Key Takeaways
- Compost works as a feeding mulch, spread around 5cm deep in spring on veg beds and at any time on borders. It is a poor weed barrier because fine, rich material makes an ideal seedbed.
- Home made compost from a heap that never reached 60°C usually carries live weed seed so using it as a mulch spreads the problem it was meant to solve.
- Applying compost as mulch suits vegetable beds and new borders where the soil needs feeding. Bark suits shrubs, trees and established borders where weeds are the problem.
A lot of the no dig crowd have gone off mulching with compost, and the reason is always the weeds. I watched it happen on more than one veg patch or bed: beautiful black compost spread in March, and by May it looked like a lawn. Whether that is a reason to stop depends on what you wanted the compost to do. If the job was feeding the bed, it worked. If you wanted weed suppression, you will need another approach.
Does compost work as a mulch?
Compost works as a mulch for feeding the soil and holding some moisture, but it is a weak barrier against weeds unless the compost is weed free and the soil beneath it is already clean. The reason is the same thing that makes it good compost: fine, dark and full of food, so any weed seed that lands on it or was already in it gets the best start it could ask for.
Bark is coarse, low in food and slow to rot, so a seed landing on it dries out. Bark laid at 7.5 to 10cm also keeps a border moist through a dry July and lasts two or three years, then wants 2.5cm of fresh chips,, while compost crusts in a hot spell and is gone into the soil within a season. Compost is a feed you spread every year. That is the trade.
| Job | Compost | Bark or wood chip |
|---|---|---|
| Blocking weeds | Poor, fine material is a seedbed | Good at 5 to 7.5cm |
| Holding moisture | Fair, crusts when dry | Good at 7.5 to 10cm |
| Feeding the soil | Good, worked down by worms | Slight, and it takes nitrogen while rotting |
| How long it lasts | One season | Two to three years |
| Weed seed in the bag | None if PAS 100, plenty from a cold heap | None |
Does compost mulch stop weeds?
Compost mulch slows weeds where it goes down 5cm or more over bare, weeded soil, but it never stops them the way bark does. Weeds appear in a compost mulch for three separate reasons:
- Seed already in the compost. A cold heap never kills seed, and the weeds arrive with the bag.
- Seed already in the soil, woken up by the disturbance and the light.
- Seed blown in afterwards, landing on the best seedbed in the garden.
Bought compost made to PAS 100, like our Green Compost, fixes the first, because the process runs hot enough to kill seed. Only a coarser mulch on top fixes the third. Slugs are the other problem, and they do increase under any organic mulch in a wet spring, compost no worse than bark; the old grapefruit skin and tile traps still work.
When does compost keep weeds down?
Compost keeps weeds down in two cases: a no dig bed that has been kept clean, and new ground where it goes over cardboard. Perennial weeds are the one exception.
- A clean no dig bed. If the compost is weed free and no weed in the bed is ever left to seed, very few weeds come up. The only ones are seeds that blow in, and they sit so shallow in loose compost that they pull out easily. This is how no dig growers use it on veg beds.
- Over cardboard on new ground. Spread the compost over the weeds first, then lay plain brown cardboard on top with the edges overlapping. Where bindweed or couch grass is thick, cover it all with black sheeting as well, because they need total darkness, and dig a trench about 30cm deep round the edge so the roots can’t feed from outside. Lay it in late February or March and lift the sheeting in autumn. The weeds underneath turn yellow and rot, and the compost is left feeding the soil.
The exception is that any perennial weeds need clearing first. These come back from the root every year: bindweed, couch grass and docks. They push through compost and any other loose mulch, bark included, so dig them out, or smother them under card and sheeting as above, before the compost goes down.
How thick should compost mulch be?
Compost mulch should go on around 5cm deep across the whole bed, which works out at about 50 litres for every square metre. Some growers go to 10cm on veg beds in spring. I never went past 7.5cm around anything woody, and I kept a 5 to 10cm gap round every stem. It has to be well rotted if it goes on in spring, because half done compost takes nitrogen out of the soil while it finishes.
PRO TIP: When it comes to my vegetable gardens, I lay it in March, not in autumn. Left on a bare bed through a wet winter, compost loses some of its nitrogen and potassium to the rain. With no roots in the ground, the worms and soil life in it slow right down too. Laid in spring, it feeds the bed just as the plants start to use it.
Is compost cheaper than bark as a mulch?
Compost is cheaper than bark to put down, but bark is cheaper over three years on a border because it lasts and compost goes on again every spring. We sell three bulk bag composts that work as a mulch, compared in the table below. Mushroom compost carries lime, so keep it away from rhododendrons, camellias and blueberries. A bulk bag of bark covers less ground, because bark needs 7.5cm. The bark is still there in year two and the compost is in the soil, so over three years bark is cheaper on a border and compost is cheaper on a veg bed you were going to feed anyway.
| Green Compost | Horse Manure Compost | Mushroom Compost | |
|---|---|---|---|
| Smallest bulk bag | around 700 litres | around 600 litres | around 1,000 litres |
| Covers at 5cm | about 14m² | about 12m² | about 20m² |
| Safe near acid lovers | Yes | Yes | No |
| Price | Price range: £100.00 through £194.00 inc. VAT | Price range: £117.00 through £242.00 inc. VAT | Price range: £117.00 through £197.00 inc. VAT |
| Choose bag size | Choose bag size | Choose bag size |
When is bark the better buy?
Bark is the better buy for shrub borders, trees and any bed where the weeds are the problem and the soil is already decent. Compost as mulch earns its place on the vegetable patch, on new borders where the soil needs building, and around roses in spring. The best of both is how I laid most ornamental borders: 2 to 3cm of compost around the plants, then 5 to 7.5cm of bark on top. The compost feeds, the bark stops the weeds, and the bed looks finished.
Next Steps
We stock green, horse manure and mushroom compost in bulk bags, composted bark fines that sit between compost and bark, and bark grades for a proper weed mulch. If you want an organic, peat free mulch in smaller 50 litre bags, RocketGro Magic Mulch feeds the bed and arrives seed free. Start with our soil improver range, and if the two are still muddled, compost vs mulch settles it.
If you would rather talk your compost mulch 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 mulched a bed with compost? How deep did you go, and did the weeds come through by summer? We would love to hear how it held up.
Frequently Asked Questions
Can you use multi purpose compost as a mulch?
Multi purpose compost can be spread as a mulch but it is sold for pots at pot prices and it dries and crusts faster than green waste compost. Better to use it up in containers and buy bulk compost for the bed.
Should compost go under bark mulch?
Compost belongs under bark, spread 2 to 3cm around the plants first with the bark laid over it so the compost feeds the roots and the bark blocks the weeds.
Can you plant straight into compost mulch?
Plants can go straight into a 5 to 10cm layer of well rotted compost mulch but seeds do better sown into the soil beneath with the compost drawn back around them once they are up.