rock wool

29Bussell & McKennie—Rockwool in horticultureNew Zealand Journal of Crop and Horticultural Science, 2004, Vol. 32: 29–370014–0671/04/3201–0029 © The Royal Society of New Zealand 2004

Review

Keywords reuse; horticultural rockwool; review

INTRODUCTION

Rockwool, a fibrous “wool” or flock material, wasfirst discovered occurring naturally on Mauna Loavolcano in Hawaii in 1850. It was first manufacturedin c. 1935 for use as an insulating material for build-ings. The manufacturing process involves meltingforms of basaltic rock at temperatures of 1500°C,incorporating additives and then feeding a stream ofthe molten mixture onto a drum that is rotating atgreat speed and which spins the molten mass intofibres (Blaabjerg 1983; Smith 1987). Insulationrockwool contains additives which prevent the prod-uct from absorbing water. Investigations into find-ing a suitable density of rockwool to grow plants inand into finding an additive (or leaving an additiveout) to make rockwool water absorbent were com-pleted in 1969 (Petersen 1994). These investigationswere the beginning of the use of water absorbentrockwool in horticulture.

The use in horticulture of water absorbentrockwool (and to a lesser extent water repellentrockwool) has expanded in the past 30 years. Thisexpansion has been first, in the number of countrieswhere rockwool is used as a substrate for the propa-gation and production of crops; second, in thenumber of crops grown in rockwool; and third, inincreasing numbers of product types to maximisecrop potential.

The first aim of this paper is to give a summaryof pertinent aspects of rockwool (which is sometimesreferred to as stonewool) in horticulture overseas andmore detailed information of rockwool in horticul-ture in New Zealand. The second aim is, using over-seas information, to consider how the current rangeof most widely used water absorbent slabs, blocks,and plugs might be reused after their first use in NewZealand. This consideration is likely to be of ben-efit for other countries with small populations usingrelatively small quantities of this material.

H03070; Online publication date 17 March 2004Received 9 July 2003; accepted 19 September 2003

Rockwool in horticulture, and its importance and sustainable usein�New Zealand

W. T. BUSSELLSchool of Landscape & Plant ScienceUNITEC Institute of TechnologyPrivate Bag 92 025Auckland, New Zealandemail: [email protected]

S. MCKENNIEVeg-Gro Supplies Ltd10 Firth StDrury, New Zealand

Abstract A brief history of the beginnings of theuse of rockwool in horticulture in overseas countries,an outline of uses overseas of the current forms ofwater absorbent and water repellent horticulturalrockwool, a summary of manufacturers of the prod-uct, a consideration of some of its advantages anddisadvantages, and a listing of the crops grown inhorticultural rockwool are given in this paper. Anoutline of the overseas experiences in both the dis-posal of and the sustainable reuse of horticulturalrockwool is also given. A history of use in NewZealand of horticultural rockwool and the cropsgrown on the product are outlined. Options, whichare both within and outside horticulture, for the sus-tainable reuse of the relatively small volume (c. 300m3/year) which becomes available for reuse in NewZealand at present are described and discussed.Many of these options may be applicable in othercountries where the volume of horticulturalrockwool used is relatively small.

30 New Zealand Journal of Crop and Horticultural Science, 2004, Vol. 32

ROCKWOOL IN OVERSEASHORTICULTURE

HistoryInitial experiments on growing horticultural crops onrockwool started in 1971 in Denmark. This firstcommercial use was in Scandinavia in 1971–72growing cucumbers, followed by tomatoes (1974)and nursery crops (1972–77) (Wikesjø 1974; JorgenBlaabjerg pers. comm.). By the late 1970s it wasbeing used commercially in Holland (Verwer 1976;Sonneveld & Voogt 1980; Verwer & Welleman1980; Blaabjerg 1983). Rockwool was first usedduring the 1980s in other countries such as Australia(Hanger 1982; Donnan & Biggs 1984; Ellyard et al.1985), Canada in 1982 and France in 1980 (Hans

Joergen Jensen pers. comm.), Great Britain (Smith1987), Israel in 1989 (Hans Joergen Jensen pers.comm.), and the United States (Hanan 1983; Ingramet al. 1993; Nelson 1998). Rockwool is now used inover 50 countries including New Zealand. Rockwoolin horticulture in New Zealand will be discussed laterin this paper.

