Mulch colour chemistry, and why cheap mulch fades by July
In short
Coloured mulch holds its colour when three things are right: the pigment is iron oxide or carbon rather than a dye, the wood is at the correct moisture content when the colourant is applied, and the particle size is consistent enough for even coverage. Get any of them wrong and the colour is bonded to the surface rather than into it, and the first hard rain takes it away.
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Pigment, not dye
The colourants that last are mineral: iron oxides for brown and red, carbon black for black. These are the same pigments used in concrete and roofing tiles, they are inert, and they do not dissolve in water. Cheaper products use water-soluble dyes, which look identical in a bag and are gone after two heavy rains.
Iron oxide is also why properly coloured mulch is safe around plantings. It is the same compound that makes soil red in large parts of the world. The concern people raise about coloured mulch is almost always really a concern about what wood was ground, not about the pigment.
Moisture at application is the variable nobody sees
Wood that is too dry when colourant is applied does not accept it — the pigment sits on the surface and sheds. Wood that is too wet dilutes the binder and produces a thin, uneven coat. There is a window, it is fairly narrow, and hitting it consistently is most of what separates a good colouring plant from a cheap one.
A plant running to schedule rather than to condition will colour whatever is in the hopper. That is why two loads from the same supplier can behave differently — not because the specification changed, but because the wood did.
Particle size and screening
| Product | Screen | Behaviour |
|---|---|---|
| Natural landscape mulch | 3/4 in minus | Even coverage, good moisture retention, minimal blow-off |
| Coloured landscape mulch | 3/4 in minus, pigmented | Colour holds through a season when applied correctly |
| Playground fibre | 1 in minus, engineered | Interlocks and compacts; graded for impact attenuation |
| Animal bedding | 3/8 in minus, low dust | Absorbent, low dust fraction |
| Hog fuel | Unscreened 2 in minus | Boiler feedstock; no screening needed |
Unscreened material contains both oversize chunks that take pigment unevenly and fines that blow away in the first dry week, taking their colour with them. Trommel screening before colouring is what makes the difference between mulch that stays where you put it and mulch that ends up on the path.
And the nails
Landscapers are rightly suspicious of mulch made from pallets, and the concerns — nails, treated wood, unidentified painted stock — are all legitimate. All three come from yards that grind indiscriminately, and all three are avoidable with process rather than luck.
- A person pulls painted, chemically stained and methyl-bromide-stamped wood at the intake belt, before it can reach a grinder.
- An overhead drum magnet on the grinder infeed removes banding and loose fasteners.
- A cross-belt magnet on the discharge catches nails freed by the grinding action itself. This is the stage most yards skip, and it is the one that matters.
- A trommel removes oversize and fines.
- Colour goes on last, at controlled moisture.
Appendix: the data behind this
Every piece on this site closes with the slice of our operating record that bears on what it argues. The full record is published at /resources/operating-record.
The fibre stream in context
Fibre is 2.9% of everything by weight. It is the last rung of the cascade, and keeping it small is the point of every stage above it.
| Stream | Share by weight | Tons | Destination |
|---|---|---|---|
| Direct reuse | 55.3% | 27,240 | Back out as a graded pallet, no tool used |
| Repair and return | 27% | 13,300 | Bench repair, regraded, banded |
| Board harvest | 9.1% | 4,483 | De-nailed lumber into our own repair stock |
| Remanufacture | 5.2% | 2,562 | Jig-built into 42×42, 36×36, 48×45 and custom |
| Fibre recovery | 2.9% | 1,429 | Mulch, playground fibre, animal bedding, hog fuel |
| Ferrous recovery | — | 1,186 | Baled nails and banding to regional scrap processors |
| Licensed disposal | 0.5% | 246 | Chemically treated, MB-stamped and non-wood debris |