effectiveness of water permeable joint filling materials for weed prevention in paved areas

Effectiveness of water permeable joint filling materialsfor weed prevention in paved areas

B DE CAUWER*, M FAGOT*, A BEELDENS†, E BOONEN†, R BULCKE*& D REHEUL‡*Faculty of Bioscience Engineering, Department of Plant Production, Weed Science Unit, Ghent University, Gent, Belgium, †TechnicalCommittee Road Pavements, Belgian Road Research Centre, Brussel, Belgium, and ‡Faculty of Bioscience Engineering, Department of

Plant Production, Unit of Plant Breeding and Sustainable Crop Production, Ghent University, Gent, Belgium

Received 12 December 2013

Revised version accepted 4 April 2014

Subject Editor: Corn�e Kempenaar, WUR, The Netherlands

Summary

The recent phase-out of herbicide use on public pave-

ments in Flanders has triggered the development of

alternative strategies for weed prevention and control.

In this study, growth chamber experiments investigated

the ability of various water permeable joint filling

materials for pavements to prevent weed growth. Joint

fillers included in the tests comprised five innovative

(iron slag sand, polymeric bound sand and three

sodium silicate enriched fillers) and eight standard

joint fillers (four fine materials, for example, sea sand,

white sand, sandstone and fine limestone, and four

coarse materials based on porphyry and limestone).

Their ability to suppress weeds was investigated by

examining seedling emergence and biomass production

of seven test species in pure or organically polluted

(5%, 10%, 20%, 40% and 80% compost by volume)

filler substrate. Selected test species were dominant,

hard-to-control weeds found on pavements. Seedling

emergence and weed biomass were lowest in iron slag

sand, polymeric bound sand and most sodium silicate

enriched fillers, irrespective of pollution level or test

species. Within standard joint fillers, pure white sand,

sandstone and the coarse materials also reduced bio-

mass, but their inhibitory effect dropped quickly once

organically polluted, in contrast to fine limestone and

sea sand for which weed suppression lasted longer (up

to 40% compost by volume). Weed suppression of

joint fillers was species specific. Our results show that

there is potential for preventing weed growth using fill-

ers that prevent the growth of a wide spectrum of

plant species over a long period.

Keywords: hard surfaces, non-chemical weed control,

weed inhibition, weed emergence, joint sealing com-

pounds.

DE CAUWER B, FAGOT M, BEELDENS A, BOONEN E, BULCKE R & REHEUL D (2014). Effectiveness of water perme-

able joint filling materials for weed prevention in paved areas. Weed Research 54, 532–540.

Introduction

To meet the European Water Framework Directive,

the Flemish Government agreed to phase out the use

of herbicides on public pavements by 2015. This

phase-out has triggered the development of new alter-

native weed control strategies on modular element

pavements. Besides the search for effective non-chemi-

cal curative methods, there is also a need for strategies

that prevent or reduce weed growth on pavements.

Newly constructed pavements can be designed to

prevent weed establishment and development (Gulde-

mond et al., 2007; Rask & Kristoffersen, 2007; Boonen

et al., 2013). Weed growth is affected by the amount of

Correspondence: Benny De Cauwer, Faculty of Bioscience Engineering, Department of Plant Production, Weed Science Unit, Ghent University,

Proefhoevestraat 22, 9090 Melle, Belgium. Tel: (+32) 92649064; Fax: (+32) 92649097; E-mail: [email protected]

© 2014 European Weed Research Society 54, 532–540

DOI: 10.1111/wre.12091

open area between pavers (joint width) and the

availability of light, water and nutrients (Benvenuti,

2004; Kempenaar et al., 2006; Fagot et al., 2011; De

Cauwer et al., 2014a). The amount of open area is

determined by the type of modular paving element (De

Cauwer et al., 2014a). The water and nutrient availabil-

ity is determined by the type of joint sealing material

filled into the joints and organic contamination level

(De Cauwer et al., 2014b). Weed growth on newly con-

structed pavements can be prevented by hermetically

sealing joints with air- and watertight Portland cement

or lime mortars, polymer-modified cementitious mor-

tars or special polymeric bound sands that are impene-

trable to roots (Boonen et al., 2013). However, to allow

local infiltration of rain water and groundwater

recharge, and to decrease fast run-off and flood risk,

water permeable joint fillers are preferred, despite their

weak weed preventive ability. Many water permeable

joint fillers with different technical (e.g. type of natural

material and grain size distribution) and chemical char-

acteristics (e.g. pH, mineral content and salinity) are

commercially available, and even special water perme-

able weed preventive joint fillers (e.g. polymeric bound

sands and sodium silicate enriched sands) have been

developed. Their effectiveness for weed prevention is

not well quantified, particularly over the long term

when joints gradually become organically polluted.

