A homeowner asking for advice on the subreddit r/landscaping was immediately warned against using a type of mulch, which commenters clarified was extremely harmful.
"Good idea to use cardboard as weedblock and cut holes for shrubs/flowers?" the homeowner asked. "Plan to use rubber mulch over cardboard."
While the commenters were generally in support of the cardboard method, everyone was quick to recommend strongly against using rubber mulch.
"Yes to your cardboard question … but please don't use rubber mulch. It's terrible for the environment," one person responded.
Others agreed. "Actual real mulch will add nutrients to the soil reducing your fertilization needs. Rubber mulch leeches crap into the soil (particularly zinc) and is a fire risk. If you want no-maintenance "mulch" use stone," another wrote.
They're correct — rubber mulch is hazardous to humans, plants, and the broader environment.
Rubber mulch is made of shredded tires. In the course of their functional lives, these tires are exposed to a number of heavy metals, particularly zinc. When those metals then leach from the shredded mulch into soil, the zinc levels are often high enough to kill plants before they can reach maturity.
Rubber mulch is also highly flammable, burning hotter and faster than wood mulch, and unfortunately, once it's been spread in an area, it can be incredibly difficult to remove.
"It is abundantly clear from the scientific literature that rubber should not be used as a landscape amendment or mulch," wrote researcher Linda Chalker-Scott, Ph.D. "There is no question that toxic substances leach from rubber as it degrades, contaminating the soil, landscape plants, and associated aquatic systems."
Fortunately, there are many superior alternatives to rubber mulch.
Choosing to rewild your yard instead with native plants creates a healthy environment for humans, pollinators, and plants alike — which is especially important if you're hoping to use your garden to grow your own food.
For all of these reasons, commenters were united in their argument against rubber mulch. "Top it with regular mulch. It's much easier to refresh it every couple of years as it slowly breaks down and improves the soil rather than picking pieces of slowly degrading … tire bits out of your grass while it leeches some nasty crap into your soil," another advised. "I wish I was being hyperbolic here."
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