Garden bark mulch has many functional uses in the landscape, including helping to suppress weeds, although it doesn’t provide an impenetrable weed barrier. It’s an organic form of mulching, conditioning the soil as it deteriorates. You shouldn’t layer bark mulch too deeply or problems with moisture and disease can put the plants’ health at risk.
Garden Bark Mulch for Landscaping
Bark mulch typically comes from milled fir, pine, redwood and spruce logs, notes Clemson Cooperative Extension. Landscape bark mulch is graded according to particle size, ranging from bark chunks or nuggets, bark granules and shredded bark. Bark chunks last the longest and granules are useful for soil conditioning. Bagged bark mulch has usually weathered enough to destroy any toxins that may harm plants. The coarse bark mulches are more effective for weed suppression; seeds can germinate in fine, moist bark mulch.
Mulch Depth to Prevent Weeds
Bark mulch applied in excessively thick layers creates a moist environment that inhibits oxygen penetrating into the soil and invites root and stem rot disease. Layers that are too thick also keep water from infiltrating the soil, depriving plants of water and oxygen. Spread coarse bark mulch — chunks or nuggets — to a uniform depth of 3 to 4 inches and fine mulch 2 inches deep, recommends University of Arkansas. Air and water enters the soil more easily through the large pieces of bark than fine bark mulch.
When used as a soil conditioner, or in tree planting, however, bark will reduce the amount of nutrients available to plants. This is not considered a major concern as when the bark decomposes, the nutrients are re-released into the soil as the bark acts as a slow release fertiliser.
The exact processes involved are not yet proven, but an anti-biotic effect is suspected to arise from micro-organisms within the bark. Mulching with bark creates a much healthier root environment, which is also thought to be a contributing factor.
To maximise the bark’s benefits, soil should be fertilised prior to, or at the time of bark incorporation.
Enhances Soil Fertility – As it naturally decomposes, bark releases a range of nutrients and organic matter that enrich the soil below, and enhance its fertility.
What is the pH of bark, and does it make the ground acidic?
Retains Soil Moisture – Bark softens the impact of rain water so that it can effectively permeate the soil, and significantly reduces evaporation from the soil surface.
Yes! Bark protects the soil and any plant roots from summer drought and the full force of thunderstorms. It also provides a layer of insulation, which means that the soil and any plant roots are less likely to succumb to the effects of fluctuating temperatures throughout the year – which is what can cause plants to “heave” out of the ground. When the bark eventually decomposes, it releases a range of nutrients and organic matter that enrich the soil below, and enhance its fertility.
All of our bark products are supplied in 1m3 bulk bags. One bulk bag will provide approximately: 20m2 coverage at a depth of 2 inches (50mm), 13m2 coverage at a depth of 3 inches (75mm), or 10m2 coverage at a depth of 4 inches (100mm).
Can the soil still breathe below the bark?
The pH of our bark is on the acid side of neutral, pH5-6. In comparison, peat is usually in the range of pH3-3.5. Because bark lies on the soil surface, it allows for the passage of rainwater and air into the soil, and therefore has no harmful effect on the soil’s pH.
Bark’s weed suppression capacity arises purely as a result of the physical barrier it creates – depriving weeds of the sunlight they need to grow. Weed seeds land on the mulch surface and are washed down to the soil by rainwater. As they try to germinate on the soil surface, the shoot is deflected underneath the bark where it runs out of vigour and dies. The coarser the bark, the greater its weed suppression capacity.
Spread Landscape fabric and cut it to fit around plants. Photo by Saxon Holt
Left unattended, weeds will quickly fill in unplanted areas and any open ground around plants. Mulch spread over the soil surface blocks the sunlight most annual weeds need to take hold. Weeds that do sprout are easy to pull because soil beneath mulch remains loose and moist. Coarse chipped or shredded bark is a good choice for large areas between trees and shrubs because it decomposes slowly and doesn’t easily blow away. For paths, a thick layer of sawdust provides good weed suppression because it depletes nitrogen in the soil.
Smother Weeds with Mulch
Think it’s an overstatement to call it the war against weeds? Here’s what you’re up against.