Not Mulch, Not Gravel — The Yard Waste That Is Being Studied For Its Weed Barrier Potential

Weeds in flower or vegetable beds don't just look unsightly — they also rob your plants of water and nutrients. Those who want to avoid the chore of regularly pulling them out generally turn to some kind of mulch, be it landscape fabric (which definitely has its downsides), gravel, grass clippings, or some other natural material. One alternative that's often considered waste is being studied to see how effective it is as a weed barrier: walnut shells (also called hulls).

We already know that some people use walnut shells as one of those food scraps that can fertilize your garden. They're high in phosphate and potassium, which are found in many commercial fertilizers. If used as a mulch, the shells will eventually break down and incorporate these nutrients in the soil.

A thick enough covering of any biodegradable material like straw or bark chips will block out vital sunlight and stop moisture getting to weed seeds already in the ground. It should also help prevent new ones from finding their way through. So why wouldn't a layer of walnut shells have the same effect? While there's a good chance they will, there are a couple of potential drawbacks.

It's usually recommended that walnut shells are broken down rather than left whole when used as mulch. You can buy them pre-crushed for around $2 per pound, but they can, of course, be crushed at home. Our research found that they are a popular mulch around trees on golf courses, so the theory would seem to stand up to practical application.

A common drawback with most organic mulches, walnut shells struggle to combat perennial weeds, which have extensive root systems and strong growth that allows them to simply push their way through this protective layer. The main con, however, is that walnut shells, (in fact, most parts of the tree) contain a toxic chemical called juglone. On one hand, it can prevent certain seeds from germinating, which is great to stop weeds from growing. On the other hand, however, it can also stop some flowers and vegetables from taking in nutrients and water, causing them to shrivel up and die in severe cases.

While there are plants that are more or less immune to juglone, many gardening stand-bys are affected by this chemical. Using walnut shells as mulch in your garden without investigating risks serious harm, so unless you are prepared to spend time checking what you are growing against a number of lists, there are other eco-friendly mulch options you'll probably want to try instead.