Uses and forms of water absorbent rockwoolThe main use of water absorbent rockwool in horti-culture is as a propagation and growing substrate inits own right (Smith 1987). For this purpose thematerial is almost exclusively formed into plugs,blocks, or slabs. Plants of most crops are propagatedin small plugs or cubes (Fig. 1) until transplantingstage when they are then transferred into blocks

Fig. 1 Diagrammatic representa-tion of the cycle of a tomato cropgrown in rockwool (drawings fromGrodan A/S leaflets).

31Bussell & McKennie—Rockwool in horticulture

(Fig. 1). These blocks are then placed onto slabs (Fig.1), where plants grow on to maturity. A few crops,e.g., cucumber, are sown directly into blocks, whichare later placed onto slabs.

A wide range of blocks and slabs are currentlymanufactured and they differ in density, fibre ori-entation, fibre diameter, size, and shape. They arealso either wrapped in plastic or not wrapped. Slabshave either a low density of rockwool fibres and ashort life (single use slabs) or a high density ofrockwool fibres and a longer life (more year slabs).The fibre orientation in rockwool slabs is eithervertical or horizontal. Slabs with horizontal fibresand of dual density (the top third having a high den-sity, the remainder a lower density) have superiorwater holding capacity and aeration when comparedto slabs with vertical fibres. The fibre orientation inrockwool blocks is more variable, but densities are

consistent. Dimensions and other details of some ofthe slabs, blocks, and loose substrates currentlyavailable are given in Tables 1 and 2 . Further de-tails are available on rockwool manufacturers’websites, e.g., Grodan (http://www.grodan.com) andCultilene (http://www.cultilene.nl).

Water absorbent rockwool is also formed intoflock or granulate in coarse (having the highest pro-portion of large particle sizes), medium, or finegrades. Further details about these grades are avail-able on rockwool manufacturers’ websites. Themajor use of all grades of granulate is as a compo-nent of growing substrates. Water absorbent granu-lates may be used alone or mixed with waterrepellent granulate and other materials. An alterna-tive use for coarse water absorbent granulate is as asoil conditioner both in the greenhouse, where, forexample, the water holding capacity of a sandy soilcan be greatly increased, and in the field.

Uses and forms of water repellent rockwoolWater repellent rockwool used in horticulture isformed only as flock or granulate in coarse, medium,and fine grades. Coarse grade water repellentrockwool may occasionally be used as a soil condi-tioner in the greenhouse. The main use of all gradesof water repellent rockwool is in container mixesalso comprising water absorbent rockwool flock orother substrates such as peat, bark, and soil. Nurs-ery, house plants, hydroponic herbs, and commer-cial cut flower crops are grown in containers withthese mixes.

Table 1 Physical details of commonly availablerockwool slabs (adapted from Smith 1998). (Mostcommonly used sizes in New Zealand are in bold.)

Length (cm) 80, 90, 100, 120, 133Width (cm) 15, 20, 25, 30Depth (cm) 7.5, 10Finish wrapped, unwrappedDensity normal, low, standard, and dual densities

Table 2 Physical details of commonly available rockwool blocks and loose substrates (adapted from Smith 1998).

Dimensions (cm)Common name Brief description length ¥ width ¥ depth How sold

BlocksPlugs (Kiem plugs) Cylindrical profile 2 ¥ 2 ¥ 2.7 240/polystyrene trayMultiblocks Cubes joined at the 2.5 ¥ 2.5 ¥ 4 200/mat(A/O cubes) top to form mats 3.6 ¥ 3.6 ¥ 4 98/mat

5 ¥ 5 ¥ 4 50/matPropagation blocks Individually wrapped blocks 7.5 ¥ 7.5 ¥ 6.5 or (10) 384 (256)/carton

with or without grooved bases 10 ¥ 10 ¥ 6.5 216/carton10 ¥ 15 ¥ 6.5 (2 holes) 144/carton

Loose substrates 10 ¥ 20 ¥ 6.5 (2 holes) 108/cartonGrocubes For pot plant or cut flower production 1 ¥ 1 ¥ 1 1100 litres/palletGranulates For sole use or incorporation Small, medium, or per 20 kg bale

into substrate large granules


Horticultural rockwool manufacturingaround the worldThe first manufacturer of water absorbent rockwoolfor horticultural use was the Danish companyGrodania A/S, now Grodan A/S which is the horti-cultural marketing subsidiary of Rockwool Interna-tional A/S, the largest rockwool manufacturer in theworld. Over the years this company has developeddifferent products in flock or granulate, block orplug, and slab form. Water absorbent rockwool prod-ucts are the major part of Grodan’s production. Thecurrent range of Grodan rockwool (stonewool) prod-ucts is described on the website: http://www.grodan.com.

