Mulch does three jobs — holds water in, keeps weeds down, and feeds the soil as it goes. Hemp is unusual in doing the third one properly rather than just sitting there for five years.
Two numbers and one month. Almost every disappointing mulch is a thin one, applied at the wrong time of year.
| Where | Depth | When | Watch for |
|---|---|---|---|
| Mixed border | 5–7 cm | Spring, soil damp | Keep it off stems |
| Around shrubs and trees | 7–10 cm | Autumn or spring | Never against the trunk |
| Vegetable beds | 3–5 cm | After planting out | Pull back to sow |
| Pots and containers | 2–3 cm | Any time | Leave the rim clear |
| New planting | 5 cm | Water first, then mulch | Mulch traps what is there |
Woody material has a high carbon-to-nitrogen ratio, so the microbes decomposing it draw nitrogen from their surroundings. This is real. What is usually overstated is the depth: it happens in the first centimetre or two where mulch meets soil, and established plant roots are well below it.
It matters in two situations. Direct-sown seedlings, whose roots are exactly in that layer — so pull mulch back to sow. And hungry crops in their first season, where a handful of pelleted manure or a nitrogen feed under the mulch settles it entirely.
For borders, shrubs and established beds it is not a practical problem, and the trade is a good one: hemp mulch breaks down in a season or two into material the soil keeps, where bark sits inert for years.
Bark lasts longer and looks more formal. Hemp breaks down faster into something that improves soil structure, and it is far lighter to carry — a real consideration at volume.
Choose bark for paths, hemp for bedsCompost feeds better and suppresses weeds worse, because it is a fine seedbed itself. Many gardens do both: compost first, hemp on top.
Compost feeds, mulch coversBoth suppress weeds well and add nothing. Membrane in particular tends to end up as shredded plastic in the soil within a decade.
Neither improves the groundA season to two, depending on depth and weather. That is shorter than bark and it is the point: what disappears has gone into the soil rather than into a skip.
It is light when dry and stays put once watered in, so water it after spreading. In a very exposed site a coarser grade holds better than a fine one.
Less than they like damp compost or straw — a dry hemp surface is unpleasant to cross. It is not a slug barrier and anything claiming that is overselling.
Compost it first. Used bedding carries nitrogen-rich manure that will scorch plants fresh, but after three to six months on a heap it becomes an excellent soil improver — which is the neatest thing about keeping animals and a garden on hemp at the same time.