When asked if you should use weed fabric under mulch, I think of P.T. Barnum. Before P.T. Barnum was known for circuses (circii?), he operated the American Museum in New York City. While it had some legitimate exhibits, like taxidermied animals, Revolutionary War relics, and even whales in a tank in the basement, he also exhibited things like a “mermaid skeleton”. He also, quite famously, labeled a door “this way to egress” which led people – who didn’t know the word, so they thought it was an exhibit – right out onto the sidewalk.
Were P.T. Barnum alive today, he would be quite the weed barrier fabric salesman.
What happens when you use weed fabric under mulch
The use of weed fabric under mulch seems like a no-brainer, doesn’t it? Roll out the fabric, put down a healthy 4” of an organic mulch, and spend the summer sipping juleps on the back porch while the dog mows the lawn.
Weed fabric doesn’t stop weeds
The idea that when you use weed fabric under mulch you’re totally blocking weeds relies on a few flawed assumptions. It assumes weeds are ONLY coming up from under the fabric, and it assumes that the fabric itself is virtually bulletproof. The reality is that weed fabric does nothing to stop weed seeds, carried by birds or the wind, from landing in the mulch above the fabric and taking root. Also, the fabric can get small rips during installation, and you have to cut holes in it anyhow to accommodate plants. The longer weed fabric is in place, the more it gets torn up.
What ends up happening is the weeds root into the fabric. Not only are you still pulling weeds, but now it’s harder because they have something to grab onto. And by pulling weeds rooted through fabric, you can either tear the fabric or even pull up big chunks of it.
Weed fabric is bad for your plants and your soil
“Dave, Dave, quite contrary, how does your garden grow?”
Well ideally, my plants have plenty of room to spread out, and the soil is constantly getting enriched from my organic mulch breaking down and being worked into the soil from all the insects and microbes that live there. If you use weed fabric under mulch, that’s not happening. Organic matter can’t pass through the fabric. Many fabrics are so tightly woven that it’s even difficult to get water to pass through to the soil. That results in fewer beneficial critters in the soil, which means plants aren’t getting the nutrients they need.
Weed fabric can also choke out woody plants if there’s not enough space around the trunk to allow for growth. The fabric also makes it harder for perennials to spread and thrive. In my planting designs I use perennials and groundcovers that will fill in, reduce the amount of mulch needed, and out-compete weeds. That’s called working smarter!
How do I stop weeds then?
The best way to keep weeds down is by mulching with a heavy (4 to 6 inches) layer of organic mulch, being sure to keep the mulch away from trunks. Good organic mulches include shredded hardwood mulch, leaf compost, or arborist wood chips. If you stay on top of things and weed before they can get established, it’s really not that hard.
You can also use a granular pre-emergent herbicide, installed before you mulch, to help keep weeds down. A pre-emergent works by preventing weed seeds from germinating, so it won’t do anything to weeds that have already sprouted. Just read the manufacturer’s instructions on the label for frequency, application rate, and anywhere that you shouldn’t use it.
What if I’m using a gravel or rock mulch?
If you’re in, say, the Southwest, do what you have to do. Here in Virginia though, I don’t know why you would want to. I know people think an inorganic mulch (rock, tumbled glass, rubber mulch) is going to be way easier in the long run, but it’s not. As I’ve explained, weeds are still going to pop up on, under, and through the weed fabric. Now, though, you’re tearing up your hands trying to pull weeds out from between pieces of gravel.
When we create a dry streambed, we do use a barrier fabric underneath the river jack. It’s a very thick filter fabric, though, one that’s used under paver patios. Using that as a weed fabric would be a terrible idea.
Bottom line: at ten to twenty bucks a roll, plus the time it takes to install something that actually makes your gardening life worse, don’t do it. If that money’s burning a hole in your pocket I’m sure you can find a better way to spend it!
By the way, if you want to learn more about P.T. Barnum and all the crazy stuff he did, The Dollop did a hilarious podcast episode on the subject here.