Mulch is a protective layer of material, such as wood chips or gravel, spread over the soil surface. It serves multiple purposes, primarily retaining soil moisture, regulating temperature, and improving aesthetics. To significantly enhance weed control, a physical barrier is often placed directly on the soil underneath the mulch layer. This underlayment works by blocking the sunlight needed for weed seeds to germinate and grow and preventing perennial weeds from pushing through.
Preparation: Clearing and Grading the Area
A successful weed suppression barrier begins with thorough ground preparation. The first step involves completely clearing the area of all existing weeds, debris, and stones to create a smooth surface. Large perennial weeds must be removed entirely, either by hand-pulling the root system or by applying an herbicide and waiting until they are dead.
After clearing, the soil should be lightly raked to remove remaining small organic material and to level the ground. A smooth, even surface ensures the barrier material lies flat, preventing air pockets or wrinkles where weed seeds could settle. Proper grading, ensuring a gentle slope away from structures, also helps maintain good drainage beneath the barrier.
This preparation is important because a clean, level base prevents the underlayment from shifting or tearing, which would create weak points for weeds to exploit. Skipping this initial step can severely compromise the longevity and effectiveness of the entire mulching system.
Primary Underlayment Options for Weed Suppression
The choice of underlayment material depends on the specific needs of the landscape and the desired lifespan of the weed control. Landscape fabric, often made of woven or non-woven polypropylene, is a popular choice for long-term applications like flower beds or around trees. Woven fabric is generally more durable, making it suitable for use under heavy materials like gravel or stone mulch.
A key advantage of landscape fabric is its permeability, allowing air, water, and nutrients to pass through to the soil below. However, a significant drawback is that the fabric eventually collects fine dirt and decomposed organic matter on its surface, providing a new medium for airborne weed seeds to germinate. When weeds root into the fabric, they become difficult to remove, often resulting in the fabric being torn during weeding.
For vegetable gardens or areas prioritizing soil health, biodegradable options like cardboard and newspaper are highly recommended. Cardboard, especially unprinted, non-waxed sheets, can be laid directly over the ground to smother existing weeds by blocking sunlight. This material decomposes over a single season, improving soil structure and attracting beneficial organisms like earthworms.
Newspaper functions similarly to cardboard but requires multiple layers, typically four to five sheets thick, to achieve light blockage. Both cardboard and newspaper must have all plastic tape and glossy inserts removed before application to ensure clean decomposition. Unlike landscape fabric, these organic barriers break down completely, requiring annual reapplication but avoiding the long-term issue of plastic residue in the soil.
Plastic sheeting, usually a thick black polyethylene film, offers excellent initial weed suppression by completely blocking light and water. Its primary use is often in vegetable gardening to warm the soil for heat-loving crops. However, plastic is discouraged for permanent landscape beds because its impermeability can lead to waterlogging or excessive dryness beneath. The lack of water and air exchange creates an unhealthy environment for permanent plantings. Furthermore, the plastic does not decompose, creating a challenging removal process later on.
Installation Techniques for Effective Barrier Placement
Proper installation ensures the underlayment material performs its function without creating pathways for weed growth. When using rollable material like landscape fabric, pieces must be overlapped significantly, typically by 6 to 12 inches at the seams, to prevent weeds from finding the gap. The fabric is secured using U-shaped landscape staples or pins, pushed through the material and into the ground every few feet, with extra pins placed along seams and edges.
For areas with existing plants, the barrier material should be cut carefully to fit snugly around the base of the stems. The best method involves cutting a small X-shaped slit where the plant will emerge, rather than a large circle, which minimizes the exposed soil area. After planting a new specimen, the flaps of the X-cut should be folded back down to lie close to the stem.
When using cardboard or newspaper, the layering technique is important for creating an effective barrier. The sheets must overlap generously, with edges layered several inches over one another to avoid any exposed soil. Wetting down the material after laying it helps it conform to the soil contours and prevents shifting by wind while the mulch is being applied.
The final step for any underlayment is to cover it promptly with a layer of mulch, which protects the barrier from sunlight and degradation. A mulch layer of at least two to three inches is recommended to completely hide the underlayment and add to the weed suppression effect. For edges, the material can be tucked into a shallow trench dug around the perimeter or secured with edging material.