le

ft

an

d r

igh

t:

rit

a p

el

cz

ar

. c

en

te

r:

co

ur

te

sy

of

ut

el

ite

54 the American Gardener

CGREEN GARAGE®

WHEN I MOVED to westernNorth Carolina nearly fiveyears ago, I was introduced to

Carolina clay—very different from thesandy loam of southern Maryland, whereI had gardened for 20 years or so. Thesticky, reddish soil seems better suited formaking pottery than for gardening—atleast until it’s amended.

Organic matter—large quantities ofcompost, manure, and pine fines—thatI had added regularly to my sandy Mary-land soil to help retain moisture, I nowadd to my heavy clay for the opposite rea-son, to improve drainage and porosity.But organic matter breaks down andneeds to be regularly replenished. That’snot a problem for the vegetable gardenand other annual plant ings, but for morepermanent plantings, I needed a longer-lasting solution.

Many public gardens across the coun-try have employed expanded aggregates toresolve a variety of soil problems and to in-corporate into “designer soil mixes” forspecific uses. A bit of research and a trial ofmy own has convinced me of the value ofexpanded aggregates in the home garden.

PUFFED PARTICLES When a slate, shale, or clay particle is ex-posed to very high heat, it expands. The re-sult is a stable, lightweight granule—oraggregate—with lots of pores. Whenadded to soil, the pores provide space for

the air and water necessary for healthyplant growth, offering a long-term solutionto opening up heavy soils, adding porosi-ty, and preventing root compaction.

Expanded aggregate products haveother landscape uses: a component in thegrowing mixes for rooftop gardens andcontainer gardens; a porous medium forrain gardens; root bridges to protect treeroots; a lightweight option for backfillingretaining walls; and for porous walkways.They also provide a physical barrier to pro-tect plants from vole damage. Sold undera variety of brand names, many are re-gionally distributed, although some brandsare nationally available. The following arejust a few of the brands that have foundwide usage in horticulture.

Stalite PermaTill® and Espoma’s SoilPerfector™ are derived from slate heat-ed to over 2,000 degrees Fahrenheit. It isthen crushed and graded by size. UteliteE-Soil® and TXI TruGro® are derived

from shale. There are several otherbrands of expanded slate and shale.

Hydroton® and Turface® are madefrom clay and the resulting expanded par-ticles are round. They are commonly usedin growing media for plants such as orchidsand bonsai. They are also used in terrari-ums and hydroponic systems.

EXCELLENT DRAINAGE REQUIREDAt Mount Vernon Estate, Museum, &Gardens in Alexandria, Virginia, Per-maTill has been used in several gardenareas, including the large boxwood parter-res, where excellent drainage is essential.“Our flourishing boxwood parterres havesurvived for 13 years without losing aplant. I credit much of the success in theseformal plantings to the benefits of Per-maTill,” says Dean Norton, Mount Ver-non’s director of horticulture. “Theexpanded aggregates improve soil struc-ture, allowing for roots to breathe.”

At the Dallas Arboretum in Texas,Jimmy Turner, senior director of gardens,considers expanded shale “an indispens-able component of our standard soil im-provement regimen…if you have clay it’sthe miracle cure.”

Turner also recommends mixing ex-panded shale with potting soil to increasedrainage and reduce weight in large con-tainers. He adjusts the ratio of shale topotting soil based on the plant. “I havesome agaves and yuccas planted in 80

by Rita Pelczar

Expanded Aggregates Solve Soil Problems

SourcesEspoma Soil Perfector,www.espoma.com.Growco Indoor Garden Supply,www.4hydroponics.com.Stalite PermaTill Horticultural Aggregates, www.permatill.com.TXI TruGro, www.txi.com.Utilite E-Soil, www.uteliteesoil.com.

Slate aggregate Shale aggregate Clay aggregate

percent shale; for most plantings though,I do 30 percent,” says Turner.

At the Gardens at Post Hill in Morris,Connecticut, Ron Burch propagates, dis-plays, and sells orchids. He uses a combi-nation of Soil Perfector and Turface forsome tropical orchids grown in pots andtemperate terrestrial orchids in the garden.“We use these materials for orchids thatrequire high moisture: Phragmipediumand Maxillaria in pots, and Cypripediumin the garden,” says Burch. “These orchidsrequire consistent moisture but they mustalways have excellent air capacity aroundthe roots. Use of expanded aggregates al-lows for both conditions.”

RAIN GARDEN In 2003, PermaTill was used in the con-struction of the Baker Exhibit Center RainGarden at the North Carolina Arboretum

in Asheville, which was designed to capturestormwater runoff. Downspouts and un-derground pipes direct runoff from a por-tion of the Exhibit Center roof to theadjacent Events Lawn. The storm -water fills perforated culverts below thesurface of the lawn, which are supportedby bioretention media composed of Per-maTill and recycled brick chips. Waterthen drains into the rain garden nearby,which also collects runoff from the sur-rounding parking area. In addition to itsfunction as a sub-grade filtering material,PermaTill, combined in this garden withcomposted leaf mold, provides a porousgrowing medium.

