Landscape fabric or cardboard goes under mulch to block weeds and slow decomposition

The layer you put under mulch depends on what problem you're trying to solve. If your main goal is stopping weeds, use landscape fabric (also called weed cloth) or overlapping cardboard. If you want to improve soil as the mulch breaks down, skip the barrier and let mulch contact the earth directly. If you're dealing with a wet spot or compacted ground, add a drainage layer first, then fabric, then mulch on top.

Most homeowners use landscape fabric because it's durable, lets water through, and lasts several years. Cardboard works just as well for one or two seasons and costs almost nothing if you save boxes from deliveries. The choice comes down to how long you want the barrier to last and how much you want to spend.

Key Takeaways

  • Landscape fabric blocks weeds while letting water and air reach the soil below, and typically lasts three to five years before breaking down.
  • Cardboard is free or cheap, stops weeds just as well as fabric for the first season or two, and breaks down into the soil as it decays.
  • Bare soil under mulch lets the mulch feed the soil as it decomposes, which is better for plant health but allows more weeds to grow.
  • Wet or compacted ground needs a drainage layer (gravel or sand) before you add fabric or cardboard, so water doesn't pool under the mulch.
  • Overlap fabric or cardboard by at least 6 inches at the seams so weeds can't push through the gaps.

When to use landscape fabric

Landscape fabric is the standard choice for flower beds, around shrubs, and in areas where you want a long-term weed barrier. It's a thin, woven material that blocks light so weed seeds can't germinate, but water and air still move through it. Most fabric lasts three to five years before UV light and soil microbes break it down enough that weeds start pushing through again.

Buy fabric rated for your climate—thicker fabric (3 to 4 ounces per square yard) lasts longer than thin stuff (1 to 2 ounces), but costs more and is harder to cut and install. For a typical flower bed, 2 to 3 ounce fabric is a good middle ground. Lay it down, overlap seams by at least 6 inches, cut X-shaped slits for plants, and pin it down with landscape staples or rocks before you add mulch on top.

When to use cardboard instead

Cardboard works just as well as fabric for blocking weeds in the first year or two, and it's free if you save boxes from online orders or ask a grocery store for produce boxes. Lay it down in overlapping sheets (overlap by at least 6 inches), wet it so it stays in place, cut holes for plants, and cover it with mulch. As it breaks down over one to three years, it adds organic matter to the soil, which improves drainage and feeds earthworms and beneficial microbes.

The downside is that cardboard eventually falls apart, so you'll need to add more mulch or replace the barrier after a couple of seasons. It also tears easily during installation if the ground is rough or rocky. For temporary beds, newly planted areas, or anywhere you're willing to refresh the mulch regularly, cardboard is the cheapest option.

When to skip the barrier and use mulch alone

If you're planting in an established bed with good soil and you want the mulch to feed the earth as it decomposes, put mulch directly on the soil with no barrier underneath. This lets the mulch break down into humus, which improves soil structure, water retention, and nutrient content. Over time, your soil gets richer and plants grow stronger roots.

The trade-off is more weeds. Without a barrier, weed seeds blow in and land on the mulch surface, where they sprout easily. You'll need to pull weeds by hand or spray them as they appear. This approach works best in small beds where you don't mind doing regular weeding, or in areas where you're replanting every season anyway.

Dealing with wet ground or poor drainage

If the spot where you want to mulch stays wet or has compacted clay soil, add a drainage layer before the fabric or cardboard. Spread 2 to 3 inches of coarse gravel or sand, then lay your weed barrier on top, then add mulch. The gravel lets water drain away instead of pooling under the mulch, which prevents root rot and keeps the mulch from breaking down into a soggy mat.

This is especially important in low spots, near downspouts, or in areas with heavy clay. If you skip the drainage layer and just put fabric and mulch over wet ground, the mulch will stay damp, fungi will grow, and the barrier will break down faster. The extra step takes an hour and costs very little.

How to install a barrier correctly

Preparation matters more than the material you choose. Clear the bed of existing weeds and debris, rake it smooth, and remove any large rocks or sticks that could poke holes in the fabric. If the ground is very compacted, loosen the top inch or two with a garden fork so water can drain.

Lay fabric or cardboard in strips, overlapping each seam by at least 6 inches—this is the most common mistake people make, and gaps let weeds push through. For fabric, use landscape staples every 2 to 3 feet to hold it down. For cardboard, wet it thoroughly so it stays in place and doesn't blow away. Cut X-shaped slits for plants (not circles—the flaps help hold the barrier down). Finally, add 2 to 4 inches of mulch on top, keeping it a few inches away from tree trunks and shrub stems so bark doesn't rot.

Mulch depth and material choices

The barrier underneath doesn't change how deep your mulch should be, but it's worth getting right. Most plants do best with 2 to 3 inches of mulch. More than 4 inches can trap moisture against stems and roots, leading to rot and fungal problems. Less than 2 inches doesn't suppress weeds well and breaks down too fast.

Use whatever mulch suits your landscape: wood chips, shredded bark, pine needles, or compost. Wood chips last longest and look neat. Shredded bark breaks down faster and adds organic matter quicker. Pine needles work well in acidic soil. Compost feeds the soil when ready but breaks down in one season. The barrier material (fabric or cardboard) works the same way regardless of what mulch you put on top.

Frequently Asked Questions

Can I put mulch directly on grass without removing it first?

No. Grass will grow up through the mulch and compete with your plants. Remove the sod first by cutting it away with a spade or smothering it with cardboard for a few weeks. Once the grass is dead, you can lay your barrier and mulch on top.

Do I need to replace landscape fabric every year?

No. Good-quality fabric lasts three to five years. When weeds start pushing through easily, it's time to replace it. Cardboard breaks down faster—usually one to three years—so you'll need to add more mulch or lay new cardboard sooner.

What happens if I don't overlap the seams enough?

Weeds will push through the gaps. Overlap by at least 6 inches so there's no space for light to reach the soil. This is the most common reason barriers fail.

Can I use plastic sheeting instead of landscape fabric?

Plastic blocks weeds but also blocks water and air, so soil stays wet and roots can rot. Landscape fabric is better because it lets water and oxygen through. If you use plastic, drill holes every foot or so to let water drain.

Should I add compost on top of the barrier before mulch?

You can, but it's not necessary. A thin layer of compost (1 inch) under the mulch adds nutrients, but the barrier will eventually break down and feed the soil anyway. Save the compost for spring top-dressing if your budget is tight.