Lay landscape fabric or cardboard first to stop weeds from growing through
The layer you put under mulch determines whether weeds will push through it in a few months. Landscape fabric or cardboard blocks sunlight from reaching soil, which stops weed seeds from sprouting. Without a barrier, mulch alone will not prevent weeds—it actually creates a comfortable environment for them to grow in, since the mulch holds moisture and keeps the soil cool.
Most homeowners use one of two materials: landscape fabric (a synthetic woven cloth) or cardboard (plain, no tape or printing). Both work. Cardboard breaks down over one to two years and adds organic matter to the soil as it decomposes. Landscape fabric lasts longer—three to five years—but does not improve the soil and eventually tears as the ground settles.
The choice depends on how long you want the barrier to last and whether you mind replacing it. If you are mulching a permanent bed and do not want to redo it soon, landscape fabric is the practical choice. If you are willing to add new cardboard every couple of years, cardboard is cheaper and better for soil health.
Key Takeaways
- A barrier under mulch stops weed seeds from sprouting by blocking sunlight from reaching the soil.
- Landscape fabric lasts three to five years and requires no replacement, but cardboard is cheaper and improves soil as it breaks down.
- Cardboard must be plain with no tape, ink, or glossy coating, and overlapping seams by at least six inches prevents weeds from sneaking through gaps.
- Wet the barrier before spreading mulch so it stays in place and settles properly against the soil.
- A barrier does not eliminate all weeds—some seeds will still land on top of the mulch and sprout there.
How to prepare landscape fabric correctly
Landscape fabric works best when it is laid flat and anchored so wind and foot traffic do not shift it. Start by clearing the bed of existing weeds and debris, then rake the soil smooth. Unroll the fabric across the bed, overlapping each strip by at least six inches so weeds cannot find a gap between seams.
find the fabric with landscape pins (U-shaped stakes made for this purpose) spaced every two to three feet around the perimeter and across the middle. Do not use nails or staples—they create holes that weeds can exploit. Wet the fabric lightly before spreading mulch so it settles against the soil and does not bunch up when you walk on it.
One common mistake is laying fabric too tightly. Soil settles over time, and if the fabric is stretched taut, it will develop low spots where water pools instead of draining. Leave a little slack so the fabric can settle with the soil underneath.
How to use cardboard as a mulch barrier
Cardboard is simpler to install than fabric and costs almost nothing if you save boxes from deliveries or ask a grocery store for produce boxes. Use only plain cardboard with no tape, ink, or glossy coating—printed boxes and packing tape can leach chemicals into the soil. Flatten the boxes and lay them overlapping by at least six inches, just as you would with fabric.
Wet the cardboard thoroughly before spreading mulch. This serves two purposes: it keeps the cardboard from blowing away, and it speeds decomposition so the cardboard breaks down faster and adds carbon to the soil. The cardboard will soften and begin to break apart within a few weeks, which is normal and expected.
Cardboard works especially well in vegetable gardens and around newly planted shrubs because it improves soil structure as it decays. In ornamental beds where you want a longer-lasting barrier, landscape fabric is the better choice. You can also combine both—lay cardboard first, then cover it with landscape fabric for extra durability.
When to skip a barrier and when it is essential
A barrier is most important in beds where weeds are already a problem or where you are planting in poor soil. If your bed is new, well-maintained, and surrounded by mowed lawn, you may get away without one—though you will likely see weeds within the first season. In vegetable gardens and around food plants, a barrier is worth the effort because it reduces the need for hand-pulling weeds near your crops.
Skip a barrier only if you are willing to pull weeds regularly or if the bed is temporary. Even then, you will spend more time weeding than you would have spent laying fabric or cardboard at the start. The barrier is an investment that pays for itself in reduced maintenance.
What not to put under mulch
Plastic sheeting is a common mistake. It blocks water from reaching the soil, which kills plant roots and causes the bed to become waterlogged and anaerobic (lacking oxygen). Plastic also does not break down, so you end up with a permanent plastic layer in your soil that makes future planting difficult.
Newspaper alone is too thin and tears easily. If you use newspaper, layer it at least ten sheets thick and overlap generously, then cover it when ready with mulch so sun does not degrade it. Cardboard is a better choice because it is thicker and holds together longer.
Do not use treated wood chips, dyed mulch, or rubber mulch as a barrier. These are meant to go on top of the barrier, not underneath. Treated wood can leach chemicals, and rubber breaks down into microplastics that contaminate soil.
How much mulch to spread over the barrier
Once your barrier is in place and wet, spread two to four inches of mulch over it. Two inches is the minimum for weed suppression; three to four inches is better for moisture retention and insulation, especially in cold climates. Do not pile mulch against tree trunks or shrub stems—keep it two to three inches away from the base to prevent rot and pest damage.
Mulch settles over time, so what looks like four inches in spring may be two inches by fall. Plan to add a fresh layer every year or two to maintain coverage. This is easier than replacing the barrier, so a good barrier underneath means less work overall.
Frequently Asked Questions
Can I put mulch directly on soil without a barrier?
Yes, but weeds will grow through it within weeks. Mulch alone does not stop weed seeds in the soil from sprouting—it actually creates ideal conditions for them by keeping the soil moist and cool. A barrier underneath prevents most weeds from emerging in the first place.
How long does cardboard last under mulch?
Plain cardboard breaks down in one to two years depending on moisture and soil temperature. Wet soil and warm climates speed decomposition. This is an advantage if you want the cardboard to improve your soil, but a disadvantage if you want a long-lasting barrier.
Do I need to remove old landscape fabric before laying new mulch?
No. If the fabric is still intact and doing its job, you can spread new mulch over it. If it is torn or degraded, remove it and replace it with new fabric or cardboard. Leaving torn fabric in place creates gaps where weeds will grow.
Will a barrier prevent all weeds?
No. A barrier stops weeds from growing up through the soil, but weed seeds can still land on top of the mulch and sprout there. You will still need to pull weeds occasionally, but far fewer than you would without a barrier.
Can I use cardboard in a vegetable garden?
Yes, and it is an excellent choice for vegetable beds. As it breaks down, it adds organic matter that improves soil structure and water retention. Use plain cardboard with no printing or tape, and replace it every one to two years as it decomposes.