Many gardeners look for budget-friendly alternatives to traditional gardening supplies, often considering a plastic tarp as a substitute for landscape fabric to suppress weeds. Landscape fabric is typically a woven or non-woven textile designed to cover the ground while still allowing moisture and gases to pass through. Conversely, a tarp is generally a heavy-duty sheet made of high-density polyethylene or similar materials engineered to be waterproof and completely impermeable. Understanding the fundamental differences in material function is necessary to determine if this substitution is practical.
The Critical Difference: Water and Air Permeability
Landscape fabrics are specifically engineered with tiny pores in their woven or spun structure, allowing rainfall and irrigation water to filter slowly through to the soil below. Most commercial fabrics have a measured permeability rate designed to manage moisture without causing saturation, which is necessary for root health. A standard plastic tarp, however, is made from non-porous, high-density polyethylene and acts as a complete barrier to liquid water.
When a tarp is used, water cannot penetrate the surface, leading to pooling and runoff in surrounding areas rather than absorption into the covered ground. This surface pooling can become a breeding ground for certain insects and can promote the growth of surface molds on the tarp itself. The impermeable barrier drastically alters the local moisture dynamics both above and below the soil line.
Beyond water, landscape fabric permits the exchange of atmospheric gases, which is necessary for healthy root respiration and soil organism activity. This gaseous exchange includes the release of carbon dioxide produced by roots and microbes, and the intake of fresh oxygen. A plastic tarp prevents this necessary air exchange, trapping humidity and gases like methane and sulfur compounds beneath its surface.
This trapped moisture and lack of ventilation can create an environment conducive to anaerobic decomposition and fungal growth immediately below the barrier. The constant, saturated environment created by trapped moisture under the tarp quickly leads to the suffocation of any underlying plant roots. This restriction demonstrates why the materials are not interchangeable for active planting beds.
Long-Term Impact on Soil Health
The lack of gas exchange caused by an impermeable tarp creates anaerobic conditions within the soil after only a short period. This environment, devoid of free oxygen, immediately stresses and kills beneficial aerobic microorganisms, including bacteria and fungi responsible for breaking down organic matter and cycling nutrients. Instead of healthy decomposition, the soil begins to undergo fermentation and putrefaction. The healthy, earthy smell of soil is quickly replaced by the sour, rotten odor of these harmful anaerobic processes.
Larger soil life, such as earthworms and beneficial nematodes, require oxygen to survive and thrive. By suffocating the soil, the tarp drives these organisms away or causes their death, effectively halting their roles in soil aeration and nutrient distribution. Earthworms are particularly important for creating tunnels that improve soil structure and water infiltration, functions that cease under the plastic barrier.
When the soil biology dies off, the natural processes that maintain soil structure—like the creation of stable aggregates held together by microbial secretions—are disrupted. Without the constant mixing and tunneling activity of earthworms and other fauna, the soil beneath the tarp becomes heavily compacted and dense. This lack of structure makes it extremely difficult for future plant roots to penetrate the profile and severely limits water infiltration once the plastic barrier is eventually removed. This biological disruption leads to a failure in nutrient cycling. Prolonged tarp use essentially sterilizes the top layer of soil, leaving behind a biologically dead substrate that requires significant amendment and time to recover its fertility.
When is a Tarp Acceptable?
Despite its drawbacks for garden beds, a tarp is highly effective when used temporarily for soil solarization. This technique involves covering moist soil with clear or black plastic during the hottest months to trap solar heat, raising the soil temperature high enough to kill weed seeds and certain soil pathogens. This intense heating process is typically performed over a period of four to eight weeks before any planting occurs.
A plastic tarp is also an acceptable barrier when preparing a base for non-living structures like patios, sheds, or walkways where soil health is irrelevant. For long-term weed suppression in active areas where budget is a concern, permeable, biodegradable alternatives are better. These include thick layers of organic mulch, overlapping sheets of cardboard, or multiple layers of newspaper. The key distinction remains that any barrier placed over living soil must allow for gas and water exchange.