Key Takeaways
- Compost is organic matter that has heated up as it rotted, to around 65 to 71°C at the core, and its job is to feed and improve the soil beneath.
- Mulch is any layer laid on top of the soil to hold moisture and block light from weed seeds, and it does that best when it is coarse and slow to rot.
- Garden compost can be spread as a mulch, but a 5cm layer of compost feeds the bed far better than it stops weeds.
- Bark or wood chip mulch needs a minimum of 5 to 7.5cm to control weeds and 7.5 to 10cm to hold moisture through a dry spell.
- A bed that needs both gets compost first, worked in or spread thin, and the mulch laid over the top.
I remember getting a call from a client who had bought two bulk bags of “bark” for composting some new borders, asking me if she’d actually bought the wrong thing. The stuff seemed like the garden compost she’d seen in smaller retail bags, but she just wasn’t sure what the difference was. When I looked at the materials, the coarse, woody texture made it fairly obvious what it was intended for. I explained that her bark was meant as a surface material, so she spread it over the finished beds as a mulch, rather than digging it into the soil. It made the borders look tidier straight away, but the nutrients obviously we’re not going to reach the soil for months to years from then, so she took my advise and gave the borders a liquid feed as well. It was one of those times where knowing what’s in the bag saves you from trying to make the wrong material do the wrong job.
What is the difference between compost and mulch?
Compost is rotted organic matter that goes into the soil to feed it, while mulch is a layer that sits on top of the soil to protect it. The test I use is heat. Proper compost has been through a hot rot, with the middle of the heap running at 65 to 71°C for weeks. That heat breaks the material down to a fine, dark crumb and kills most weed seed in it. If the heap never got hot, it is a pile of rotting stuff: useful, but a mulch in waiting rather than compost. Outdoor Aggregates horse manure compost is heat treated to ensure that there are no weed, for example.
Mulch has not heated. Bark, wood chip, leaf mould, straw and gravel all count. A mulch has to let water and air through and sit heavily enough to choke weed seedlings underneath it. Fine, rich material fails that second part, which is the whole problem with treating compost as if it were mulch.
What is compost for?
Compost feeds the soil and changes its structure, so it belongs in the ground or spread thinly on top where worms can pull it down. On heavy clay it opens the soil up. On sand it helps the soil hold water and nutrients that would otherwise drain straight through. The rule of thumb I worked to was 7.5cm over the whole bed once a year, which is 75 litres for every square metre. Dig it in during autumn, or spread it in spring and let the worms do the digging.
Compost spread in spring needs to be fully rotted. I learned that at a client’s expense, spreading a half done load in April and watching the bed sulk for a month while the soil life finished the job and took the nitrogen with it. You can lay fresher compost in October without issues because it has all winter to finish.
What is mulch for?
Mulch holds moisture in the soil, blocks light from weed seeds and keeps the roots under it cooler in summer and warmer in winter.
Bark or wood chip needs 5 to 7.5cm to control weeds, and 7.5 to 10cm if the main job is keeping a border damp through a dry summer. AAn 80 litre bag at 7.5cm covers about 1m², which is why I always told clients to measure the bed before they bought bags.
Bark goes two or three years before it needs a rake through and 2.5cm of new chips. The mistake I saw most was a fresh 10cm layer every spring on top of the old one, until shrubs sat in 15cm of rot with their stems buried.
| Question | Compost | Mulch |
|---|---|---|
| Has it heated up? | Yes, 65 to 71°C at the core | No |
| Where does it go? | Into the soil, or thin on top | On the surface only |
| Main job | Feed the soil, improve structure | Hold moisture, block weeds |
| How much | 5cm a year, one barrow per 25m² | 5 to 7.5cm for weeds, 7.5 to 10cm for moisture |
| How long it lasts | One season, then gone into the soil | 2 to 3 years for bark, less for finer types |
| Weed seed risk | Low if hot made, high from a cold heap | Low for bark and chip |
When do you need both compost and mulch?
A bed needs both compost and mulch when the soil is poor and the weeds are relentless, because one material cannot do both jobs well. The order is fixed. Compost goes down first, dug in or spread a couple of centimetres deep around the plants, and the mulch goes over it. The other way round, the compost sits on top of the bark and feeds nothing but the weed seed that lands on it. Compost in the planting hole and across the bed, then 7.5cm of bark over the lot, is the method I would still use on any new border.
Vegetable beds are the exception. Bark ties up nitrogen as it rots and gets in the way of sowing, so compost alone does the mulching there, spread in spring, and you weed a bit more for it. That is a big enough subject that I cover it separately in using compost as a mulch.
Which should you buy, compost or mulch?
Buy compost when the problem is the soil, and buy mulch when the problem is weeds, watering or a border that looks unfinished.
Yellowing leaves, clay that puddles, sand that dries out in a day: compost problems. Weeds back a fortnight after you cleared them, a hose out every evening in July, bare soil showing between the plants: mulch problems. If you only have money for one, feed the veg beds with compost and mulch the ornamental borders, because that is where each one pays back fastest.
For a bed I would buy green waste compost over multi purpose, which is priced for filling pots rather than covering borders. Green waste compost made to the PAS 100 standard has been through the hot process, so it arrives weed free. The Green Compost bulk bag from Outdoor Aggregates is PAS 100 and starts at £100, which covers about 9m² at 7.5cm.
For the mulch side, bark for borders, shrubs and trees, and composted bark fines from our compost mulch range if you want something finer that still feeds a little. The full run down of every type is in what is mulch.
Next Steps
We stock the bulk bag composts for feeding a bed and the bark grades for covering it, delivered to the kerb. If you are still unsure which one your bed needs, describe the soil and the weeds when you get in touch and we will tell you straight, including when the answer is that you need nothing.
If you are still torn between compost and mulch and would rather talk it through with a person, ring Outdoor Aggregates on 0330 043 7585. The office is open 7am to 7pm, seven days a week.
Have you ever spread compost thinking it was mulch, or the other way round? What did you put down, and how did the bed look by the end of summer? Let us know in the comments.
Frequently Asked Questions
Is mulch the same as compost?
Mulch and compost are different things; compost has been through a hot rot and feeds the soil, while mulch is any layer laid on top of the soil to hold moisture and block weeds.
Can you use compost as a mulch?
Compost can go on top of a bed as a mulch and it feeds the soil well there, but it blocks weeds far less well than the same 7.5cm of bark so only mulch with it where you are happy to hoe.
Is leaf mould compost or mulch?
Leaf mould is a mulch; leaves rot cold and never go through the hot stage that makes compost. After a year it makes a rough mulch, and after two it is fine enough to sow into.
Does compost go under or on top of mulch?
Compost goes under the mulch, worked into the soil or spread thin on it, with the bark laid over the top. Compost laid on top of bark feeds the weed seed and nothing else.