The Case for Lightweight Aggregate in Parks

Lightweight aggregates are of great value to the management of park and recreation venues. Not only do they ease the maintenance burden, but they play a key role in environmental stewardship. In a 2022 NRPA Agency Performance Review report it was stated that “a typical park and recreation agency dedicates 45 percent of its operating budget to park management and maintenance.” Also cited in the review was on average, 56 percent of capital budgets were designated for renovation, while 30 percent was geared toward new development. The desire to control stormwater runoff and reduce watering are additional issues for park districts.

Park and recreation programming is now more diverse than ever. The number of programs keeps rising and urban parks have exploded to meet the needs of 80% of Americans are now living in urban areas. There are dog parks, deck parks, disc golf, expansions at zoos, community gardens, skate parks, urban trails, arboretums, multi-use courts, nature centers, and water parks. Cities are also creating spaces for special events like weddings and outdoor concerts to supplement budgets. These activities all share concerns for a quality experience, safety and aesthetics, maintenance, and environmental stewardship.

What is so special about lightweight aggregates (expanded shale, clay and slate) for park use? Simply put, they improve soil conditions by incorporating more oxygen and moisture into the root zone. The network of pores present in lightweight aggregates alone contributes to over 60% total pore space. As the name implies lightweight aggregates are light in weight, typically half the weight of sand or pea gravel making them easy to install. The rotary kiln heating process produces an inert ceramic free of weed seeds and pathogens, and the crushing and screening process delivers consistent gradations. The durability of lightweight aggregates adds lasting value to soil mixes because they will not break down or degrade. Lastly, lightweight aggregates drain exceptionally well yet retain moisture and nutrients in the porous ceramic aggregate, eventually releasing these as the soil dries out. And because soil infiltration is improved with lightweight aggregates, roots are encouraged to grow deeper to find water thereby limiting evaporation at the soil surface. This, in turn, saves water. Greater infiltration also limits stormwater runoff into engineered pipe systems which are the source of pollution and water treatment costs. Finally, the deeper rooting promotes healthier plants which saves money in plant removal and replacement.

While lightweight aggregates are produced for many applications, several gradations are available for park and recreation use. Larger gradations (3/8″–3/4″) are specified in structural soil for trees beneath pavement, permeable turf for overflow parking, and utility access and fire lanes. Medium gradations (1/4″–3/8″) do the lions share in the landscape and are used for lightweight soils on structure, bioretention and bioswales, performance lawns, amending clay soil and lightweight soil for container plantings. Finally, smaller gradations (course sand size) are used for turf topdressing, they have been used in rootzone mixes for golf tees and have a long track record as infield conditioners for baseball and softball.

Lightweight aggregates are becoming a new tool for park managers. Managers have initiated designs with landscape architects using lightweight aggregates in bioswale media to collect and filter stormwater from parking lots. I have seen infiltration basins created in parks to reduce offsite stormwater, heavy in nitrogen and phosphorus, from reaching a nearby river.

Lightweight aggregates were specified in structural soil placed beneath permeable paving for urban trees at the World Trade Center 911 Memorial and for lightweight soil at the Klyde Warren Deck Park. When used with a good quality compost they can turn heavy clay soils into quality growing media important at city arboretums and zoos.

Need an event lawn that drains quickly after a rain yet supports vehicles loads? Auditorium Shores in Austin used a combination of lightweight aggregates and sandy clay in their event lawn. It is used for various festivals with heavy foot traffic, semi-trucks often drive on it to unload tents, concert stages, and band equipment.

How about a driving range that doubles as a stormwater filtration basin? Commissioned by New York City’s Departments of Environmental Protection and Parks & Recreation, Mosholu Golf Course has a 9-acre driving range made in part by lightweight aggregates used for infiltration. The site brings together landscape and building design strategies, together with stormwater systems designed to treat runoff at the recreation site. The 12-story water treatment plant below the site delivers up to 30 percent of the city’s water supply.

The City of New Orleans removed compacted soils beneath mature Live Oak trees along historic Esplanade Avenue years ago then added a soil mix containing lightweight aggregates to reduce compaction, adding drainage and oxygen to improve tree performance.

Lastly, lightweight aggregates have been used by countless park districts, universities, and professional baseball teams like the Kansas City Royals to improve the safety and playing conditions of their fields.

Parking lot (left), World Trade Center Memorial (right)
Linear bioswales or infiltration strips designed in parking lots (left) filter runoff and reduce the load on engineered systems. At the World Trade Center Memorial (right) lightweight aggregates were used in the structural soil beneath the permeable paving, providing another stormwater collection method.
Klyde Warren Park (left), Croton golf course and driving range (right)
Urban parks incorporate lightweight aggregates in their growing media for different reasons. At the Klyde Warren Park (left) lightweight aggregates were used in the lightweight soil mix to reduce dead loads. At the Mosholu Golf Course and driving range (right) lightweight aggregates were part of the solution to treat and infiltrate stormwater.

The Expanded Shale, Clay and Slate institute is your source for additional information. You can find producers, project profiles, and specifications on their website at