Bark mulch is seasoned bark from spruce and pine, laid as a protective layer on top of the soil. The layer stops weed seeds from germinating, slows evaporation and insulates against ground frost in winter. How well it works depends almost entirely on the depth, and 5 cm and 10 cm are two different things. Here you get the measurements, the quantities and the situations where bark does more harm than good.
How deep the bark layer should be
The recommendations vary, and there is a reason for that. The Royal Horticultural Society in the UK sets the minimum for biodegradable mulches at 5 cm and recommends 7.5 cm. Bark producers state at least 10 cm. Both figures are correct, because they answer different questions. A thinner layer holds moisture, a thicker layer stops weeds.
| Layer depth | What the layer does | Choose it for |
|---|---|---|
| 5 cm | Holds soil moisture and covers bare ground | Newly planted perennials, pots and containers |
| 7-8 cm | Stops most weed seeds from germinating | Beds you weed every year |
| 10 cm | Full cover against weed seeds, insulates against ground frost | Shrub and rose plantings, autumn cover |
| 20 cm over cardboard | Smothers couch grass and ground elder, but only after several years | Ground you want to take out of use altogether |
At Florea we lay 8 cm in beds that are tended every season and 10 cm around shrubs we would rather not weed. Below 5 cm it is rarely worth the effort, because weed seeds germinate straight through a layer that thin.
Working out how many bags you need
The quantity is easy to work out in your head. The number of litres of bark per square metre is the layer depth in centimetres times ten. A layer of 8 cm therefore takes 80 litres per square metre, and 10 cm takes 100 litres.
Bags in the shops usually hold 40 litres. One such bag covers about 0.8 square metres at 5 cm, 0.5 square metres at 8 cm and 0.4 square metres at 10 cm. A bed of 6 square metres that is to have 8 cm of bark therefore comes to 480 litres, which is twelve bags. Buy one bag too many rather than one too few. An uneven layer lets weeds through exactly where it is thinnest.
How to spread the bark
Clear the ground completely: take out the weeds with their roots, especially couch grass, ground elder and dandelion.
Feed first if it is needed: apply fertiliser or compost before the bark, because you cannot reach the soil afterwards.
Lay it on moist soil: water thoroughly or wait for rain, and never lay bark on frozen ground. The layer holds moisture in, but it adds none.
Spread it out and rake it level: measure in three places and compare against the 8 or 10 cm you settled on, because the weeds come up where the layer is thinnest.
Keep the bark away from the stems: thin the layer down to almost nothing closest to stems and trunk bases. Bark lying against a stem keeps it damp and leaves it open to fungal attack.
Nitrogen in the soil and when it matters
This is the part that is most often misunderstood. As the bark breaks down, micro-organisms use nitrogen, and that nitrogen is taken out of the soil. The effect is real, but it sits in the boundary layer between bark and soil, that is in the top few centimetres. Further down, where established roots sit, it goes unnoticed.
That gives a decision rule that is easy to use. If a plant's roots go deeper than the 8 to 10 cm of bark you have laid, it is not affected. Perennials, shrubs, roses and bulbs cope. Direct-sown annuals and most vegetables do not, because their roots sit exactly where the nitrogen is tied up.
Two things soften the problem. Seasoned bark draws less nitrogen than fresh chippings, which should sit for a few weeks before they are used. If you still need to cover ground planted with hungry species, you can put fresh grass clippings or chicken manure under the bark. How to choose the right fertiliser is covered in the article on plant nutrition.
Plantings that suit bark and plantings that suffer
A good fit: shrub roses, ornamental shrubs, soft fruit bushes, perennials and bulbs and tubers for autumn planting, which are glad of a 10 cm layer over winter.
A poor fit: the vegetable bed and hedges, where the nitrogen demand is high and the roots sit shallow. Cover the vegetable bed with grass clippings or compost instead.
No fit at all: ground you are about to sow directly. The whole point of bark is that seeds do not germinate in it, and it does not tell weed seeds from your own.
Take care with: drought-tolerant Mediterranean plants such as thyme, lavender and sage. The bark holds moisture around stems and leaves, which is exactly wrong for them. A layer of gravel or shingle works better there.
Bark mulch compared with other mulches
The choice of material is governed by what you grow on the ground, not by the look of it. A free-draining sandy soil dries out faster and gains more from 10 cm of bark than a heavy clay does. If you are unsure which soil you have, read about soil types first.
| Material | Nitrogen effect | Topping up | Best suited to |
|---|---|---|---|
| Seasoned bark mulch | Ties up nitrogen in the layer closest to the soil | Lasts about five years | Shrubs, roses, perennials and bulb plantings |
| Grass clippings | Add nitrogen | Several times a season | Vegetable beds and raised beds |
| Compost | Adds nutrients | Every spring | Beds where the soil needs improving |
| Gravel and shingle | None at all | Does not break down, laid once | Thyme, lavender and sage, which want it dry |
When to lay the bark and when to top it up
Two points in the year work. One is late spring, in May, once the soil has warmed up and the perennials are away. The other is October and November, before the ground freezes, when the aim is to insulate against ground frost and protect bulb plantings.
A bark layer lasts about five years before it has broken down, but it settles and gets thinner long before that. Check it every spring. Rake the layer up, measure in a few places and top it up before the depth drops below 5 cm. The raking is worth something in itself, because a layer that has compacted lets less water through.
Bark on weedy ground is the commonest mistake
The most common disappointment with bark mulch is that the layer goes down over ground already full of weeds, in the hope that the bark will solve the problem. It will not.
Seed weeds the bark handles. Seeds that land on top of the layer germinate poorly, partly because they never reach the soil, partly because the nitrogen in the boundary layer is tied up. Weed suppression and the nitrogen effect are the same thing seen from two sides.
Perennial weeds are another matter. Couch grass, ground elder and field bindweed carry reserves in their roots and push up through 5 to 10 cm of bark without much trouble. Using bark to smother them takes around 20 cm of bark or chippings, preferably over a layer of cardboard, and it has to stay in place for several years and be topped up along the way. That is a method for taking ground out of use, not for tidying up a bed.
The decision rule is simple. The bark deals with seed weeds for you, perennial weeds have to go before you lay it. If you want to read more about covering the soil instead of digging it, the no dig method is in the Gardening School.
Author: Emma Vogiatzi
Fact-checked by Erik Hoekstra
Last updated 2026-09-25