Mulching or bare soil? Strategies for greater biodiversity

Key points at a glance

• Organic mulch protects perennial beds from drying out and encourages humus formation.
• About 70% of native solitary bees nest in open, uncovered areas of soil.
• Structural diversity is crucial: mulch selectively around shrubs and trees, but keep nesting areas clear for insects.
• Bare soil patches require no maintenance, whereas meadows must have their clippings removed after mowing.

Why a mosaic of habitats is necessary

In gardening, the question often arises whether soil areas should be mulched or kept bare. The answer is not an "either-or", but a matter of location and purpose. While organic mulch reduces evaporation in perennial beds and supports soil organisms like earthworms, it presents an insurmountable barrier for specialised solitary bees.

Since around 70% of native wild bee species build their nests underground, unplanted, sunny and well-drained soil spots are essential for their reproduction. A garden that covers every square metre with woodchip or dense planting deprives these species of their habitat.

The ecological significance of soil conditions

Mulch material – whether grass clippings, leaves or woodchips – simulates natural soil cover. This benefits decomposers and regulates the microclimate on the soil surface. However, a nature garden requires a nuanced approach. Wildflower meadows, for instance, require nutrient-poor (low-fertility) conditions. A layer of mulch would introduce nutrients that encourage the growth of vigorous grasses, crowding out species-rich wildflowers.

Comparison: Mulch vs. bare soil

How to properly design your garden

1. Selective mulching: Use organic material exclusively in perennial and vegetable beds to stabilise soil moisture during the summer months of July and August.
2. Create nesting areas: Identify sunny, unplanted spots. These should remain free of mulch and dense vegetation so that solitary bees can dig their burrows.
3. Meadow maintenance: Mow wildflower meadows and remove the clippings after a few days. This is crucial to avoid nutrient build-up and to maintain the low-fertility conditions necessary for native species.
4. Encourage structural diversity: Avoid large-scale monocultures. A mosaic of mulched areas, bare soil patches, deadwood and leaf piles offers the widest range of ecological niches.
5. Minimise disturbance: Do not dig over soil areas used as nesting sites. The brood cells of ground-nesting species are often only a few centimetres below the surface and would be destroyed by tilling.

Frequently asked questions

Why is mulch harmful on wildflower meadows?

Mulch enriches the soil with nutrients. However, many native wildflowers require nutrient-poor conditions to survive against competitive grasses.

How large does a bare soil patch need to be for solitary bees?

Just a few square metres is enough. The most important factors are a sunny location, loose soil structure, and the absence of mulch or dense planting.