Furthermore, there is a need to evaluate effectiveness

over a wide range of weed species, because weeds may

show species-specific responses to the joint filling mate-

rials (De Cauwer et al., 2014b). In this study, we deter-

mined the weed preventive or suppressive ability of

different standard and innovative joint filling materials.

Materials and methods

Species included in the trials

Eight dominant, hard-to-control weed species found

on pavements, as indicated by Fagot et al. (2011), have

been tested. Nomenclature of species follows Van Der

Meijden (2005). Selected test species were Taraxacum

officinale F.H. Wigg. (dandelion), Poa annua L.

(annual meadow-grass), Plantago major L. (greater

plantain), Trifolium repens L. (white clover), Cerastium

fontanum subsp. vulgare (Hartm.), Greuter & Burdet

(common mouse-ear), Conyza canadensis (L.) Cronq.

(Canadian fleabane/horseweed), Polygonum aviculare

L. and Sagina procumbens L. (procumbent pearlwort).

Growth chamber trials

Between May 2010 and September 2012, eight separate

pot trials, one for each plant species, were established

to evaluate weed suppressiveness of joint fillers. For

reasons of reproducibility and comparability, all spe-

cies were tested under similar tightly controlled and

reproducible experimental conditions. All pot trials

were conducted in a growth chamber at fixed diurnal

temperature and light regime. The growth chamber

was set for a daily 16-h day – 8-h night photoperiod

using Gro-lux cool white fluorescent lamps (F58W/

GRO-T8, Sylvania, Erlangen, Germany) with a red:far

red (R:FR) ratio of 98 and a light intensity of

100 lmol m�2 s�1 at the soil level. Daily daytime tem-

perature increased from 15 to 25°C at a rate of 2.5°Cper hour, was kept at 25°C for 8 h and decreased from

25 to 15°C at a rate of 2.5°C per hour. The daily night

time temperature was kept at 15°C. Pots were watered

twice a day by automatic sprinkler irrigation (1.1 mm

water day�1) to avoid moisture stress. Hence, weed

suppressive ability of joint filling materials was

assessed under optimal temperature and moisture con-

ditions for plant growth.

Experimental plots comprised all factorial combina-

tions of thirteen joint filling materials and seven purity

levels, in terms of added dry compost, arranged in a

randomised complete block design with four replica-

tions. The joint filling materials comprised eight ‘stan-

dard’ joint filling materials and five innovative fillers.

Tested standard joint filling materials (with their range

of grain size indicated in brackets) were as follows:

white sand (0–2 mm), sea sand (0–2 mm), porphyry

(grain size fractions 0–6.3 and 2–6.3 mm), limestone

(fractions 0–2, 0–6.3 and 2–6.3 mm) and sandstone

(0–2 mm). Innovative fillers comprised four ‘weed pre-

ventive’ fillers that were specifically designed for weed

prevention and one ‘experimental’ material (Linz-Do-

nawitz slag sand). The weed preventive fillers were as

follows: Rompox�-Easy (polymeric bound sand), Dan-

sand� block paving sand (sodium silicate enriched

quartz sand), Dansand� stone dust (sodium silicate

enriched crushed granite) and Biozand� (sodium sili-

cate enriched quartz sand). The Linz-Donawitz (LD)

iron slag sand is a secondary by-product of the steel-

making process, which is not yet commercialised as a

joint filler. For all these materials, a set of physical

(Table 1) and chemical (Table 2) characteristics was

determined: water permeability (column test as devel-

oped by OCW, 1968), silt/clay fraction (grain

size < 0.063 mm), sand fraction (grain size 0.063–

2 mm) (obtained by sieving), amount of plant available

water (PF2–PF4.2; as described by Cornelis et al.,

2005), pH (KCl), electric conductivity (measured with

electrode) and mineral composition [N (extraction in

KCl, measurement with Continuous Flow Autoanaly-

ser), K, P, Ca, Na and Mg (extraction in ammonium

lactate, measurement with inductively coupled


Paving joint filling materials to inhibit weeds 533

plasma)]. All tested joint filling materials were strongly

water permeable (Table 1).

