Mulch depth is a drainage decision, not a tidiness one

Every spring, a great many Louisiana beds get another two inches of mulch laid over last year's two inches, which was laid over the two inches before that. Nobody removes anything. After eight or ten years the bed is six inches higher than the lawn, the crowns of the shrubs are buried, and the house has a raised, saturated collar of organic material sitting against its foundation.

What the right depth actually does

At two to three inches, mulch does three useful things: it blocks enough light to suppress most annual weed germination, it slows evaporation from the soil surface, and it moderates soil temperature through the worst of the summer. All three benefits are essentially maxed out at three inches.

What too much does

• Sheds water. Deep, matted, partly decomposed mulch becomes hydrophobic — rain runs off it rather than through it, and the plants underneath go thirsty in a wet season.
• Rots bark. Mulch piled against a trunk keeps the bark permanently damp. Bark is not root tissue and is not built for it.
• Buries crowns. Shrubs planted at the correct height ten years ago are now three inches too deep, which is a slow way to kill them.
• Dams the foundation. A bed higher than the lawn holds water against the slab instead of letting it leave.

How to reset a bed

Pull the old material back rather than adding to it. Find the original soil line and the root flare on any tree in the bed — the flare should be visible, the point where the trunk widens as it meets the ground. Re-cut the edge to a clean spade line so the bed is level with or slightly below the lawn. Then mulch to two or three inches and stop.

A bed reset like this looks better for the whole season than one that has simply been recoloured, because the edge holds and the plants stop struggling.

Pine straw, hardwood or dyed

Pine straw knits together and stays put on a slope, which matters on any bed with fall across it. Shredded hardwood breaks down into the soil and does the most for structure over time. Dyed mulches hold colour longest and contribute the least. All three work at the right depth; none of them works at six inches.