Photo of grass vergeThe most common green spaces in our cities are mown pasture on road verges, parks and private lawns.  We think adding biowastes to these ubiquitous green spaces could improve the ‘ecosystem’ services they deliver, for example retaining storm water (helping reduce flooding) and supporting pollinators (bees) and beneficial predator insects. Woody organic wastes and composted greenwastes are used in large volumes overseas to enhance degraded urban soils. Spreading and/or incorporating minimum depths and qualities of organic soil conditioners is now mandated in parts of USA and Canada through rules and policies[1] and was mandated in Long Bay, North Shore City[2]. Poor quality urban pasture can have high ongoing financial costs (for mowing) and environmental costs where compacted, or intensively irrigated, fertilised and/or sprayed.

In 2015 a corner of old pasture at the Auckland Botanic Gardens (ABG) was divided into 49, 2 by 2 m plots. Two rates of coarse wood chip from macrocarpa trees, were spread over 14 of the plots.  Five other treatments included a control (untreated) and additions of sawdust, sugar and nitrogen.  A high rate (8 litres/m2) was defined as the maximum ground staff considered was practical without smothering the turf or impeding mowing; we also used a half rate of 4 litres/m2 9. We wanted to maintain public use of the area and so didn’t want to disturb the surface by rotary hoeing. Rotary hoeing is the usual way organic amendments are added, and happens at the time of turf renewal or renovation. We hypothesised that an annual addition of woody biowaste to the surface would:


  • lower soil nitrogen levels and increase competitiveness of a variety of herbs, including ones used by beneficial insects (clovers, plantains and wild carrots)
  • reduce turf growth, hence reduce the mowing frequency (saving $)
  • increase infiltration rates as the wood chip broke down, reducing surface runoff and flooding, and improving drought resilience

Picture of macrocarpa mulch

Unfortunately, mowing plots with the high rate of coarse wood chip damaged mower blades, so in the second year we substituted a much finer biowaste (compost). An advantage of the compost was that rain could wash it down to the soil surface - this minimised visual impact, improving aesthetics compared with the coarse mulch.







We measured foliar and soil nutrient concentrations, plant composition and soil infiltration rate. Foliar measurements indicated application of both levels of mulch had slightly reduced Nitrogen concentrations in grasses (1.99 and 2.07 % N in high and moderate mulch treatments compared with up to 2.2 % N in control treatment) in the autumn growth flush (April). This is because the high C:N ration of the mulch meant microbes removed nitrogen as they decomposed the mulch. Soil measurements confirmed both mulch treatments had lowered nitrogen concentrations, so less N was available for plant growth. The abundance of browntop (Agrostis capillaris), a thin-leaved, short grass adapted to low fertility increased in mulched treatments at the expense of ryegrass.  These positive browntop-dominant swards require less frequent mowing and are usually more diverse. However at ABG, the composition of herbs did not change significantly over the first year, and neither did overall pasture biomass.

Picture of measuring infiltration

We measured infiltration rates in late 2016 in 5 of the 7 replicate plots for control and high-mulch treatments. Infiltration rates in both treatments were very good, over 280 mm/hr, with no plots less than 150 mm/hr.   

Low infiltration rates are typically less than 18 mm/hr, and bio-filtration devices (raingardens, bioswales, planters) are usually designed to have infiltration rates of 50 to about 400 mm/hr. This reflects the history of dense pasture cover and infrequent trafficking – there was no clear influence of adding mulch on infiltration. A high infiltration rate means high intensity rainfalls can soak in without surface runoff – helping to reduce flooding in downstream urban areas.

Applying up to 5 litres/m2 is practical if fine mulch is used. A coarser mulch can also be used if mowing height can be increased to above 80 mm, and preferably 100 mm (our lawn mower blades couldn’t be lifted higher than 80 mm).  The trial helps support applying tree pruning’s from parks over grassed areas used for passive recreation (not playing fields) to avoid transport and disposal costs and is likely to deliver local ecosystem benefits; indeed, motorways now routinely spread mulched pruning’s straight back onto adjacent batters. Best results are likely with a fine mulch (e.g. higher mulch rotation speed) and where soils are compacted or have low initial water holding capacity. The soil physical conditions at our trial were pretty good to start with, so were hard to improve, and the plant diversity was relatively high!

Overseas research shows turf plant and soil communities take several years to respond to organic additions and changes in nutrients, so we plan to continue the trial, at least to see how soils may change, and include plant diversity if another University of Auckland masters student can be found to follow Curtis Lubbe[3] (who has started a PhD in the United States).


Table 1: Chemical properties of the mulches used, the soil to which it was added, and a common manufactured garden soil



% w/w


% w/w

C:N ratio








Wood chip 1





Sawdust 1










Arborist mulch





ABG Turf Soil 2

6.2 ± 0.3

0.58 ± 0.01

11 ± 0.2

1032 ± 97

Garden Mix 3





1 the source material in both cases was macrocarpa. The ‘tipping point’ for C immobilisation is C:N 25 to 27

2 This is soil from the whole trial site, sampled in 7 blocks (each block has 1 treatment replicate)

3 Garden mix is a manufactured product with tight Quality Control made from greenwaste-based compost and sand


Benefits of organic mulches

Benefits of organic mulches diagram

 These benefits occur when applied over the soil surface at 50-100 mm depth but depend on underlying soil moisture, fertility and drainage conditions

  • moderate soil temperature fluctuations (cooler in summer, warmer in winter)
  • protect soils from compaction
  • supress weeds (especially weeds growing from small seeds)
  • improve plant establishment by helping conserve soil moisture, and enhance soil moisture storage once broken down (reducing vulnerability to drought) but will also keep dry soils from wetting up/ and wet soils from drying out)
  • reduce runoff and erosion by absorbing water and preventing development of ‘crusts’ by physically protect soil from raindrop impacts
  • influence soil fertility (depending on the proportion of fines, C:N and C:P ratio)
  • enhance contaminant removal (heavy metals, oils, greases)

  For more information please contact Robyn Simcock


[1] Recent ‘best practice’ guide from Canada. link)

[2] However soil conditioning has been less used than raingardens and detention tanks. Wadan P, Clements O and Kettle D. 2013. Construction of low impact design solutions at Long Bay. 8th South Pacific Stormwater Conference and Expo. link)

[3] Lubbe C. 2016. Unpublished M. Sc. Thesis. University of Auckland. link)


Page updated 8/9/2017

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