What Is Garden Mulch? – Advice from Expert Tree Surgeons
Garden mulch is a layer of material spread over the soil surface around plants.
It covers a variety of material. From organic options like bark, compost, and manure, to inorganic materials such as gravel and landscape fabric.
But when you think of mulch in the UK, you are probably thinking organic.
Organic vs inorganic mulch
Organic mulch decomposes over time and releases rich nutrients into the soil.
Inorganic options like gravel and landscape fabric don’t break down and won’t improve soil structure.
But a benefit it that they last longer and are useful for permanent plantings where you want minimal maintenance.
If you’re covering a larger area, loose loads of wood chip are often more cost-effective.
Better yet, if you’ve just had tree surgery done, some tree surgeons will let you keep the woodchips produced to use as mulch.
Different Types of Mulch
Bark Mulch and Wood Chip
Bark mulch is the most widely used biodegradable mulch in UK gardens.
Bark chippings are slow to decompose, so they hold their depth on the soil surface longer than finer materials and don’t need replacing as often.
Woodchip mulch breaks down faster because it includes more sapwood alongside the bark, but it adds more organic matter to the soil as it does.
Both suppress weeds well and look tidy in beds and borders around shrubs.
Fresh wood chip can temporarily draw nitrogen from the top of the soil as it decomposes, but this affects only the soil surface and doesn’t harm established plant roots at lower depths.
Compost and Manure
If you have a compost bin, homemade garden compost is hard to beat as a mulch because it feeds the soil at the same time as covering it.
Applied to the soil surface as a thick layer of organic matter, it feeds microorganisms in the soil, improves drainage in heavy ground, and encourages earthworms to work through the beds.
Manure and compost together make a powerful combination for vegetable gardens and flower beds where feeding the soil is as important as weed suppression.
Both need to be well-rotted before you apply them as mulch.
Fresh manure releases ammonia and can scorch plant roots.
Well-rotted manure is dark, crumbly, and smells earthy rather than sharp.
Grass Clippings and Leaf Mulch
Grass clippings from the lawn mower are a free source of mulch that adds nitrogen to the soil as it decomposes.
Don’t apply them too thickly.
A heavy layer of fresh clippings mats down, cuts off airflow, and creates a habitat for slugs.
Spread them thinly and let each layer dry before adding more.
Leaf mulch, made from shredded autumn leaves left to decompose over winter, is another free option. It is better-suited to woodland-style or cottage garden planting schemes.
Inorganic Mulch
Gravel, slate, and landscape fabric are the main inorganic options.
They won’t release nutrients into the soil or improve the health of the soil over time, but they’re long-lasting and effective at reducing weed growth without regular topping up.
Landscape fabric is often used under bark or gravel to block weed seeds from the soil below.
Gravel works well in contemporary or Mediterranean gardens and around drought-tolerant plants.
Neither is the right choice for a vegetable garden or beds where soil improvement is the goal.
Benefits of Mulching Your Garden
Suppresses Weeds
A layer of mulch 5cm or more deep blocks light from reaching weed seeds sitting in the soil below, which stops them germinating.
Reducing weed growth this way means fewer hours spent weeding through summer and more time enjoying the sun.
Plus, any weed seeds that land on top of the mulch and try to root are far easier to pull than those growing directly in the ground.
When mulch is applied around trees and shrubs, by suppressing weeds, it reduces competition for water and nutrients in the soil.
Retains Moisture in the Soil
Mulch slows evaporation from the soil surface, keeping moisture in the soil longer between rain or watering.
On lighter, free-draining soils this makes a noticeable difference through summer.
Retain soil moisture effectively and plants cope better through dry periods without needing daily attention.
Even in Scotland, where prolonged dry spells are less common than further south, a good mulch layer prevents the rapid drying that follows a few warm days in June.
Improves Soil Structure
Organic mulch decomposes steadily.
As it does, it feeds microorganisms in the soil that convert it to humus.
Humus makes soil better no matter what type of soil you have.
It helps heavy clay drain more easily, and helps sandy soil hold onto water longer.
Earthworms also help by pulling the broken-down mulch deeper into the soil as they feed, creating channels that let water move through more freely.
