Lay cardboard or landscape fabric first, then add mulch on top
Mulch alone will not stop weeds from growing through it. Weeds already in the soil will push up through loose mulch, and new seeds will land on top and sprout. The standard method is to create a barrier between the soil and the mulch: lay cardboard, newspaper, or landscape fabric directly over the weeds, then spread mulch over that. The barrier blocks light and prevents seeds from reaching soil, while the mulch holds the barrier in place and breaks down over time.
Cardboard is the cheapest option and works well for one to two seasons. Landscape fabric (also called weed cloth) lasts longer but costs more. Newspaper works in a pinch but tears easily when wet. Whichever you choose, the weeds underneath will die from lack of light within a few weeks, and you will not see them again as long as the barrier stays intact.
Key Takeaways
- Cardboard or landscape fabric must go down before mulch to block weeds from pushing through and new seeds from reaching soil.
- Overlap cardboard sheets by at least six inches and weight them down with mulch or rocks so they do not shift or curl up.
- Spread mulch three to four inches deep over the barrier; thinner layers let weeds break through, and thicker layers can rot plant stems.
- Cardboard breaks down in one to two seasons and needs replacing; landscape fabric lasts three to five years but costs two to three times as much.
- Weeds under the barrier will die within weeks, but new seeds that land on top of the mulch will still sprout unless you pull them regularly.
Prepare the area and choose your barrier material
Before laying any barrier, mow or cut down the existing weeds as short as possible. You do not need to pull them out by the roots—the barrier will kill them—but tall weeds will create lumps under your mulch and make the surface uneven. If the weeds are very thick, a weed trimmer or brush cutter works faster than a mower.
Next, decide between cardboard and landscape fabric. Cardboard is free or nearly free if you collect boxes from stores or deliveries. It breaks down in twelve to twenty-four months, so you will need to replace it or add more mulch on top after a year. Landscape fabric costs $0.50 to $2.00 per square foot depending on thickness, but it lasts three to five years and does not need replacing. For a small garden bed, cardboard is usually the better choice. For a large area or a permanent installation, landscape fabric saves money over time.
Lay the barrier without gaps or overlaps that are too small
If you are using cardboard, remove all tape, staples, and plastic tape from the boxes. Lay the sheets flat on the ground and overlap each piece by at least six inches—this prevents weeds from finding gaps between sheets. Stagger the overlaps so they do not all run in the same direction; think of how bricks are laid in a wall. Wet the cardboard as you go; damp cardboard conforms to the ground better and stays in place more easily than dry cardboard.
For landscape fabric, unroll it across the bed and overlap edges by at least four inches. Landscape fabric is lighter than cardboard, so weight it down with mulch, rocks, or landscape pins as you work. Do not stretch it tight; leave a little slack so it can settle into low spots without tearing.
Around trees and shrubs, cut an X in the barrier material and fold the flaps back, leaving a two-inch gap around the stem. This prevents the barrier from touching the bark, which can trap moisture and cause rot. Do the same around any perennials or plants you want to keep.
Spread mulch three to four inches deep and keep it away from stems
Once the barrier is down and weighted, spread mulch over it. Wood chips, bark mulch, and shredded hardwood are all common choices. Aim for a depth of three to four inches. Thinner mulch (one to two inches) will not block light effectively, and weeds or new seeds may break through. Thicker mulch (five inches or more) can trap moisture against plant stems and cause them to rot, especially in wet climates.
Keep mulch at least two to three inches away from tree trunks and plant stems. Mulch piled against wood promotes fungal growth and insect damage. Create a small ring of bare ground around each plant, then fill the rest of the bed. As the mulch settles and breaks down over the first few months, you may need to add more to maintain the three-to-four-inch depth.
Replace or refresh the barrier as it breaks down
Cardboard typically lasts twelve to twenty-four months before it becomes soft and breaks apart. When that happens, you have two choices: lay new cardboard on top of the old, or add a fresh layer of mulch on top of what is already there. Adding mulch is faster and does not require new cardboard, but it will eventually compact and settle. Most gardeners replace cardboard every year or two.
Landscape fabric lasts much longer—three to five years depending on sunlight exposure and weather. When it finally breaks down, you can pull it up and replace it, or straightforward add more mulch on top. Some gardeners leave old landscape fabric in place and add new fabric over it, though this creates a thick, hard-to-work-through layer that can eventually prevent water from reaching plant roots.
Pull new weeds that sprout on top of the mulch
Even with a good barrier, new weed seeds will blow onto your mulch and sprout on the surface. These are much easier to pull than weeds rooted in soil, because they have no anchor below the barrier. Pull them by hand when they are small, or let them dry out on the surface and they will die on their own. A light layer of extra mulch can bury small seedlings, but do not keep adding mulch indefinitely—you will eventually build up a layer so thick that water cannot reach plant roots.
Check your beds every few weeks during the growing season and remove any weeds you see. This takes only a few minutes and prevents them from setting seed. If you let them grow, they will eventually root into the decomposing cardboard or landscape fabric and become harder to remove.
Frequently Asked Questions
Can I just use mulch without a barrier underneath?
Mulch alone will not stop existing weeds from pushing through, and new weed seeds will sprout on top of it. You will spend more time pulling weeds than you save by skipping the barrier. A barrier is worth the small extra effort.
How long does it take for weeds under the barrier to die?
Most weeds will be dead within two to four weeks once the barrier blocks light. Perennial weeds with deep roots may take longer, but they will eventually exhaust their energy reserves and die. You will not see them growing back as long as the barrier stays intact.
Is newspaper as good as cardboard?
Newspaper works but is not ideal. It tears easily when wet, and it breaks down faster than cardboard—usually within six to twelve months. Use it only if you have no cardboard available, and lay it at least ten sheets thick to give it enough strength.
What if I want to plant something new in the mulched area?
Cut an X through the barrier material where you want to plant, fold back the flaps, and dig a hole through the barrier into the soil. Plant as normal, then fold the barrier flaps back around the stem. The barrier will not harm the new plant's roots as long as you keep it away from the stem itself.
Can I use landscape fabric under a vegetable garden?
Yes, but be aware that landscape fabric can make it harder to amend the soil later. If you plan to add compost or fertilizer to the bed, you will need to pull back the fabric or work around it. Cardboard is often better for vegetable beds because it breaks down and becomes part of the soil over time.