Other manufacturers of water absorbent rockwoolfor horticultural use who, like Grodan A/S, also sellproduct in New Zealand include: Cultilene(France—http://www.cultilene.nl)—this companyproduces propagation plugs, cubes, blocks, slabs,and granulate (flock); Growool (Australia)—thiscompany primarily produces horticultural propaga-tion cubes, blocks, slabs, and granulate (flock); andVacroc (Slovak Republic)—this company, previ-ously produced a product known as Isotone, pro-duces cubes, blocks, and slabs.

There are many other manufacturers of horticul-tural rockwool throughout the world. Details of mostof these are available on Internet search engines.

Advantages for crop productionin the rockwool manufacturing processThe high temperatures used in the manufacture ofrockwool ensure that the material is essentially ren-dered chemically and biologically inert, making itfree of any potential pests, diseases, and weed seeds.This high degree of sterility is a major advantageover some alternative substrates used in greenhousecrop production.

Other advantages of rockwool include a manufac-turing process that produces a very consistent prod-uct and that some products are able to be sterilisedfor reuse. However, losses in productivity and mar-ginal economics see only small volumes being steri-lised for reuse.

Rockwool slabs and blocks can be wateredfrequently as they drain freely and can thus bemanaged to provide an optimum ratio between airand water for crop production through the season.These products do not modify or restrict nutrientavailability, with more than 98% of the water andfertiliser applied being readily available for uptakeby the crop, and they can quickly respond to changesin liquid feeding (i.e., electrical conductivity (EC)

or pH) requirements delivered by the irrigation tothe root zone of a hydroponic crop. Smith (1998)outlines some of the changes made to the charac-teristics of rockwool slabs to improve crop produc-tion. Further improvements continue to be made,with the latest information on manufacturers’websites.

Disadvantages of rockwoolA perceived disadvantage of rockwool use is find-ing an acceptable and sustainable second use afterits first use. We consider this matter in more detaillater in this paper. Health concerns, particularly inrelation to some lung cancers, had been expressed(Drent et al. 2000). However, in 2001 the Interna-tional Agency for Research on Cancer (IARC) con-cluded a re-evaluation of the carcinogenic risk ofairborne man-made vitreous fibres including allmineral wool fibre products such as rockwool. As aresult of this re-evaluation these products are nowclassified as non-carcinogenic to humans (IARC2001—http://www.iarc.fr/pageroot/PRELEASES/pr137a.html, IARC monographs programme re-evaluates carcinogenic risks from airborne man-made vitreous fibres).

Crops grown in rockwoolThe major food crops grown in rockwool are tomato,pepper (capsicum), cucumber, melon, climbing greenbeans, egg plant (aubergine), lettuce, and strawberry.Descriptions of cultural practices for these crops,which are mostly grown on slabs or in blocks, aregiven in a number of publications including Smith(1998), websites of rockwool manufacturers, andwebsites of extension and advisory organisations.Other food crops (including asparagus, citrus, grape,loquat, olive, and yard long bean) have been grownon rockwool, mostly for propagation only.

The major ornamental and nursery crops grownin rockwool are rose (grown on slabs), gerbera(grown on slabs, blocks, and grocubes), andcymbidium orchid (grown in granulate). Descrip-tions of cultural practices for these crops are givenin a number of publications including Smith (1998)and websites. Other ornamental and nursery cropsgrown on rockwool include anthurium, Australiannative plants, big trees in shopping malls, bouvardia,bulbs, camellia, carnation, chrysanthemum,euphorbia, hibiscus, lily, pelargonium, pine andother timber trees, poinsettia, rhododendron, andsnapdragon (Ellyard et al. 1985; Hansen 1988;Contrisciano & Holcomb 1993; Hansen et al. 1993;Heiskanen 1994; Smith 1998).


Disposal of rockwoolDisposal of used horticultural rockwool in a landfillhas been carried out in all countries where crops havebeen grown on rockwool. Such a disposal methodis either increasingly less available or regarded asinappropriate or unsustainable. However, it is stillsometimes the best solution to handling usedrockwool when an analysis of the total environmen-tal impact (including costs of transport to centralcollecting depots and costs of alternative solutions)through a product life-cycle analysis is taken intoaccount. At present used rockwool is commonlydisposed of in a landfill in many developing coun-tries, in the United States and other developed coun-tries where appropriate sites, e.g., disused quarries,are readily available, or in situations where the vol-ume of rockwool used is small.