Seven purity levels were tested: 100% (unpolluted),

95%, 90%, 80%, 60%, 20% and 0% (completely pol-

luted) by volume, where pollution was taken to be

addition of organic matter. The joint fillers were mixed

with air-dried fine compost with maximum particle size

of 3 mm (chemical composition is given in Table 2) up

to 0%, 5%, 10%, 20%, 40%, 80% and 100% by vol-

ume respectively. Joint filling materials were evaluated

in pure and organically polluted state to mimic in situ

organic pollution in joints. It is the case that joints

accumulate organic material over time (Boonen et al.,

2013). Preliminary research indicated that organic mat-

ter content (estimated by loss on ignition) in narrow

joints (2–5 mm) of 33 old in situ pavements ranged

between 1.8% and 11.3% by weight, with a median of

5.4% by weight. This median corresponds with a pur-

ity level of about 80% by volume, depending on joint

filling material. Joint filler and compost fractions were

thoroughly mixed in the right proportions and distrib-

uted over four plastic pots (one for each replicate) with

the aid of a splitter. In this way, the organic pollution

was evenly distributed over the four pots (or replicates)

and the particle size distribution of the joint filling

materials was not affected.

Prior to sowing, the pot substrate was compacted

manually and moistened to field capacity. Pots

(7 cm 9 7 cm, 8 cm high) were seeded with 25 fresh,

viable seeds of a single weed species per pot. Seed

Table 1 Physical characteristics of the joint filling materials

Joint filler

Water

permeability

(m s�1)

Range of

grain size

(mm)

Fraction small

particles

<0.063 mm (%)

Fraction

0.063-2-mm

(%)

Plant available

water* (volumetric %)

White sand 3.33 9 10�4 0–2 0.2 99.8 1.18

Sea sand 3.37 9 10�4 0–2 0.1 98.9 0.33

Porphyry 0/6.3 1.25 9 10�3 0–6.3 9.3 35.8 7.46

Porphyry 2/6.3 1.60 9 10�3 2–6.3 1.4 1.2 0.12

Limestone 0/2 4.88 9 10�5 0–2 22.1 67.0 10.83

Limestone 0/6.3 3.04 9 10�3 0–6.3 14.4 32.4 8.10

Limestone 2/6.3 1.98 9 10�3 2–6.3 1.9 1.1 0.35

Sandstone 1.10 9 10�2 0–2 5.8 86.8 4.71

Rompox�-Easy NA NA NA NA

Dansand� block paving sand 3.33 9 10�4 0–2 1.8 98.2 1.73

Dansand� stone dust 1.67 9 10�4 0–2 3.7 91.2 3.16

Biozand� 7.55 9 10�4 0–2 0.3 99.6 0.75

LD iron slag sand 9.97 9 10�6 0–2 11.1 88.8 7.81

*Difference in soil water content between PF2 and PF4.2.

Table 2 Chemical characteristics of the joint filling materials and fine compost

Joint filler

Mineral N

(mg/100 g)

P

(mg/100 g)

K

(mg/100 g)

Ca

(mg/100 g)

Na

(mg/100 g)

Mg

(mg/100 g)

EC

(lS cm�1)

pH

(KCl)

White sand <1 0.5 2.4 8.5 2.2 <1 12.5 7.55

Sea sand <1 9.4 6.0 2230 60.6 25.5 457 8.52

Porphyry 0/6.3 <1 2.6 10.4 862 8.6 31.0 115 8.55

Porphyry 2/6.3 <1 2.5 18.8 654 9.2 43.8 68 8.13

Limestone 0/2 <1 1.7 6.4 17452 50.0 187.1 236 8.52

Limestone 0/6.3 <1 1.2 4.8 17474 47.6 194.8 811 8.39

Limestone 2/6.3 <1 2.0 8.4 17208 49.4 199.1 188 8.57

Sandstone <1 <0.5 2.5 21550 <1 545.4 190 7.90

Rompox�-Easy <1 0.9 2.4 21 <1 <1 112 4.03

Dansand� block

paving sand

<1 10.1 2.4 26.8 402 <1 1070 9.57

Dansand� stone

dust

<1 8.3 3.2 254 178 7.4 754 9.34

Biozand� <1 <0.5 0.4 88.2 53.2 0.3 311 8.92

LD iron slag

sand

<1 <0.5 0.4 136.3 <1 170.7 7930 12.70

Fine compost 48.8 23.7 258 1280 142.2 143.3 1275 6.04


534 B de Cauwer et al.

viability of all sown seedlots was above 95%. Seeds of

the extremely small-seeded S. procumbens were not

countable or transferable without losses; hence, each

pot was seeded with an equal volume (0.4 mL) of

S. procumbens seeds. Sowing depth depended on plant

species. Seeds from small-seeded species (P. annua,

C. fontanum, C. canadensis and S. procumbens) were

left uncovered at the surface of the pot substrate.

