Plastic barriers are often considered for long-term weed suppression. Plastic can be used to block weeds, but the material choice determines the outcome for the soil and surrounding plants. The term plastic covers a range of materials with vastly different characteristics regarding water flow, air exchange, and overall garden health. Understanding the distinction between these options is necessary for making an informed decision for your landscape.
Distinguishing Between Barrier Types
Plastic weed barriers are categorized by their permeability: impermeable sheeting and permeable fabric. Impermeable sheeting is a solid, thin film typically made from polyethylene, often called plastic mulch or black plastic. This material is completely non-porous, blocking light, water, and air from reaching the soil below. It works by smothering weeds and retaining soil moisture for short-term agricultural applications.
Permeable fabric, commonly known as landscape fabric, is a woven or non-woven geotextile usually made from polypropylene. Unlike solid sheeting, this material allows air and water to filter through its structure while blocking sunlight, preventing weed seeds from germinating. This durable barrier can last multiple seasons, making it a popular choice for permanent landscape beds or under hardscaping.
Negative Impacts of Impermeable Sheeting
The use of solid, impermeable plastic sheeting carries significant drawbacks for soil health. By blocking water penetration, the plastic causes runoff and prevents rainfall from reaching plant roots. This necessitates installing drip irrigation systems beneath the sheet, as overhead watering will not pass through the barrier.
The lack of air exchange beneath the sheeting starves the soil of oxygen, which is detrimental to microbial activity and nutrient cycling. This anaerobic environment can lead to soil compaction and a decline in the overall quality of the growing medium. Shallow-rooted plants are vulnerable because their feeder roots may grow directly beneath the plastic, making them susceptible to drought and heat stress.
The dark color of black plastic absorbs solar radiation, which can superheat the soil, sometimes by as much as 20°F. While this effect can warm the soil early for heat-loving crops, it risks thermal stress during peak summer. The sheeting creates a temporary, artificial environment that ignores the biological needs of a healthy soil ecosystem.
Comparing Landscape Fabric and Organic Mulch
Long-term weed control often involves choosing between permeable landscape fabric or natural organic mulch, such as wood chips or straw. Landscape fabric offers superior longevity, often lasting several years, which reduces the need for reapplication. However, the fabric must be covered with a layer of mulch or decorative stone to protect the synthetic material from ultraviolet (UV) light degradation.
Organic mulch requires annual or biennial replenishment as it naturally breaks down and decomposes. This decomposition adds organic matter and nutrients back into the soil, improving its structure and supporting beneficial earthworms and microorganisms. Landscape fabric, being an inert, synthetic material, offers no nutritional or structural improvement to the soil below.
Both methods are effective for weed suppression, but they work differently. Landscape fabric blocks the growth of aggressive perennial weeds and rhizomes from below. Its long-term effectiveness is compromised when organic debris, dust, and decomposed mulch accumulate on top, creating a shallow layer where weed seeds can germinate. Organic mulch, applied at a depth of three to six inches, suppresses annual weeds by blocking light and creating a physical barrier while enriching the soil beneath.
Installation and End-of-Life Management
Proper site preparation is necessary before installing any plastic barrier. The area must first be cleared of existing weeds, debris, and sharp stones to prevent punctures and ensure the material lies flat against the soil. Once the barrier is laid out, sections must be overlapped by 6 to 12 inches to prevent weeds from exploiting the seams. The material should be secured using specialized landscape staples or pins.
For planting, small X-shaped cuts are made in the material where plants will be inserted, ensuring the opening is only as large as necessary to limit weed access. Impermeable plastic is typically a single-season product that must be removed entirely after harvest. Landscape fabric is designed for multi-year use but eventually breaks down and must also be removed.
A primary concern for both types of plastic is their end-of-life management, as they are not easily recyclable through standard municipal programs. Polyethylene sheeting often fragments into microplastics in the soil, which are nearly impossible to remove and can remain in the environment for decades. Landscape fabric, made of polypropylene, is also difficult to recycle due to contamination with soil and organic material, meaning both plastic barriers usually end up in a landfill.