SUSTAINABLE REUSE OF ROCKWOOLIN OVERSEAS COUNTRIES

Reasons for investigating sustainable reuse of hor-ticultural rockwool in overseas countries now in-clude: in Europe a general waste regulation has ledto soil amendment with “good” wastes (includingrockwool) being abandoned in many countries dueto the difficulty of distinguishing good from “bad”wastes such as “sewage sludge” containing heavymetals; landfill costs are becoming increasinglyexpensive or landfill sites are becoming increasinglyunavailable in some countries; rockwool has fewercultural demands and generally produces more re-producible results than other substrates used in com-mercial greenhouses (Pieters et al. 1998), andtherefore use is increasing; and rockwool manufac-turers are increasingly wishing to contribute to agood image for their product.

Research on the reuse of rockwool has been andcontinues to be conducted by rockwool manufactur-ers, particularly Grodan A/S and Grodan BNF. TheGrodan studies to date are summarised in the“Grodan solutions” listed in Table 3. Two of these,solutions 1 and 5, are now used widely in the UnitedKingdom and Holland. In both countries nearly allof the used rockwool from crops is now collectedtogether and processed at large facilities (Anon.2000; Neefjes 2001). Within these large facilities theused slabs and blocks are ground with their plasticsleeves still on them. The ground material is left toweather for a period so the root material partiallydecomposes. Plastic and rockwool is then separatedeasier. The majority of the ground rockwool fromthese facilities is made into bricks for houses orremanufactured into both horticultural and insulationrockwool.

Research has been conducted, using many speciesof ornamental plants, on the incorporation of groundused rockwool into potting mixes containing com-post or other substrates (“Grodan solutions” 3 and 4in Table 3) in Poland (Strojny 1997) and Korea(Jeong 2000; Kim et al. 2000; Shin & Jeong 2000;Lim & Jeong 2000; Jeong & Hwang 2001; Kim &Jeong 2003). The other substrates have includedpeat, bark, perlite, rice hull, pinewood chips, andchestnut woodchips. Rockwool incorporation intocompost or other substrates results in improved aera-tion and increased water holding capacity of themixture (Fonteno & Nelson 1990; Choi et al.1999,2000). The Korean research has emphasised the needto grind rockwool and other components of the mix-tures to a suitable grade, to have certain substrates,particularly bark and wood chip, weathered orsoaked to remove chemicals harmful to plant growth,and to steam sterilise ground used rockwool duringthe preparation of mixtures.

ROCKWOOL IN NEW ZEALANDHORTICULTURE

HistoryHorticultural rockwool, in the form of the productGrowool manufactured by Bradford Insulation Ltdin Australia, was first introduced to New Zealand ata Vegfed Conference in Christchurch in the 1980s.New Zealand greenhouse growers did not take upGrowool then. The growers instead began usingnutrient film techniques, sawdust modules, or bagsin soilless culture. It was felt that Growool was

Table 3 Means of re-using rockwool investigated andreported by Grodan A/S.

Solution no. Title of solution

1 Recycling to rockwool factory2 Soil amendment3 Mixing into other substrate, e.g., peat4 Mixing into compost5 Brick production7 Landfill cover8 Mining cover/recultivation9 Mushroom casing


comparatively too expensive and there was a hint ofquality variability and collapse during use, which hassince been eliminated. Individual growers undertooka few trials but it failed to be generally accepted.Bradford Insulation also failed to appoint an appro-priate agent in New Zealand and therefore their prod-uct was neither adequately nor at all aggressivelymarketed.

The horticultural rockwool product Grodan hasbeen commercially distributed in New Zealand since1996. Before a few growers had imported a smallamount of Grodan more year slabs on their ownaccount. The volume of Grodan rockwool used hasincreased considerably since it became commer-cially available and is likely to continue to increase.In the year to March 2003, 17.5 ha of the c. 250 haof crops in greenhouses were grown on Grodanrockwool. Single use slabs (which comprised 95%of the slabs imported in the year to March 2003)rather than more year slabs are likely to be the mostwidely used slabs in New Zealand in future. This isbecause single use slabs are pest and disease free forthe start of a crop, they are homogeneous from slabto slab, and EC and water content can be controlledwith greater accuracy.