Seeds from large-seeded species (T. officinale, P. major,

T. repens and P. aviculare) were covered by �2 mm of

the appropriate joint filling mixture, provided that

maximum grain size of the joint filling material did not

exceed 2 mm as is the case for white sand, sea sand,

limestone 0/2, sandstone, Dansand� block paving

sand, Dansand� stone dust, Biozand� and LD iron

slag sand; otherwise, seeds were left uncovered. Prior

to sowing, seeds of P. aviculare were stratified at 4°Cfor 3 weeks to relieve dormancy. Sagina procumbens

was only tested in white sand, sand stone, Rompox�-

Easy, Dansand� block paving sand, Dansand� stone

dust, Biozand� and LD iron slag sand.

Recorded parameters

Plant responses to joint filling materials were investi-

gated by assessing seedling emergence (%) and above-

ground dry biomass (mg pot�1) 30 days after sowing.

Weed seedling emergence was calculated as the number

of live seedlings expressed as a percentage of the num-

ber of viable seeds planted. Aboveground biomass was

clipped at surface level and dried at 70°C for 24 h. For

S. procumbens, biomass was estimated by species cov-

erage (pot area covered by living biomass expressed in

number of green pixels plot�1); coverage was recorded

30 days after sowing by taking pictures of each pot at

right angles from a height of 40 cm above the ground

and quantified using ImageJ software.

Data analysis

Emergence and biomass data obtained from each pot

trial were analysed with the open source language and

environment R (version R2.15.1; R Development Core

Team, 2012) and/or its dose–response curves extension

package drc (Ritz & Streibig, 2005) based on Knezevic

et al. (2007).

Due to a significant species by joint filler by purity

level interaction (P < 0.05), the influence of joint filler

on seedling emergence and weed biomass was analysed

within species and most prevalent purities (0%, 5%,

10% and 20% compost by volume) using one-way

ANOVAs. To determine the significant differences

between group means, the Tukey HSD test (for

normally distributed data) or the Bonferroni test (for

non-normally distributed data) was used.

To estimate long-term weed suppressive ability of

joint fillers, a proxy approach was used. Using the

dose–response technique and associated ED50 typically

used for herbicide evaluation, we applied this using %

filler purity as the causative factor and raw biomass

data (i.e. aboveground dry biomass per pot) as the

response variable. Whilst % purity does not directly

affect weed biomass, organic material accumulates in

joint fillers over time (Boonen et al., 2013) and the

derived ED50s allow relative comparison of fillers.

Weed biomass can exactly be predicted by percentage

of compost added to the filler if seeds, light and water

are in place. No dose–response curves could be fitted

to data of the innovative joint fillers because of their

high weed suppressive ability even at low purities.

Dose–response curves for all standard joint filling

materials were fitted simultaneously for each species.

The initial regression model was the four parameter

Weibull model (Streibig et al., 1993):

cþ ðd� cÞ expf� exp½bðlog x� eÞ�g ð1Þ

where Y represents aboveground dry biomass per pot

(g), at dose x (i.e. the purity or% joint filling material by

volume). The parameter b denotes the relative slope

around the inflection point of the curve (e), and the

upper and lower limits of the curve are d and c respec-

tively. This model was retained for C. canadensis, and

the parameters c and d were defined as equal for all

curves (i.e. joint filling materials). For all other species,

the initial model was reduced to the three parameter

Weibull model, with c being zero. For T. officinale,

P. annua, P. major, T. repens and C. fontanum, the

parameter d was defined as equal for all curves, because

there was no lack-of-fit between the models with and

without similar parameter d. A Box-Cox transformation

was applied on the data of T. officinale, P. annua,

P. major, C. fontanum, C. canadensis and Po. aviculare

to obtain variance homogeneity (Streibig et al., 1993).

