Mulch is a protective layer placed over the soil, primarily intended to benefit the ground beneath it. The water must reach the soil and the plant’s root zone. Watering the mulch itself is rarely the goal, as its main function is to conserve moisture and reduce the frequency of watering. Exceptions exist when the mulch is newly applied or has become water-repellent, preventing water from reaching the soil below.
How Mulch Changes Soil Hydration
Mulch acts as a physical barrier that drastically reduces the rate of water loss from the soil surface through evaporation. Evaporation can account for significant water loss, sometimes up to 50% in un-mulched areas, especially during hot, dry weather. By shading the ground and creating a buffer layer, mulch limits the direct exposure of soil moisture to solar radiation and wind.
The layer of organic material also helps to regulate the soil’s temperature. Mulched soil typically remains cooler in the summer and warmer in the winter compared to bare soil. Keeping the soil temperature more consistent and cooler further reduces the rate of evaporation, lessening water stress on plant roots.
Over time, organic mulches, such as wood chips or compost, break down and improve the soil’s structure. This decomposition adds organic carbon and humus, which increases the soil’s capacity to hold water and nutrients. This improved structure enhances water infiltration, meaning less water is lost to surface runoff and more soaks deeply into the root zone.
Proper Watering Techniques Under Mulch
The presence of mulch necessitates a shift from frequent, shallow watering to a deep, infrequent schedule. Since mulch conserves moisture, the goal is to saturate the root zone, typically the top 6 to 8 inches of soil, and then allow a period of drying before watering again. This encourages the plant to develop a deeper, more robust root system that is less dependent on surface moisture.
To confirm when watering is needed, you must check the moisture level under the mulch layer. The “finger test” involves pushing a finger through the mulch and into the soil. If the soil 2 inches below the surface feels dry, it is time to water.
Effective watering methods focus on delivering water directly to the base of the plant or the soil, bypassing the main mulch layer. Drip irrigation systems or soaker hoses placed beneath the mulch are the most efficient ways to achieve this, delivering water slowly and precisely to the root zone with minimal waste. If using a hose or watering can, aim the water directly at the base of the plant to ensure it penetrates the mulch and reaches the soil beneath.
When to Apply Water Directly to the Mulch Layer
While the general principle is to water the soil, the mulch layer itself requires moisture in two specific scenarios. The first is immediately after applying a new layer of organic mulch, such as fresh wood chips or straw. New organic materials are often very dry and can absorb a considerable amount of water, effectively “stealing” it from the soil until they become saturated.
A thorough initial soaking ensures the new material is saturated, preventing it from wicking moisture away from the soil below. This initial watering also helps settle the material and prevents light organic mulches from being blown away by the wind.
The second situation occurs when an established mulch layer becomes “hydrophobic,” or water-repellent. This happens when certain organic mulches, particularly wood products, decompose and compact over time, forming a waxy crust on the surface. When this crust dries out completely, water beads up and runs off rather than penetrating the layer to reach the soil.
To fix hydrophobic mulch, you must break the surface tension to allow water penetration. A light, direct spray of water over the affected area can sometimes moisten the top layer and allow subsequent, deeper watering to pass through. For severe cases, a commercial wetting agent or soil wetter, which acts like a mild detergent, may need to be applied directly to the mulch surface before watering.