Crops grown on rockwoolThe vegetable crops currently grown on rockwoolin New Zealand are tomato, capsicum, cucumber,egg plant, lettuce, and herb vegetables. The orna-mental crops are rose, gerbera, and orchid. Seedlingsand cuttings of many species of cutflower and nurs-ery plants, plants which have been tissue cultured,and aquatic plants are also grown on rockwool.

These crops are grown on one or more of the fol-lowing rockwool products manufactured byGrodan—slabs, grocubes, granulates, kiem plugs,blocks, and A/O cubes. The rockwool products areeither wrapped in plastic or not wrapped.

Reuse of rockwool in New ZealandThere have been no formal studies to date on thereuse (2nd use) of rockwool in New Zealand. A pi-lot study involving placing used slabs (with the plas-tic sleeves removed) as a mulch around avocadotrees has been conducted. The mulch has success-fully suppressed weed growth for 2 years and cropgrowth has not been affected. New tree roots havegrown into the slabs causing breakdown of the slabs.Mulching of avocado and other tree crops (e.g., cit-rus) with surface roots may be a worthwhile andappropriate reuse (2nd use) of rockwool in NewZealand. A second crop in the now widely usedsingle use slabs is unlikely except for cucumbers,where three crops could be grown in the same green-house in a calendar year. The disadvantages ofreusing single use slabs include being broken moreeasily when moved than more year slabs, the possi-bility of incomplete steam sterilisation betweencrops and the occurrence of variable amounts of rootmatter in used slabs making water content and ECcontrol much more difficult than in new slabs. Thevolume of rockwool available for reuse, namely allof the slabs and c. 10% of the blocks and plugs usedeach year, is relatively small and will always besmall compared to some countries.

We can examine possibilities for sustainable useof a product by conducting a life cycle analysis

Table 4 Patents for uses of waste from insulation rockwool having possibleapplication in the reuse of horticultural rockwool in New Zealand.

Brief patent title Patent application

Treatment of waste rock wool JP 89-180809 19890712 (1991)CN 95-10070 19950106 (1996)

Fibrous mats for pressing FR 90-15279 19901206 (1992)Method for treating waste rock wool CN 95-10070 19950106 (1996)Manufacture of artificial soil from JP 95-199224 19950712 (1997)waste rock wool

Mineral wool waste cement US 85-789691 19851021 (1987)Asbestos-free, fibre reinforced DD 89-329861 19890622 (1990)composite cement products

Fillers for bituminous mixtures, US 95-568791 19951207 (1997)especially asphalt mastic

Adhesive mastic SU 85-3928669 19850716 (1987)


Fig. 2 Proposed rockwool recycling system for New Zealand.

(LCA), the objective of which is to provide a quickand reliable insight into the environmental profilesof a crop production system throughout its “lifecycle”. The parts include extraction of raw materi-als, processing and manufacture of a product (in thisinstance rockwool), distribution of the product, cropproduction in the product, and finally the discard-ing or reuse of waste material. A stylised partial LCAfor rockwool in New Zealand, including assumedvolumes, is given in Fig. 2. It starts at the point ofsale in New Zealand (since there is no manufacturerhere) and ends at the point of granulation and reuse(using incorporation of the used slabs by a compostmanufacturer as an example of a reuse option).

The three key people in the life cycle of rockwoolin New Zealand are: wholesaler/distributor, whosells the product(s) to the grower; grower, who pur-chases the product, wraps it (if not ordered and soldpre-wrapped), carries out the cultural practices ofhydration, planting, production (which adds water,fertiliser, and root matter from the plant to the sys-tem), crop pull out at the conclusion of the growingcycle, de-sleeves the product and finally transportsthe used product to the place of granulation; andcompost manufacturer (or other end user), whogranulates the used product, separates any waste, andincorporates the granulated material into composts(or any other end product) for sale or use.

The detail of preparing rockwool for reuse andhow it might be reused may become quite specificfor New Zealand. It is likely that some of the“Grodan solutions” (Table 3) and some of the othercurrent European means of reuse, e.g., brick mak-ing, will not be immediately applicable for NewZealand. Further clues for the reuse of horticulturalrockwool in New Zealand may be provided by someof the patents developed (Table 4) for the use ofwaste from the manufacturing of insulationrockwool. These matters await further investigation.

ACKNOWLEDGMENTS

We thank the Ministry of Agriculture and ForestrySustainable Farming Fund; Tomato Sector, VegFed; Veg-Gro Supplies Ltd; and Grodan A/S for financial support.

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