| Compost | Mulch | |
|---|---|---|
| Primary purpose | Feeds the soil | Protects the soil surface |
| Made from | Decomposed organic matter | Bark, wood chips, straw, gravel, etc. |
| Where it goes | Into or on top of soil | On top of soil only |
| Weed suppression | Moderate (if used as top dressing) | Good to excellent |
| Moisture retention | Excellent | Good to excellent |
| Nutrient content | High | Low (bark/wood) to moderate (straw) |
The Key Difference
The simplest way to think about it: compost feeds the soil, mulch protects it.
Compost is fully decomposed organic matter — kitchen scraps, garden waste, manure — that’s broken down into a dark, crumbly material rich in nutrients and beneficial microorganisms. Its job is to improve soil structure, add fertility and boost biological activity. You can dig it in or spread it on the surface as a top dressing.
Mulch is any material laid on the soil surface to suppress weeds, retain moisture and regulate temperature. It can be organic (bark, wood chips, straw, leaf mould) or inorganic (gravel, slate). It stays on top and breaks down slowly — or in the case of gravel, not at all.
The confusion arises because compost can be used as mulch. When you spread a thick layer of compost on the soil surface, it does everything mulch does — plus it feeds the soil underneath. But not all mulch is compost, and not all compost is ideal as mulch.
When to Use Compost
Compost is the better choice when your goal is improving soil health and fertility:
- Vegetable gardens: Hungry crops like courgettes, tomatoes and brassicas thrive with a generous layer of compost. Apply 5–8 cm annually before planting season. See our vegetable garden mulching guide for more on this approach.
- New beds: When establishing a new border or converting lawn to growing space, compost builds soil structure far faster than bark mulch.
- Poor or compacted soil: Heavy clay or thin sandy soil benefits enormously from annual compost applications. Worms pull it down into the soil, improving drainage in clay and water retention in sand.
- Rose and shrub beds: A spring top dressing of compost around flower beds feeds plants steadily through the growing season without the risk of nitrogen robbery.
When to Use Mulch
Mulch is the better choice when your priority is weed control, moisture retention or aesthetics:
- Ornamental borders: Bark mulch looks neat, suppresses weeds and lasts 2–3 years before needing a top-up. It’s the classic choice for flower beds where appearance matters.
- Around trees: A ring of bark or wood chip mulch around trees protects roots, retains moisture and prevents mower damage — without encouraging excessive surface rooting the way compost can.
- Weed-prone areas: Where weed pressure is high, a thick layer of bark mulch (8–10 cm) is more effective than compost. For the toughest weeds, combine mulch with cardboard underneath or a targeted weed killer. Our guide to the best mulch for weed prevention covers this in detail.
- Paths and play areas: Wood chips and bark create natural-looking pathways. Gravel works for more permanent paths and driveways.
- Winter protection: Straw and bark insulate roots from frost far better than compost. See our winter mulching guide for material choices.
Can You Use Compost as Mulch?
Absolutely — and many experienced gardeners do exactly this. The no-dig method, popularised by Charles Dowding, uses compost as the primary mulch on vegetable beds. You spread 5–8 cm of compost on the surface each year and let the worms incorporate it naturally.
However, compost has some limitations as a mulch compared to bark or wood chips:
| Factor | Compost as Mulch | Bark/Wood Chip Mulch |
|---|---|---|
| Weed suppression | Moderate — weeds can root into it | Good — hostile rooting medium |
| Longevity | Breaks down in 6–12 months | Lasts 2–3 years |
| Soil feeding | Excellent | Minimal (can tie up nitrogen) |
| Appearance | Dark, earthy — less tidy | Neat, uniform — looks maintained |
| Cost | Free (homemade) or £3–5/bag | £4–8/bag |
The biggest drawback of compost as mulch is that it can actually encourage weed growth. Because it’s nutrient-rich and has a fine texture, weed seeds that land on compost germinate easily. Bark mulch, by contrast, is a poor growing medium — seeds struggle to establish roots in it.
For veg beds and feeding borders, compost as mulch is brilliant. For long-term weed suppression in ornamental areas, you’re better off with bark. Use our mulch calculator to work out how much of either you need.
The Best of Both: Layer Them
The ideal approach for many situations is to use both. Apply a 3–5 cm layer of compost as a top dressing first, then cover it with 5–8 cm of bark mulch. This gives you:
- Soil feeding from the compost layer below
- Effective weed suppression from the bark layer on top
- Excellent moisture retention from the combined depth
- A neat, tidy appearance that lasts
This layered approach works particularly well for flower beds and established borders where you want both nutrition and aesthetics. Refresh the compost layer annually in spring, and top up the bark every 2–3 years as it breaks down. Check our guide on how often to apply mulch for timing advice.
Frequently Asked Questions
Is compost better than mulch?
Neither is universally better — they serve different purposes. Compost is better for feeding soil and improving structure. Mulch is better for weed suppression and moisture retention. For the best results, use both: compost underneath, bark mulch on top.
Can I use shop-bought compost as mulch?
Multi-purpose compost isn’t ideal as mulch — it’s designed for container growing and tends to dry out and blow away when used as a surface layer. Garden compost or composted green waste works much better because it’s heavier, coarser and richer in beneficial organisms.
Does mulch turn into compost?
Organic mulches like bark and wood chips do eventually decompose into humus, similar to compost — but very slowly. Bark mulch takes 2–3 years to break down significantly. This gradual decomposition is why you need to top up mulch regularly.
Should I put compost down before mulch?
Yes, this is the ideal approach. Spread 3–5 cm of compost first, then cover with 5–8 cm of mulch. The compost feeds the soil while the mulch suppresses weeds and retains moisture. It’s the best of both worlds.
Can I use grass clippings as mulch or compost?
Both, but differently. Thin layers of grass clippings as mulch work well around vegetables. Thicker layers should go in the compost bin — they mat down and go slimy if piled too deep as mulch. Mix with brown materials like cardboard for best composting results.
What about mushroom compost — is that mulch or compost?
Spent mushroom compost is technically a compost (soil improver) but can serve as mulch. It’s alkaline, so avoid it around acid-loving plants like rhododendrons and blueberries. It works well on vegetable beds and around roses where its lime content isn’t a problem.