Effect dosage ED50 (i.e. the purity, expressed in%,

required for 50% biomass reduction) and selectivity

indices (SI) as relative potencies between two dose-

response curves were derived from the regression model

utilising the delta method (Van der Vaart, 1998). SI (50,

50) (i.e. the ratio between ED50 for one dose–responsecurve and ED50 for another dose–response curve) was

used to compare the relative differences in ED50

amongst curves. ED50 was chosen because ED70 and

ED90 estimates for several curves were outside the

observed dose range (purity range 0–100%).



Results

Effect of joint filler on seedling emergence

Seedling emergence was lowest in the innovative fillers

Rompox�-Easy, Dansand� block paving sand, Dan-

sand� stone dust and LD iron slag sand, irrespective

of purity or species (Table S1). At 100% purity, seed-

ling emergence was significantly lower than in all stan-

dard materials, except for a small number of cases. At

80% purity, the level found in old in situ pavements,

and the reduction in germination from the novel mate-

rials, apart from Biozand�, was clear (Fig. 1). Within

the purity range 80–100%, no seedlings emerged in the

sodium enriched quartz sands Dansand� block paving

sand and Dansand� stone dust, irrespective of species,

except for Po. aviculare. Seedling emergence in pure

LD iron slag sand and pure Rompox�-Easy ranged

between 0–18% and 0–7%, respectively, well below the

seedling emergence range of standard fillers. Within

sodium enriched quartz sands, seedling emergence was

significantly higher in Biozand�, except for P. major

and P. aviculare at purities 95 and 100% and for

C. canadensis at 100% purity.

Seedling emergence in pots filled with pure joint fil-

ler depended on species (Table S1). In general, seedling

emergence was lower in coarse-grained standard fillers

(grain fractions 0–6.3 and 2–6.3 mm) than in the fine-

grained standard fillers white sand and sandstone.

White sand showed highest emergence of T. officinale,

P. annua, P. major, C. canadensis and C. fontanum.

Extremely low (<10%) emergence percentages were

obtained for C. canadensis in limestone 2/6.3 and

Po. aviculare in limestone 0/6.3. However, emergence

in these coarse-grained standard fillers quickly

increased once organically polluted. Within fine-

grained standard fillers, sea sand showed lowest seed-

ling emergence of T. officinale, P. annua, P. major and

C. fontanum, irrespective of purity. Contrary to coarse-

grained materials, sea sand exhibited a slow increase in

seedling emergence with increasing pollution. Emer-

gence of P. aviculare was lowest in sandstone, irrespec-

tive of purity.

Effect of joint filler on aboveground dry weed

biomass

Similar to seedling emergence, weed biomass was low-

est in the innovative materials Rompox�-Easy, Dan-

sand� block paving sand, Dansand� stone dust and

LD iron slag sand, illustrated for 80% purity in Fig. 2.

This was the case irrespective of purity or species

(Table S2). Within the purity range 90–100%, no bio-

mass was produced in the sodium enriched quartz

90

100

50

60

70

80

Taraxacum officinale

20

30

40

% g

erm

inat

ion Poa annua

Plantago major

Trifolium repens

Conyza canadensis

0

10

Cerastium fontanum

Polygonum aviculare

Fig. 1 Percentage germination of seven weed species after 30 days in 13 joint fillers, each mixed with 20% compost by volume. See

Table S1 for significances.



sands Dansand� block paving sand and Dansand�

stone dust, irrespective of species. Biomass produced in

pure LD iron slag sand and pure Rompox�-Easy was

extremely low (ranging between 0–4.0 and 0–2.3 mg,

respectively) compared with pure standard fillers (rang-

ing between 2.4 and 83.2 mg). Within sodium enriched

quartz sands, weed suppressiveness was lowest for

Biozand�.

