If you want to prevent weeds in your garden, one common solution is to use a weed mat. This barrier is made of a porous plastic material that allows water to move down into the soil, but prevents new weeds from growing up into the garden. A weed mat can indeed be useful in many situations, but it is not a one-size-fits-all solution by any means!
A 3” layer of bark mulch will also prevent weeds in your garden, because seeds cannot penetrate the mulch and take root in the soil. You might think that a combination of both methods would be idea, because with a weed mat and bark mulch, you could prevent weeds from invading your garden from both above and below. And yet, we typically advise against this course of action for one very important reason.
Remember that another function of bark mulch is to provide nutrients to your growing plants. As the mulch decomposes, it releases valuable nutrients into the soil. If a weed mat is present underneath the mulch, it will actually block this important action. Your plants won’t receive those important nutrients.
Weed mats can also be problematic in gardens which include a lot of annuals and perennials. These types of plantings like to spread as they grow, and the weed mat could potentially strangle them. Another possible outcome is that the weeds grow underneath the weed mat, and push it upwards in the garden, creating even more problems for you because removal of weed matt is required in order to remove the weeds.
If mulch is needed in a garden space, we prefer not to combine it with a weed mat. But weed mats can indeed be useful underneath gravel (where there are no plants included). For example, if you want to create a maintenance free side yard, you might use a gravel or rock base without plants. In this case, a weed mat might be a great solution but over time the weed matt usually pokes through the gravel creating an unsightly mess.
The bottom line is that weed mats have their uses, but don’t become overzealous! Just because a landscaping tool is useful in one circumstance does not mean it will work well in other situations.