Landscape fabric and cardboard are the two most effective barriers to put under mulch

Landscape fabric — the woven or non-woven material sold in garden centers — stops most weeds from pushing through mulch when laid directly on soil before you spread the mulch on top. It lets water and air reach plant roots while blocking light that weed seeds need to sprout. Cardboard (plain, no tape or printing) works the same way and costs less, though it breaks down over one to two years and needs replacing.

Fabric lasts longer — typically three to five years depending on thickness and sun exposure — but costs more upfront. Both work best when overlapped at the seams by at least 6 inches so weeds cannot find gaps. Neither blocks weeds completely; some seeds still land on top of the mulch and sprout there. The barrier stops the ones already in the soil from reaching light.

Plastic sheeting (black or clear) also blocks weeds but traps moisture and heat, which can damage plant roots and kill beneficial soil organisms. It is not recommended for garden beds where you want healthy soil long-term.

Key Takeaways

  • Landscape fabric or cardboard laid on bare soil before mulch stops soil-based weed seeds from sprouting by blocking light.
  • Overlap seams by at least 6 inches so weeds cannot grow through the gaps between pieces.
  • Landscape fabric lasts three to five years; cardboard lasts one to two years and costs less but needs replacing sooner.
  • Mulch alone, without a barrier underneath, does not stop weeds — seeds still sprout in the mulch layer itself.
  • Avoid plastic sheeting because it traps moisture and heat, which damages roots and harms soil health.

How to lay landscape fabric correctly

Clear the bed of existing weeds and debris first. Lay the fabric directly on bare soil, not on top of old mulch. Unroll it across the entire bed, overlapping each strip by at least 6 inches where pieces meet. find the edges with landscape pins, rocks, or soil so wind does not lift it.

Cut X-shaped slits where plants already grow so you can pull the fabric around the stem. For new plantings, cut individual holes for each plant before laying the fabric. Once the fabric is in place and secured, spread mulch 2 to 4 inches deep on top. The fabric stays underneath; you do not see it once the mulch covers it.

Using cardboard as a weed barrier

Flatten cardboard boxes and lay them flat on soil, overlapping edges by at least 6 inches. Remove tape, labels, and any plastic — only plain cardboard works. Wet the cardboard lightly so it stays in place and breaks down faster. Spread mulch 2 to 4 inches deep on top.

Cardboard breaks down within one to two years depending on moisture and soil activity. When it degrades, it adds organic matter to the soil, which improves texture and water retention. You will need to reapply a barrier after it breaks down if weeds return. This makes cardboard a good choice for temporary beds or when you want to build soil over time.

Why mulch alone does not stop weeds

Mulch slows weed growth but does not prevent it. Weed seeds blow in from neighboring yards and land on top of the mulch. If the mulch stays moist — which it usually does — those seeds sprout in the mulch layer itself, not in the soil below. You end up pulling weeds from the mulch surface all season.

A barrier underneath stops the seeds already in the soil from sprouting, but it does not stop airborne seeds. The combination of barrier plus mulch gives you the best control: the barrier handles soil-based seeds, and the mulch makes it easier to pull the few weeds that do sprout on top.

Thickness and type of mulch to use over barriers

Spread mulch 2 to 4 inches deep over the barrier. Thicker mulch blocks more light and makes pulling weeds easier, but too much (over 4 inches) can trap moisture against plant stems and cause rot. Wood chips, shredded bark, and straw all work over barriers. Avoid dyed mulch if you are concerned about chemicals; natural wood chips break down faster but need replacing more often.

Do not pile mulch against tree trunks or plant stems. Leave a 2- to 3-inch gap around each stem so air circulates and moisture does not collect. This prevents rot and pest problems.

Cost comparison: fabric versus cardboard

Landscape fabric costs between $0.10 and $0.30 per square foot depending on thickness and brand. A 50-foot roll of standard-weight fabric runs $15 to $40. Cardboard is free if you save boxes from deliveries or ask grocery stores and appliance shops for flattened boxes.

If you buy cardboard, plain kraft paper rolls cost $10 to $20 for enough to cover a medium bed. Fabric costs more upfront but lasts longer, so the per-year cost evens out over time. For a one-time project or a bed you plan to redesign in a year or two, cardboard makes sense. For permanent plantings, fabric saves money and labor in the long run.

Dealing with weeds that still appear

Even with a barrier, some weeds will sprout — usually from seeds that land on the mulch surface after you lay it. Pull them by hand when they are small, before roots establish. If you pull when the soil is moist, the roots come out more easily.

If weeds are thick, a light layer of additional mulch can suppress new sprouts temporarily, but it does not solve the problem. The barrier underneath is doing its job; the weeds you see are coming from above. Spot-treating with a contact herbicide (one that kills on contact without moving through soil) works on surface weeds, but hand-pulling is safer around plants and does not add chemicals to the bed.

Frequently Asked Questions

Can I put landscape fabric over existing mulch?

No. Fabric works only when it touches bare soil. Lay it before you mulch. If you already have mulch down, pull it back, lay the fabric on soil, and replace the mulch on top. This takes more work but gives you the weed control you want.

How long does landscape fabric last?

Standard-weight fabric lasts three to five years. Heavy-duty fabric (thicker and more expensive) can last seven to ten years. Sunlight breaks down the material over time, so fabric in shaded beds lasts longer than fabric in full sun. Once it tears or degrades, weeds start coming through the holes.

Do I need to remove old fabric before laying new fabric?

You can lay new fabric over degraded old fabric, but pulling out the old material first gives you better results. Old fabric can shift or tear, creating gaps where weeds grow. Removing it takes time but ensures the new barrier sits flat on soil.

Will cardboard hurt my plants?

No. Cardboard breaks down into organic matter that feeds soil. It does not release chemicals or harm roots. Make sure you remove tape and plastic before laying it, since those do not break down.

Can I use newspaper instead of cardboard?

Yes, but use multiple layers — at least five to eight sheets thick — and overlap them well. Newspaper breaks down faster than cardboard, sometimes in just a few months. It works in a pinch but is not as durable as cardboard boxes.