In general, weed biomass in pure coarse-grained

standard fillers was statistically not significantly differ-

ent from biomass in the best weed-suppressing pure

innovative fillers (Dansand�, Rompox�-Easy and LD

iron slag sand), except for porphyry 0/6.3. However,

weed biomass in these fillers increased quickly once

organically polluted; at a purity of 95%, biomass was

significantly higher in the coarse-grained standard fill-

ers than in the best weed-suppressing innovative fillers,

irrespective of species, except for T. officinale and

P. aviculare in limestone 2/6.3.

At 100% purity, no significant differences in bio-

mass were found between fine-grained standard fillers

white sand, sea sand and limestone 0/2, irrespective of

species, except for P. annua with higher biomass in

limestone 0/2 than in sea sand. Compared with white

sand, sea sand and limestone 0/2, biomass in pure

sandstone was equally high (T. repens, C. fontanum

and P. aviculare) or higher (P. major and C. canaden-

sis). At lower purity, weed suppressive ability was

quickly lost, particularly in white sand. Compared with

pure white sand, white sand at 95% purity had a 2- to

20-fold higher biomass, depending on species. At 80%

purity, white sand was the least weed suppressive filler

amongst standard fine-grained fillers, irrespective of

species, except for C. canadensis. Similar to coarse-

grained fillers, no differences in biomass were found

between sea sand and the best weed-suppressing inno-

vative joint fillers when tested in pure state, irrespective

of species, except for T. repens and C. canadensis.

However, contrary to coarse-grained fillers, weed sup-

pressive ability of sea sand lasted longer under polluted

condition; at 90% purity, biomass of T. officinale,

P. annua, P. major, C. canadensis and C. fontanum

was not significantly different from biomass in the best

weed-suppressing innovative fillers.

Coverage by S. procumbens depended on purity

level and joint filler (Table 3). Within the purity range

60–100%, weed coverage was significantly lower in the

innovative materials Rompox Easy, Dansand stone

dust, Dansand block paving sand and LD iron slag

sand than in standard materials and Biozand.

Purity levels required for 50% biomass reduction

(ED50) in standard joint filling materials

Standard joint fillers differed largely in terms of ED50

values (Table 4). Differences in weed suppressiveness

250

275

300

150

175

200

225

Taraxacum officinale

50

75

100

125

Abo

ve-g

roun

d dr

y bi

omas

s (m

g po

t–1)

Poa annua

Plantago major

Trifolium repens

Conyza canadensis

0

25

Cerastium fontanum

Polygonum aviculare

Fig. 2 Above-ground dry biomass (mg pot�1) of seven weed species after 30 days in 13 joint fillers, each mixed with 20% compost by

volume. See Table S2 for significances.



were most pronounced for T. repens (up to a sevenfold

difference in ED50 value) followed by T. officinale,

P. major and P. aviculare (up to threefold differences

in ED50 value). For C. fontanum, P. annua and

C. canadensis, differences were less pronounced.

Taraxacum officinale and P. annua had lowest ED50

values in limestone 0/2. ED50 values for Pl. major and

P. aviculare control were lowest in limestone 2/6.3. Tri-

folium repens, C. fontanum and C. canadensis showed

lowest ED50 values in porphyry 0/6.3, sea sand and

sandstone respectively.

Weed suppressive ability of white sand was amongst

the worst with ED50 values >83%, except for P. major

and C. fontanum (Table 4). Sea sand showed a good

suppressive ability (ED50 < 77%) for most species,

except for P. aviculare. Weed suppressiveness of lime-

stone-based fillers depended on their granulometry,

irrespective of species, except for C. canadensis. The

finer-grained limestone fillers 0/6.3 and 0/2 had signifi-

cant lower ED50 values for T. officinale, P. annua,

P. major and C. fontanum than the coarser-grained

limestone 2/6.3. Similarly, porphyry 0/6.3 exhibited

lower ED50 values for T. repens and P. major than

porphyry 2/6.3.

Discussion

Dansand� block paving sand, Dansand� stone dust,

Rompox�-Easy sand and Linz-Donawitz (LD) slag

sand showed the highest weed suppressiveness, irre-

spective of pollution or species (Fig. 2). Weed suppres-

siveness of sodium enriched Dansand� fillers is caused

by the excess of sodium salt (Table 2), which increases

the osmotic pressure of the soil water, hence prevent-

ing seeds and roots from absorbing water and that

provokes sodium toxicity (physiological drought)

injury to scarcely emerging seedlings. Weed germina-

tion and growth may be hampered by toxic concentra-

tions of trace elements (LD iron slag sand) and/or

extreme substrate acidity (Rompox�-Easy) or alkalin-

ity (LD iron slag sand). Extreme pHs directly affect

(micro)nutrient availability and/or substrate concentra-

tion of plant-toxic minerals (e.g. aluminium). In addi-

tion, Rompox�-Easy is impenetrable to plant roots

(Boonen et al., 2013).