If you spread a thick layer of organic matter over bare soil each spring, the whole bed becomes easier to dig and grows healthier plants over time
Prevents Soil Erosion
Bare soil takes the full impact of heavy rain, which can wash the surface away on sloped beds and compact it flat on level ground.
A covering of mulch absorbs the impact of heavy rain before it can displace the surface or drive water straight through the soil.
In a vegetable garden, where you’re working the ground regularly, mulch between rows also prevents the compaction that builds up from regular foot traffic.
Prevent soil erosion consistently and you’ll find the soil stays loose and drains well even through a wet Scottish winter.
Warms the Soil in Spring
Dark organic mulch absorbs heat from the sun and can warm the soil a few degrees earlier in spring than unmulched ground.
In Scotland, soil temperatures often stay low well into April, so the warming effect of dark organic mulch in late winter is worth having.
Apply mulch in late winter or early spring, before growth begins in earnest, and it gives newly planted trees and shrubs a slight head start with root development.
It also encourages plant roots to establish deeper, where moisture levels are more stable through summer.
How to Apply Mulch
Clear the area
Clear the area of existing weeds before you start.
If you’re clearing overgrown ground or working near outbuildings, check for bats first, as they’re a protected species under UK law and disturbing a roost is an offence.
Mulch the top
Mulch over the top of established weeds and you’ll slow them down, but you won’t get rid of them.
Pull or hoe them out first, then apply mulch to suppress regrowth from weed seeds that remain in the soil below.
Mulch depth
Spread a layer of mulch 5-8cm deep and use a rake to level it and distribute it evenly around plants.
A shallower layer won’t suppress weeds effectively.
Going much deeper than 8cm can restrict water from reaching the soil surface and may limit airflow around roots.
Apply mulch to the surface of soil between plants, not on top of them.
Careful around bases and trunks
Keep mulch pulled back from the immediate base of stems and tree trunks.
Piling it against stems holds moisture against the bark, which encourages rot and can invite fungal disease.
Around trees, aim for a ring that reaches toward the drip line but leaves a clear gap around the trunk itself.
What Not to Mulch
Freshly seeded areas need light to reach the soil surface for germination, so don’t apply mulch until seedlings are well up.
Ivy is sometimes left to spread as a living groundcover rather than mulched over, but it grows aggressively and can quickly smother smaller plants if left uncheck.
Waterlogged ground shouldn’t be mulched either. Adding a layer over saturated soil traps moisture the ground can’t drain quickly enough to shed, which can damage plant roots sitting in it.
Acid-loving plants, including rhododendrons, azaleas, and heathers, need a mulch that helps lower the pH of the soil rather than raise it.
Composted pine bark or conifer-based wood chip suits them because it maintains or reduces the pH as it breaks down. Standard composted manure would work against them by raising the pH over time.
Mulch vs Compost
Mulch and compost do different jobs.
Compost is a soil improver worked into or applied directly on the soil surface where worms and microorganisms incorporate it.
Mulch sits on top of the soil. Its job is to block light from weed seeds, slow evaporation, and cushion the surface from heavy rain and foot traffic.
Compost or bark can both be used as mulch.
Garden compost is the better choice when you want to actively feed the soil at the same time as covering it. Bark mulch and wood chip mulch last longer on the surface and do a better job of suppressing weeds through a full growing season.
Both are useful, and using them in different parts of the garden depending on what each area needs is a straightforward approach.
Need Help With Mulching or Garden Maintenance?
Mulching your own garden is a straightforward job on smaller beds.
But if you’re dealing with a large area, a newly landscaped garden, or overgrown ground that needs clearing first, it’s worth getting professionals in to do it properly.
Getting the depth right, preparing the ground correctly, and choosing the right mulch materials for your soil type and planting all make a difference to how well it works.
Our landscaping team work across Glasgow on everything from full garden makeovers to seasonal maintenance and mulching visits.
If you’re thinking about a low-maintenance garden that holds its structure season to season, mulching is the single most effective change you can make to cut the hours you spend weeding and watering each season, and we can advise on materials and approach as part of a wider garden plan.
Call our friendly team on 07946 322717 or get in touch for a free quote and we’ll take it from there.