A layer of mulch on a garden bed serves multiple purposes, including providing a neat appearance, moderating soil temperature, and retaining moisture. While mulch offers moderate weed suppression by blocking sunlight, many gardeners place a material beneath it to enhance this function or improve soil health. This underlying layer acts as a foundation, either as a physical barrier to prevent weed emergence or as a source of nutrients. Selecting the right underlayer depends on the specific goals for the space and the desired longevity of the application.
Preparing the Area for Mulch
Proper preparation is necessary to ensure the underlayer and mulch perform effectively. Before laying down any barrier, all existing weeds must be manually removed, including their root systems. For stubborn perennial weeds, a non-chemical spot treatment may be applied, taking care to avoid affecting desirable plants.
Defining the edges of the planting bed helps contain the mulch and underlayer, establishing a clear line between the garden and surrounding areas. It is also beneficial to water the area lightly before application. A moist base is conducive to the function of both soil amendments and biodegradable barriers, maximizing effectiveness against future weed growth.
Materials Used for Weed Suppression
The most common reason for using an underlayer is to create a robust defense against weeds, accomplished using either synthetic or organic barriers. Synthetic options, such as woven landscape fabric, provide a long-lasting physical block that allows water and air to pass through, though often at a reduced rate. Over time, decomposing organic mulch can clog the fabric’s pores, impeding the flow of moisture and oxygen to the soil below. Furthermore, as the top mulch decomposes, it creates a new layer of soil on top of the fabric, where wind-blown weed seeds can easily germinate and take root.
An impermeable option, like heavy-duty plastic sheeting, is reserved for solarization, a temporary process that uses intense heat to eliminate weed seeds and pathogens. Unlike fabric, this material completely blocks all air and water exchange and is not appropriate for beds containing plants intended to thrive long-term.
Conversely, organic barriers like corrugated cardboard or newsprint offer a temporary, biodegradable solution that smothers existing weeds by blocking light. These materials must be laid down in thick, overlapping layers—a minimum of a single layer for cardboard or about ten sheets for newspaper—and wetted thoroughly to keep them in place. They decompose naturally within a season or two, adding carbon to the soil and improving its structure, but they require reapplication more frequently than synthetic materials.
Pre-Mulch Soil Amendments
Some underlayers are applied to directly enrich the soil before the protective mulch is added, rather than focusing on weed-blocking. High-quality compost serves as an excellent foundational layer, supplying organic matter and microbial life to the soil. Applying a layer one to two inches deep introduces slow-releasing nutrients that improve soil structure, increasing its capacity to hold both air and moisture.
Aged manure, specifically well-rotted cow, horse, or poultry manure, can also be used to provide a concentrated boost of nutrients, particularly nitrogen, phosphorus, and potassium. It is important to ensure the manure is fully composted, as fresh manure can burn plant roots and may contain viable weed seeds. Targeted slow-release fertilizers or mineral amendments may be lightly incorporated into the topsoil based on the results of a soil test. These amendments address specific deficiencies, offering a focused nutrient supply to support plant establishment and long-term health beneath the mulch layer.
When to Apply Mulch Directly to Soil
In certain situations, adding material beneath the mulch is unnecessary or counterproductive. Applying a thick layer of organic mulch directly onto well-prepared soil is sufficient for weed suppression in areas already free of persistent weeds. This approach allows the mulch to break down naturally at the soil surface, integrating humus and nutrients directly into the topsoil.
For annual vegetable gardens or flower beds, where the soil is turned over and replanted every season, installing a permanent barrier like landscape fabric is impractical. A physical barrier can also impede the necessary exchange of gases and moisture in areas requiring frequent soil aeration or deep watering. Forgoing an underlayer allows the natural decomposition process to continuously improve soil tilth and fertility.