14 December 2018

A poo story: Making mulch at Kew

Our mulch supports the growth of our thousands of incredible plants. Find out how we make it and the secret ingredient that makes it so great.

By Ellen McHale

A pile of mulch

The magic happens in the stableyard, which is where we make our wonderful mulch. 

This is a yard in the middle of the Gardens, where we make an incredible 3,500 tonnes of mulch a year. It's used across the Gardens to feed our plants and keep them growing happily. 

Sorting fertiliser in the Stable Yard
Making mulch at Kew, Ellen McHale © RBG Kew

What goes in it?

The base ingredient for mulch is plant waste, which is a mixture of branches, leaves and grass cuttings. This comes from all the pruning, cutting and planting that our horticulturalists work hard on throughout the year. 

The waste is brought to the stableyard from different areas of the Gardens and divided into huge piles of woody and leafy material. It's shredded in our shredder, which increases the surface area so that bacteria and fungi can begin to break it down. 

The secret ingredient

We get lots of used stable bedding (which includes lots of plant-friendly horse poo) from the Royal Horse Artillery Barracks. We get 50 cubic metres per week - that's a lot of poo! 

Once this is mixed into the shredded plant waste, the mixture is formed into massive heaps and watered and turned by machines to kick-start the deomposition. 

A pile of mulch
Making mulch at Kew Ellen McHale © RBG Kew

Temperatures inside the piles can reach a steamy 85 degrees centigrade. This is because the piles are being chemically broken down by fungi and bacteria, which creates heat. 

Hot air is released and creates condensation when it meets cold air. The condensation makes the piles look like they're steaming, just like when you breath out on a cold day. 

Happy soil, happy plants

After about 10 weeks of decomposition, the mulch is ready to be used. 

It's distributed back to the horticulturalists to use in the Gardens. It's used around mature trees, shrub beds, the Palm House bedding display, the Broadwalk, and some of the glasshouses and nurseries. 

At this time of year you'll often see horticulturalists "mulching", which is where they're adding mulch to the soil's surface. 

But why is mulch so great for the plants and the soil? 

It conserves the soil's moisure and reduces weed growth. It also contains acids that make plant roots more permeable, which allows plants to take in water and nutrients better. 

Mulch is a great food source for microorganisms and insects that make up the soil's ecosystems. Keeping these microorganisms happy means healthier soil, and in turn healthier plants

A digger sorting mulch in the Stable Yard
Making mulch at Kew, Ellen McHale © RBG Kew


Making our own mulch is environmentally friendly, as it eliminates any manufacturing process which would use a lot of energy. 

Making it on site saves an enormous amount of transportation, as the raw materials to make mulch and the final product aren't travelling miles to get to Kew. We also resuse stable bedding which would otherwise go to waste. 

We can control exactly what goes into the mulch to make sure it's top quality for our plants. We don't use any chemicals in the processing - just water and air. 

Our top quality mulch gives our plants lots of nutrients to help them thrive - the plants in our Great Broad Walk and Rhododendron Dell love it.

Crimson rhododendrons in the dell

Help us beat our record

Every year you can recycle your Christmast tree with us, and help us make more of our fabulous mulch. We'll shred the trees, mix them with manure, and create a mineral-rich mulch for our treasured plants. 

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