A weed barrier, commonly known as landscape fabric, is a synthetic or natural material laid over soil to block sunlight and prevent weed growth. The fundamental design premise is to suppress unwanted plants while remaining permeable to water and air. This permeability is a deliberate engineering choice to ensure the underlying soil remains hydrated and aerated, which is necessary for the health of planted vegetation. Therefore, a properly installed landscape fabric is explicitly designed to allow water to pass through it.
The Intended Function of Water Permeability
The ability of landscape fabric to let water through is based on its construction as a geotextile, which features millions of microscopic openings. This structural characteristic, known as porosity, allows liquid water molecules to filter through the barrier while preventing the passage of larger solid particles like soil or weed roots. The fabric acts as a separation layer, maintaining the integrity of the soil structure below it. This controlled water passage ensures that moisture from rain or irrigation can reach the roots of desired plants, preventing surface runoff. The porous nature also facilitates the exchange of gases, allowing oxygen to reach the roots for respiration and preventing the soil from becoming anaerobic.
How Material Type Affects Water Flow
The specific construction of the landscape fabric determines the rate and volume of water that can pass through. The two primary types are woven and non-woven fabrics, each offering distinct permeability characteristics.
Woven Barriers
Woven barriers are created by interlacing flat strands of material, typically polypropylene, in a pattern that leaves small, visible gaps at the thread intersections. These defined gaps often result in a faster initial flow rate, making woven fabrics suitable for areas where rapid drainage is a concern, such as under gravel paths. If the weave is too tight, it can restrict the overall volume of water passing through.
Non-Woven Barriers
Non-woven barriers are manufactured by bonding or needle-punching fibers together into a single, felt-like mat. This material has a random, labyrinthine structure of micro-pores rather than defined holes. Water passes through this dense network in a more diffused and slower manner, which is advantageous for reducing runoff and encouraging prolonged soil absorption. Non-woven fabrics are often preferred in drainage applications like French drains because their structure is highly effective at filtration.
Common Reasons Water Stops Passing Through
Despite their design, landscape fabrics frequently experience a reduction or complete failure in water passage due to external factors that compromise their porosity over time.
Clogging
The most frequent issue is clogging, where fine soil sediments, dirt, and decaying organic matter accumulate on the fabric’s surface. As mulch and leaves decompose, the resulting fine particles settle into the tiny pores of the fabric, effectively sealing them shut. This accumulation turns the permeable fabric into a solid barrier that prevents water from reaching the soil.
Hydrophobic Barrier Formation
Another common problem involves the formation of a hydrophobic barrier on top of the fabric, often caused by certain types of top dressings. Degraded organic mulches, particularly aged bark or fine wood chips, can become highly water-repellent when they dry out. This creates a surface layer that causes water to bead up and run off the sides before it can contact the fabric pores.
Soil Compaction
Issues with the underlying soil can also exacerbate drainage problems, leading to the appearance of fabric failure. Heavily compacted soil beneath the barrier, especially clay-rich compositions, naturally restricts the movement of water. Even if the fabric itself is allowing water to pass, the compacted soil below acts as a secondary, impermeable layer, causing water to pool on the fabric’s surface.