“PermaTill has been incorporated intoseveral other projects at the Arboretum, in-cluding an undersurface layer in sod re-placement areas, incorporated into soilin landscape beds, and as a planting media

for plants requiring very sharp drainage,”says Clara Curtis, director for design andexhibit assets at the Arboretum.

SCREE GARDENSharp drainage is a necessity for the ScreeGarden that was developed in 2006 at theJC Raulston Arboretum in Raleigh, NorthCarolina. “We tilled about six inches ofPermaTill into the beds,”explains assistantdirector and curator of collections, MarkWeathington, “then built berms composedof one third PermaTill and two thirds goodsifted topsoil.” A PermaTill mulch pre-vents soil splashing onto plant foliage.

During the severe drought of 2007,the scree beds actually retained moremoisture than other gardens at the ar-boretum. “This was presumably due tothe capillary space in the slate particles,”says Weathington.

Based on the positive results in theScree Garden, expanded aggregates havebeen used as a soil amendment in otherprojects at the arboretum including theexpanded Xeric Garden, the Asian ValleyGarden, and the Rooftop Garden.

ROOFTOP GARDENSExpanded aggregates are a good fit for roofgardens. “Being one-third to one-half theweight of regular sand, soil, or rock, ex-panded shale reduces the weight load re-quirements,” says Scott Jensen, landscapeand horticulture sales manager forUtilite Corporation. In Salt Lake City,Utah, Utilite E-Soil—expanded shalemixed with compost and topsoil—wasused to create a five-acre, low-maintenancerooftop garden at the Latter-Day Saint’s

co

ur

te

sy

of

de

an

no

rt

on

. b

ot

to

m:

co

ur

te

sy

of

no

rt

h c

ar

ol

ina

st

at

e u

niv

er

sit

y (

2)

55May / June 2012

Soil aggregates have been added to the soil underlying Mt. Vernon’s boxwood parterre.

Left: The Baker Exhibit Center Rain Garden atthe North Carolina Arboretum in Ashevillesupports a variety of plants. Above: During thegarden’s construction in 2007, slate aggre -gate was a component of its foundation soil.

Conference Center. It was also used for anintensive rooftop garden at the Salt LakeCity Main Library, where it supports a va-riety of small trees, shrubs, and grasses.“Expanded shale will not degrade, decom-pose or break down over time—even infreeze-thaw climates,” says Jensen. “Itmaintains its porosity and drainage prop-erties, is consistent and predictable.”

At the Atlanta Botanical Garden (ABG)in Georgia, nearly 50 percent of the roofarea of the Visitor Center is a garden, andPermaTill is an important component ofthe growing medium. “The engineered soilhas held up very well since it was installed,”says Amanda Campbell, ABG’s managerof display gardens. PermaTill has been em-ployed in other areas of the garden, in-cluding the conservatory, as a componentof various container mixes. It is also beingused as a root bridge to protect large treeroots. “In the Camellia Walk, the best lo-cation was chosen for the sidewalk but thatstill meant getting closer than we’d like toa couple of large trees,” says Campbell.“The sidewalks are actually built on a mixthat includes PermaTill in order to protect

the tree roots, ensuring water and nutrientavailability because it resists compaction.”

FOR HOME GARDENSExpanded aggregates work as well in homegardens as they do in public gardens. Bycombining them with other ingredients,such as compost and good topsoil, design-er soils can be created to suit specific plants.

For heavy or compacted soils, Turnersuggests applying a two- to three-inch layerof the aggregates over the intended plant -ing area, tilling it in six to eight inchesdeep, then working in three more inches ofcomposted organic matter. The resultingbed will be slightly raised, further improv-ing drainage.

Their light weight means that contain-er plants grown in a medium that containssignificant amounts of aggregates will beeasier to move. I also found that mulchingwith Soil Perfector prevents wind fromdrying out and blowing away loose soilfrom the surface of large containers.

Last year I gave PermaTill a try in myred clay. I incorporated it along withcomposted manure into a new bed nearmy driveway. So far, the results aregood—sinking a shovel in the preparedbed was a breeze. And despite a very wetspring, the bed has handled the moistureeasily, drainage is good, and my winterjasmine is growing well. �

Rita Pelczar is contributing editor for TheAmerican Gardener.

co

ur

te

sy

of

ut

el

ite

56 the American Gardener

A worker shovels a shale-aggregate soil mixon the roof garden of the Latter-Day Saint’sConference Center during its installation.