Mulch Math

Mulch is not decorative. It is one of the highest-leverage things you can do for your beds, and the fact that it also looks good is a bonus, not the point. A proper layer of mulch does at least four things at once: it holds moisture in the soil, it suppresses weed germination, it insulates roots from temperature swings, and it prevents soil from splashing up onto your plants during rain. That last one matters more than people realize. Soil splash is how many fungal diseases reach plant leaves. A three-inch layer of mulch between the dirt and the canopy is a physical barrier between your tomatoes and the blight spores sitting in the soil surface.

Over time, organic mulch breaks down and feeds the soil from the top, which is exactly how a forest floor works. You are not adding a product. You are mimicking a process that has been running without interruption for a very long time.

Three inches is the standard recommendation, and it is the standard for a reason. At three inches you get meaningful weed suppression (light cannot reach most weed seeds), good moisture retention, and reasonable insulation. Less than two inches and you are mostly decorating. More than four inches and you risk suffocating shallow roots, trapping excess moisture against plant stems, and creating habitat for voles and other rodents who enjoy the cover.

The exception is tree rings and pathways, where four to six inches is fine because you are not mulching around plant stems and the ground underneath is not expected to breathe the same way. If you are mulching around trees, pull the mulch back from the trunk. The mulch volcano you see at commercial properties, where bark is mounded against the trunk, causes rot and invites disease into the bark layer. The correct shape is a donut, not a volcano.

Cedar contains natural oils, primarily thujone and plicatic acid, that repel or inhibit a range of insects. This is not garden folklore. It is the reason cedar is used in closets, chests, and outdoor furniture. In the garden, cedar mulch creates an environment that many soft-bodied pests find unpleasant. It will not eliminate every pest, but it makes the surface of your beds a less hospitable place for slugs, certain beetles, and some ant species. It also resists fungal growth better than most hardwood mulches, which means it holds its shape longer and does not develop the sour smell that poorly decomposed hardwood bark sometimes produces.

Cedar chips are more expensive per yard than generic hardwood mulch. They also last roughly twice as long before needing a refresh. Whether that tradeoff makes sense depends on your priorities, but the math usually favors cedar on a per-year basis, especially in beds you want to set and forget.

When rain hits bare soil, it throws tiny droplets of dirt into the air. Those droplets land on the lowest leaves of your plants and carry with them whatever was living in the top layer of soil. In most garden soils that includes fungal spores for early blight, septoria leaf spot, and other common diseases. The standard advice is to prune lower leaves to prevent contact, and that works, but it is treating the symptom. Mulch treats the cause. A layer of mulch absorbs the impact of rain and eliminates the splash. The dirt stays on the ground. The spores stay on the ground. Your leaves stay clean.

This matters most for tomatoes, peppers, and other nightshades that are susceptible to soil-borne fungal diseases. If you have ever lost a tomato plant to early blight that seemed to climb from the bottom up, you have seen what soil splash does. Mulching alone will not stop blight if conditions are right for it, but it removes one of the primary delivery mechanisms.

Bagged mulch at a home improvement store is convenient. It is also the most expensive way to buy mulch by a wide margin. A two-cubic-foot bag typically runs $3 to $6. A cubic yard is 27 cubic feet, which is 13.5 bags. At $4 per bag, that is $54 for a cubic yard of mulch. A landscape supply yard will sell the same cubic yard for $25 to $45 delivered, depending on your area and the type of mulch. The savings get larger as the volume goes up.

The tradeoff is that bulk mulch arrives in a pile in your driveway and you move it with a wheelbarrow and a pitchfork. If you need less than about two cubic yards, bags might still make sense because you can buy exactly what you need and carry them directly to the bed. Over two yards, bulk is almost always the better deal. Over four yards, buying bags is genuinely unreasonable.

One cubic yard equals 27 cubic feet. Bags are sold in cubic feet. Bulk deliveries are sold in cubic yards. This is the conversion that trips people up, and it is the reason the calculator above shows both. A cubic yard is roughly a pile three feet wide, three feet long, and three feet tall. It covers about 100 square feet at three inches deep.

If someone tells you a cubic yard does not look like much when it is on the ground, they are right. It spreads thinner than you expect, which is why the 10% extra slider exists. Between settling, edges, low spots, and the parts of the bed you forgot to measure, the math almost always comes up short. Round up.

Spring, after the soil has warmed and you have finished planting. Mulching too early in spring traps cold soil and delays root growth. In the South, that usually means late April through May. The second window is late fall, when you are putting beds to rest for winter and want to insulate perennial roots and suppress winter weeds.

If your existing mulch has thinned to less than two inches, top it off. You do not need to remove old mulch before adding new mulch unless it has gone sour (anaerobic decomposition producing a vinegar or sulfur smell). In that case, rake it out, let it dry in the sun for a day, and either compost it or reapply it once it has aired out.