask questions and use caution.

Prairie Homestead Cookbook

The Prairie Homestead Cookbook

Jill Winger of the Prairie Homestead Blog is also the author of the newly-released Prairie Homestead Cookbook

To purchase your copy of The Prairie Homestead Cookbook, click here. (On a personal note, this has become my all-time favorite cookbook!) Then after your purchase, go to and enter your receipt number to get a packet of bonuses!

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