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Environmental Responsibility and the Art of Mulching

May 28, 2024

Environmental Responsibility and the Art of Mulching

By Alex Henderson, Curator of Living Collections, Royal Botanical Gardens

As responsible gardeners we need to consider the impact our gardening choices have on the environment surrounding our gardens. Natural resources produced by ecosystem services are critical for the health of our species and those others with which we share this planet. We must carefully consider the importance of our garden practices in relation to energy conservation. Such considerations include reduction of synthetic chemical fertilizers and pesticides, recycling green waste to reduce this entering landfill, water conservation and improving soil conditions organically. Our aim should be to create beautiful and sustainable, low maintenance gardens that preserve natural resources and protect the natural landscape.

These were the challenges considered when planning the rejuvenation of RBG’s Rose Garden in 2017. Several landscape design choices were implemented to conserve energy and use of horticultural products including plant selection, low volume drip irrigation, inclusion of companion plants to attract beneficial insects, improved drainage infrastructure, soil improvements and preparation, and the use of mulch.

Earlier this spring, we applied 100 yards of mulch via hydraulic application in the Rose Garden. Mulch is a protective ground covering that saves water, reduces evaporation, prevents soil erosion, controls some but not all weeds, enriches the soil and provides an aesthetic, professional finish.  The Rose Garden mulch is made from ground pine bark and has a fibrous texture which knits together to hold it in place providing long lasting slow decomposition. Applications of mulch have a variety of garden benefits listed below.

fresh mulching throughout the Rose Garden in early spring
Fresh mulch application in the Rose Garden, April 2024
fresh mulching throughout the Rose Garden in early spring

Soil Moisture

Mulch reduces soil moisture loss via evaporation and reduces soil exposure to wind. Soil that is mulched maintains a higher moisture level than bare soil so is constantly available to plants enhancing their health by promoting root growth. This is great for water conservation, reducing the need for supplemental irrigation. Tap water irrigation may also be high in chemicals such as chlorine and salts which can negatively affect soil chemistry. A layer of mulch is both beneficial for plants and water bills.

Soil Temperature

Stabilization of soil temperatures is an important property of mulch as if applied before the ground freezes in autumn can delay the soil freezing and allow for more time for root growth. In summer, mulch can help keep soil temperatures cooler. Soil temperature fluctuations are more extreme where soil is bare, so mulch maintains more even soil temperatures making moisture available for better root growth. Vigorous root growth promotes healthy plants reducing the need to use synthetic fertilizers which over time can reduce soil fertility.

Disease Prevention

Mulch helps reduce rain splash and run off and can help prevent the spread of certain diseases. This is especially important in the rose garden as roses are susceptible to a disease called black spot which is the most important fungal disease of roses globally. Fungal spores are spread by rain, dew, or irrigation water and so mulch in a rose garden may help mitigate this, reducing reliance on harmful chemical applications of fungicides. This is great for plant health and human health.

Weed Suppression

Annual and several perennial weeds can be suppressed by a 7-10cm (3-4 inch) layer of mulch. This prevents sunlight from reaching the soil reducing the chance of weed growth, reducing the need to use herbicide applications. This has many desirable public health and environmental related benefits. Having to weed less is also better for your back!

Beneficial Insects

Mulch may also be an effective way to provide shelter for desirable predatory insects such as beetles and deter pests such as rose sawfly larvae from soil emergence. It is estimated that approximately 97% of common garden insects are considered beneficial or innocuous so mulch may encourage an increase in predator densities.  The 2004 dissertation The Effects of Landscaping Mulch on Invertebrate Populations and Soil Characteristics, published by Ohio State University suggests invertebrates were significantly more prevalent in mulched plots than in bare soil plots, demonstrating invertebrate populations can become established in less than a month in a novel mulch ecosystem.

Attracting beneficial insects to a garden has desirable outcomes for an integrated approach to pest management. Significant consequences include a reduction of pesticide applications and an increase in invertebrate populations that attract beneficial birds and mammals and significantly increase garden biodiversity.

ladybug on rose leaf
wasp on white rose

Plant Fertilizer

One of the great benefits of mulch is the slow, long lasting decomposition process. This produces rich, dark organic material called humus. Increased organic matter and associated microbial biomass has the benefit of enhancing soil food webs which are complex and often involve a very large number of species including beneficial soil dwelling organisms. The gradual breakdown of organic matter promotes consistent and complete plant fertilization, avoiding the need to use synthetic chemical fertilizers which are environmentally damaging, being derived from the petroleum industry. Synthetic fertilizers can also acidify soil killing healthy and desirable soil microbes creating negative impacts on growing conditions. The slow decomposition of mulch also improves drainage and aeration of the root zone and holds sufficient moisture to reduce the need for supplemental irrigation

When thinking about mulch, a useful comparison is the example of forest leaf litter. Each year deciduous woodlands go through seasonal cycles such as losing leaves. As leaf litter decomposes soluble nutrients are released into the forest floor soil. The nutrients are recycled and used as food by trees to produce new leaves each spring. Slow decomposition of leaf litter creates nutrient rich soil supporting the annual life cycle of the woodland. When was the last time you heard of anyone fertilizing a forest with synthetic chemicals? The answer is never, which is why mulch is so valuable in a garden setting replicating the forest floor process.

How much mulch?

A 7-10cm (3-4 inch) layer of mulch is ideal to promote all the benefits mentioned above. Check the mulch layer in spring and autumn to ensure it is maintained at this level. If for example the mulch has decomposed by 3cm (1inch), then add 3cm (1inch) to maintain the correct level.

A Sustainable Garden for the Future

By adopting this practice gardeners can play a responsible and leading role in cultivating environmentally responsible gardens that are resilient to seasonal changes. Mulching is just one of many sustainable garden practices that can help preserve and protect the environment and ecosystem services that support all life on earth. RBG’s Rose Garden is one of the most sustainable of its type on the continent and the use of mulch is just one of many compelling environmental stories associated with the collection. The Rose Garden provides an immersive experience in promoting environmental horticultural practices that are transferable for your own garden, that benefit you, your local community, and the natural environment around you.

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