However, not all alleged weed-suppressing innova-

tive fillers are equally effective nor are they ready for

application on a large scale. Indeed, the concentration

of sodium salts is critical for weed suppression of

sodium enriched fillers as shown by the poor perfor-

mance of Biozand�. The eventual success of the stud-

ied innovative materials in in situ pavements not only

depends on their weed suppression potential but also

depends on their tolerance to frost and road salt and

on their behaviour under traffic load. Polymeric bound

fillers that seal the paving joints may lose their weed

suppressiveness over time due to the appearance of

cracks caused by repeated frost-thaw cycles or high

traffic load (Boonen et al., 2013). These cracks may

provide anchorage for weeds and access to nutrients

and moisture, particularly when they become filled

with organically polluted dust. Weed suppressiveness

of sodium enriched fillers and iron slag sand may

decrease over time by dissolution and leaching of their

weed suppressive compounds. However, leaching of

sodium out of the root zone is expected to be small

under natural conditions with alternate wet and dry

weather. Indeed, sodium leached by rainfall is expected

to migrate upwards due to evaporation and capillary

movement of water during dry and hot conditions. In

addition, sodium silicate enriched quartz sands release

their sodium slowly over time.

Most pure standard joint filling materials, and

coarse materials (limestone and porphyry) in particu-

lar, are quite weed suppressive (up to 95% biomass

reduction relative to biomass obtained in pure fine

compost). Weed growth is hampered by the low con-

tent of plant essential nutrients (e.g. white sand) and/

Table 3 Substrate coverage (number of pixels pot�1) by Sagina procumbens sown into different joint filling substrates at various purities

(0%, 5%, 10%, 20%, 40%, 80% compost by volume; means � SE)

Joint filler

% dry compost by volume

0 5 10 20 40 80

White sand 36 � 4.6b 53 � 10.4a 81 � 22.5a 73 � 10.9a 314 � 63.2a 301 � 197.8a

Sandstone 117 � 37.8a 88 � 36.1a 105 � 30.8a 108 � 24.9a 379 � 55.9a 404 � 254.9a

Rompox�-Easy 12 � 5.0c 2 � 0.9b 7 � 3.8b 9 � 3.2b 13 � 8.4b 62 � 9.5a

Dansand� block paving sand 0 � 0.0c 0 � 0.0b 0 � 0.0b 0 � 0.0b 2 � 2.0b 39 � 3.0a

Dansand� stone dust 4 � 4.0c 2 � 1.5b 2 � 2.0b 2 � 2.3b 11 � 5.2b 89 � 6.4a

Biozand� 33 � 5.1b 35 � 4.8a 38 � 12.3a 70 � 13.2a 269 � 54.4a 185 � 56.3a

LD iron slag sand 1 � 0.7c 7 � 3.9b 9 � 7.4b 6 � 3.1b 5 � 2.2b 38 � 21.2a

Mean values followed by the same letter are not significantly different at P = 0.05 according to the Tukey HSD test (in case of homosce-

dasticity) or the Bonferroni test (in case of heteroscedasticity); comparison within columns only.



or suboptimal pH (e.g. sea sand, porphyry and lime-

stone) and/or low content of plant available water (e.g.

porphyry 2/6.3 and limestone 2/6.3).

With the exception of sea sand (and to a minor

extent limestone 0/2), standard materials and coarse

joint fillers, in particular, quickly lose their weed inhib-

itory ability when they become organically polluted.

Adding organic material (fine compost in our experi-

ment) lowered pH and increased soil water and nutri-

ent availability, creating better opportunities for weed

germination and growth. It is inevitable that joints

become organically polluted after time in in situ pave-

ments (Kempenaar et al., 2009). Hence, pavements

should be kept as clean as possible by regular sweeping

operations to limit organic pollution of the joint filling

materials to maintain weed suppressiveness as long as

possible.

Contrary to the other standard fillers, sea sand

showed a long-lasting inhibitory effect on weed growth

and seedling emergence: seedling emergence and above-

ground biomass remained low even at a pollution up

to 40% of compost by volume. This effect is probably

due to higher salinity and lower amount of plant avail-

able water (Table 2).

The weed inhibitory effect of a joint filler is species

dependent. Without foreknowledge of the expected

weed flora in a particular environment, it is recom-

mended to choose a filler that suppresses a broad spec-

trum of weeds even when it becomes polluted. Good

choices in this context are sea sand and all innovative

joint fillers except Biozand�.

