Mulch
Any material applied to the soil surface to retain moisture, suppress weeds, regulate soil temperature, or add organic matter. Common types include wood chips, straw, leaves, plastic film, and compost.
Mulch is any material laid on the soil surface around growing plants. It doesn’t go into the soil - it stays on top. The benefits are primarily physical: moisture retention, weed suppression, temperature moderation, and erosion prevention. Organic mulches also decompose over time, adding organic matter to the soil surface.
There is no single best mulch. The right material depends on what you’re growing, the goal, and what’s available locally.
How Mulch Works
Moisture retention: Bare soil loses water to surface evaporation continuously. A 2-3 inch layer of mulch creates a physical barrier that reduces evaporative loss dramatically - studies on mulched vs. unmulched vegetable beds show 25-50% reduction in irrigation needs under similar conditions. The soil beneath stays consistently moist rather than cycling through wet and dry.
Weed suppression: Weed seeds germinate in response to light. A 2-3 inch opaque mulch layer blocks light from reaching seeds in the soil and prevents germination. Established perennial weeds with existing root systems can push through mulch; newly arriving airborne seeds will germinate on the mulch surface if it’s fine enough to support seedling establishment. Maintain mulch depth as it compresses.
Temperature moderation: Mulch insulates soil from temperature extremes. In summer, a mulched soil surface stays significantly cooler than bare soil in full sun. In spring and fall, mulch slows the rate of temperature change. For strawberries and many perennials, a winter mulch layer of 3-4 inches of straw protects roots from freeze-thaw cycles.
Organic Mulches
Wood chips: The most durable organic mulch. Chip size matters: fine chips (screened wood chip mulch) may form a crust; coarser chips (2-3 inch) allow water infiltration while persisting 2-3 years before breaking down. Arborist wood chips are available free in many areas. Best for fruit tree guilds, perennial beds, and pathways.
Straw: Clean wheat or oat straw (distinguish from hay, which contains weed seeds). Loose, easy to apply, decomposes in one season. Good for annual vegetable beds and strawberry winter protection. Tends to blow in wind; needs anchoring in exposed sites.
Shredded leaves: Excellent mulch from a waste stream most gardeners have available. Run over with a lawn mower to shred before applying; whole leaves mat into impermeable layers. Decomposes in one season; adds organic matter. Free.
Compost: Applied 1-2 inches thick, compost functions as both mulch and soil amendment as it decomposes into the surface layer. Less effective for weed suppression than coarser materials because it’s fine enough to support weed seed germination on the surface.
Pine needles: Slightly acidic as they decompose; useful around blueberries, rhododendrons, and other acid-loving plants. Good moisture retention. Free where pines are present.
Plastic and Synthetic Mulches
Black plastic film: Warms soil (absorbs solar radiation), suppresses weeds completely, reduces soil moisture loss. Standard in commercial pepper and melon production. Requires drip irrigation underneath. Not reusable. At end of season, plastic must be removed or it persists in the soil and environment.
IRT (infrared transmitting) plastic: Transmits light in the infrared spectrum, which warms soil faster than black plastic, while blocking the visible wavelengths that weed seeds need to germinate. More expensive than black plastic; not widely available to home gardeners.
Landscape fabric: Woven or spun-bond fabric that suppresses weeds while allowing water and some air through. Appropriate for pathways and around perennial plantings. Not recommended for vegetable gardens: soil organic matter cannot build under it, it clogs with organic matter over time, and removal is difficult.
Application Depth
2-3 inches is the standard depth for most annual vegetable gardens. Less than 2 inches provides inadequate weed suppression and moisture retention. More than 4 inches can impede water infiltration and create habitat for pests and excess fungal growth in some conditions.
Keep mulch pulled back 2-3 inches from direct stem contact. Mulch in contact with stems creates persistent moisture at the crown and can contribute to crown rot, particularly in tomatoes, peppers, and cucurbits.