How to Refresh Mulch in Your Garden Beds

Why mulch matters

Mulch does four things that bare soil cannot do on its own.

It retains moisture. A 2 to 3 inch layer of mulch reduces soil evaporation by up to 70 percent, which means less watering through the summer. Bare soil in direct sun dries out within a day of watering. Mulched soil stays moist for several days.

It suppresses weeds. Mulch blocks sunlight from reaching weed seeds in the soil. Seeds that do not get light do not germinate. A consistent mulch layer eliminates most annual weeds without herbicides.

It regulates soil temperature. Mulch insulates roots from temperature swings. In summer it keeps soil cooler. In winter it prevents the freeze-thaw cycling that heaves shallow-rooted plants out of the ground.

It improves curb appeal. Fresh mulch makes garden beds look clean and intentional. It is one of the cheapest ways to make the front of a house look maintained. A $30 mulch refresh has more visual impact than most landscaping projects ten times the price.

How to calculate how much mulch you need

Measure the length and width of each bed in feet and multiply to get the square footage. For irregular shapes, break the bed into rough rectangles and add them together.

For a 2-inch depth, you need about 0.17 cubic feet of mulch per square foot of bed. A standard bag of mulch is 2 cubic feet, which covers about 12 square feet at 2 inches deep.

A quick formula: divide your total square footage by 12 to get the number of bags you need. A 100-square-foot bed takes about 8 to 9 bags. At $3 to $5 per bag, that is $25 to $45.

If you have more than 200 square feet of beds, buying mulch in bulk by the cubic yard is cheaper. One cubic yard covers about 160 square feet at 2 inches deep. Most landscape supply yards sell bulk hardwood mulch for $25 to $45 per cubic yard and will deliver for a fee.

The mulch volcano mistake

Piling mulch into a cone around a tree trunk is the single most common mulching error. Landscapers do it because it looks tidy. It kills trees slowly.

Mulch volcanoes hold moisture against the bark, which causes the bark to soften and decay. Fungi and boring insects move into the weakened bark. The tree responds by growing roots into the mulch mound instead of into the soil, and those roots eventually girdle the trunk, strangling the tree's own vascular system.

The fix is simple. Pull mulch back to create a flat, donut-shaped ring around the tree. Leave a 6-inch gap between the mulch and the trunk. The mulch should never touch bark. This applies to shrubs and perennials too. Keep a 3-inch gap around any plant stem.

Types of mulch compared

Shredded hardwood is the most common choice. It mats together so it stays in place on slopes, decomposes at a moderate rate, and costs $3 to $5 per bag. It is the default for most garden beds.

Pine bark nuggets decompose more slowly and do not mat together, so they wash out of beds on slopes or in heavy rain. They work well in flat beds around trees and shrubs. They also slightly acidify the soil over time, which benefits azaleas, blueberries, and rhododendrons.

Cedar mulch resists insects and fungus naturally and lasts longer than hardwood. It costs slightly more, typically $5 to $7 per bag. It is a good choice for beds near the house where you want to discourage termites and carpenter ants.

Pine straw is inexpensive and widely used in the southeastern U.S. It does not compact, allows good water penetration, and is easy to spread. It acidifies soil more than bark mulch, so it pairs well with acid-loving plants but may not suit vegetable gardens.

Compost or leaf mulch decomposes fast and feeds the soil aggressively. It is free if you compost at home. The downside is that it breaks down within a single season, so you need to reapply more often. It also does not suppress weeds as well as wood mulch because it is finer and lighter.

Choose based on your beds, your budget, and what is available locally. For most homeowners, shredded hardwood is the right starting point.