Weed suppressive ability of joint fillers was tested

under optimal substrate moisture conditions (irrigation

was kept frequent and at low intensity per turn) in our

experiment. This may explain the relatively high seed-

ling emergence and weed biomass in standard materials

with low content of plant available water. Our experi-

mental conditions are a good reflection of in situ con-

ditions in springtime and early summer when substrate

moisture conditions are optimal for germination and

plant growth (De Cauwer et al., 2014b). Hence, joint

fillers should preferably be weed suppressive under

optimal growing conditions indeed. As it is expected

that moisture stress may occur later in the season

owing to irregular rainfall, weed suppression may then

be better than in our experiments. Overall, weed sup-

pressiveness of joint filling materials derived from our

short-term growth chamber pot trials was in line with

weed suppressiveness found in the study of De Cauwer

et al. (2014b) in which weed suppressive ability of

Dansand�, sea sand, white sand, limestone and por-

phyry was evaluated in in situ mini-pavements over a

2-year period. Hence, weed suppressiveness over

2 years in in situ pavements may be accuratelyTable

4Estim

atedpurities

(%)required

for50%

biomass

reduction(ED

50)withSEforeightplantspeciesgrownin

eightstandard

jointfillers

Jointfiller

Taraxacu

m

officinale

Poaannua

Plantago

major

Trifolium

repens

Cerastium

fontanum

Conyza

canadensis

Polygonum

aviculare

Whitesa

nd

82.4

�4.13d

85.0

�3.24c

58.1

�11.08bc

89.0

�1.28e

70.6

�9.76b

93.2

�9.42a

90.0

�2.76c

Seasa

nd

67.9

�4.18bc

76.7

�37.6b

26.7

�7.66a

46.7

�6.26c

51.6

�7.26a

73.5

�18.25a

84.2

�3.69bc

Porphyry

0/6.3

71.0

�5.12c

83.9

�5.12bc

65.4

�4.96c

13.3

�5.85a

90.7

�4.22c

96.5

�3.67a

72.8

�9.36b

Porphyry

2/6.3

77.3

�5.07cd

89.0

�2.42c

81.2

�2.63d

28.9

�5.84b

93.3

�1.64c

87.1

�7.31a

55.5

�11.21ab

Lim

estone0/2

32.1

�7.38a

55.8

�10.58a

42.2

�6.96ab

53.4

�5.70cd

70.3

�7.07b

90.2

�5.47a

82.8

�8.47bc

Lim

estone0/6.3

57.9

�5.75ab

71.6

�7.12a

26.4

�8.59a

13.6

�17.71a

53.9

�11.38ab

85.1

�9.92a

32.2

�21.62a

Lim

estone2/6.3

72.3

�4.39c

80.7

�2.82bc

68.4

�5.41c

22.2

�5.90ab

93.7

�1.60c

83.1

�10.02a

57.9

�10.05ab

Sandstone

35.2

�9.40a

59.7

�13.78ab

41.1

�9.20ab

65.7

�5.58d

62.9

�6.32ab

60.6

�28.19a

69.8

�6.36b

Nosignificantdifferencesbetweenfigureswiththesameletter

(basedoncomputedselectivityindices

andcorrespondingP-values);comparisonwithin

columnsonly.



predicted by performing abovementioned pot trials in

growth chambers. However, further field validation

will be required to test their predictability.

In conclusion, based upon our results, a good strat-

egy to prevent weeds requires the use of fillers showing

a high and long-lasting weed suppressive ability over a

broad spectrum of species and pollution levels. The

methodology used in this study may be used as a quick

test to evaluate weed suppressive ability of new joint

fillers. Further field testing for the longevity of weed

suppression is required. If new materials are to be

tested for their weed preventive or suppressive ability,

it is important to evaluate them with a broad range of

weed species present on pavements.

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Supporting Information

Additional Supporting Information may be found in

the online version of this article:

Table S1 Seedling emergence (%) 30 days after sow-

ing different plant species sown into various joint fill-

ing substrates at different purities (100%, 95%, 90%,

80%), based on adding dry compost by volume (0%,

5%, 10%, 20%). Standard errors in brackets.Table S2 Above-ground dry biomass (mg pot�1)

30 days after sowing different plant species grown in

various joint fillers at different purities (100%, 95%,

90%, 80%), based on adding dry compost by volume

(0%, 5%, 10%, 20%). Standard errors in brackets.



effectiveness of water permeable joint filling materials for weed prevention in paved areas

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