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This Journal of Environmental Horticulture article is reproduced with the consent of the Horticultural
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Copyright, All Rights Reserved
Propagation
of
Thuja x 'Green Giant'
by
Stem Cuttings:
Effects
of
Growth Stage, Type
of
Cutting, and IBA
Treatment1
Jason
J.
Griffin,
Frank
A.
Blazich, and Thomas G. Ranney2
Department
of
Horticultural Science
North Carolina State University
Raleigh, NC 27695-7609
r-----------------
Abstract
------------------...
Stem cuttings
of
Thuja
L.
x 'Green Giant' ('Green Giant' arborvitae), consisting
of
22 cm (8.6 in) terminals (tips
of
first-order laterals)
or 20 cm (7.8 in) laterals (side shoots removed from those portions
of
terminal cuttings inserted into the rooting medium), were
collected on three dates associated with specific stock plant growth stages (softwood, semi-hardwood, and hardwood) and treated with
indolebutyric acid (lBA) in 50% isopropanol ranging from 0 to 9000 ppm (0.9%). Regardless
of
cutting type or auxin treatment,
cuttings rooted in high percentages at each growth stage. Overall rooting for softwood, semi-hardwood, and hardwood, cuttings was
85%, 86%, and 96%, respectively. Semi-hardwood cuttings were the only cuttings in which percent rooting was affected by IBA
treatment or cutting type, with lateral cuttings rooting in higher percentages than terminal cuttings (92% vs. 79%, respectively), while
IB~
concentrations
of
3000
(.0.3%)
or 6000 ppm (0.6%) proved optimal for both cutting types. Mean root number and mean root length
vaned by growth stage, cutting type, and IBA concentration. In general, rooting percentages and root length were optimized at the
har~wood
stage with either cutting type treated with
3000,6000
or 9000 ppm (0.3%,0.6% or 0.9%) IBA, resulting in >10 roots per
cuttIng.
Index words: adventitious rooting, arborvitae, auxin, indolebutyric acid, interspecific hybrid.
Significance to the Nursery Industry
'Green Giant' arborvitae is a narrow, upright, pyramidal
evergreen tree with a rapid growth rate, and lustrous dark
green foliage. Adaptability to a wide range
of
soil condi-
tions' resistance to windthrow, absence
of
any significant pest
problems, and no reports
of
damage by white tail deer
(Odocoileus virginianus Zimm.) make 'Green Giant' arbor-
vitae an ideal landscape plant. Results herein indicate that
regardless
of
growth stage, stem cuttings
of
'Green Giant'
arborvitae can be rooted successfully throughout the year at
percentages >85%. However, percent rooting and mean root
length were maximized for hardwood cuttings treated with
3000, 6000 or 9000 ppm (0.3%, 0.6% or 0.9%) indolebu-
tyric acid (IBA) in 50% isopropanol.
Introduction
Thuja L. x 'Green Giant' is a hybrid
of
Thuja plicata J.
Donn ex D. Don (western red cedar) x Thuja standishii
(Gord.) Carriere (Japanese arborvitae) (7). 'Green Giant'
exhibits rapid height growth, which can reach 1 to 1.5 m (3
to 5 ft) per year, to a mature height
of
18
m (60 ft), while
maintaining a tightly pyramidal habit. Summer foliage is a
lustrous dark green, turning a shade
of
bronze in winter. The
IReceived for publication August
17,
1998; in revised form October 15,
1998. This research was funded in part by the North Carolina Agricultural
Research Service (NCARS), Raleigh, NC 27695-7643 and by a grant from
the North
Carolina
Association
of
Nurserymen, Inc., P.O.
Box
400,
Knightdale, NC 27545. Special thanks to Richard Schock for providing all
cutting material and William
H.
Swallow for statistical guidance. From a
thesis submitted by J.1.0. in partial fulfillment
of
the requirements for the
M.S. degree.
2Graduate Research Assistant, Professor, and Associate Professor, respec-
tively.
cultivar is reported to be hardy to USDA Zone 5 and tolerant
of
a wide range
of
soils and climatic conditions (1,
2,
7,
9).
Trees do not appear to be susceptible to windthrow and have
few pest problems.
Due to several desirable landscape attributes, there is cur-
rently great interest in propagation and culture
of
'Green Gi-
ant' arborvitae. Although other investigators (3, 4, 6, 8) have
achieved rooting percentages
of
50% to 90% using auxin
concentrations
of
0 to 10,000 ppm (1.0%) for various spe-
cies
of
Thuja, little has been reported on propagation
of
'Green Giant' (1,
2,
7, 9). Therefore, the objectives
of
this
research were to study the influence
of
growth stage, type
of
cutting, and indolebutyric acid (IBA) treatment on propaga-
tion
of
'Green Giant' arborvitae by stem cuttings.
Materials and Methods
Terminal cuttings (tips
of
first-order laterals), approxi-
mately 45
cm
(18 in) in length, were collected from the lower
2 m (6.5 ft)
of
each
of
seven trees. Each tree, approximately
5.5 m (18 ft) in height, was growing under uniform fertility
at Boonville, NC, when cuttings were collected on three dates
associated with specific growth stages: August 29, 1997 (soft-
wood), October 24, 1997 (semi-hardwood), and February
15,
1997 (hardwood). Thirty softwood, and 25 semi-hardwood
and hardwood cuttings were taken per tree.
As cuttings were collected, they were placed in polyethyl-
ene bags that were placed on ice and transported to the Hor-
ticultural Science Greenhouses, Raleigh, NC. From the ini-
tial cutting material, two types
of
cuttings were prepared: 22
cm
(8.6 in) terminals and 20
cm
(7.8 in) lateral cuttings. A
lateral cutting consisted
of
a side shoot removed from that
portion
of
a terminal cutting that was inserted into the root-
ing medium. Stem tissue at the proximal end
of
hardwood
and semi-hardwood cuttings was highly lignified, dark brown
in color, and resembled that
of
mature stem tissue. Tissue at
the proximal end
of
softwood cuttings was less lignified, tan
J.
Environ. Hort. 16(4):212-214. December 1998
212
Tr
in color, and phenotypically, resembled stem tissue
of
the
more distal portion
of
the cutting.
Following preparation, the basal I
cm
(0.4 in)
of
all cut-
tings were treated for I sec with 0, 3000
(0.3%),6000
(0.6%)
or 9000 ppm (0.9%) reagent grade IBA in 50% isopropanol.
Cuttings were air dried for 15 min before inserting the basal
4
cm
(1.6 in) into a raised greenhouse bench containing a
medium
of
perlite:peat (2: 1 by vol) with bottom heat main-
tained at 24 ± 1C (75 ± 2F). Softwood cuttings received the
same treatments with an additional treatment
of
1500
ppm
(0.15%) IBA. Bottom heat was not provided during rooting
of
softwood cuttings. For each growth stage the experimen-
tal design was a randomized complete block with a factorial
arrangement
of
treatments (two types
of
cuttings x four
or
five IBA concentrations), six blocks, and six cuttings per
treatment per block.
Cuttings were maintained under natural photoperiod and
irradiance with day/nights
of
24 ±
5C
(75 ± 9F)1l8 ± 5C (65
± 9F). Intermittent mist operated daily 5 sec every 6 min
from sunrise to sunset. Mist was applied by deflection type
nozzles with a capacity
of
32.2 liters (8.5 gal) per hour. As a
preventative measure, Daconil (chlorothalonil), a broad spec-
trum fungicide, was applied weekly, as a spray application
to runoff, at a concentration
of
4 ml/liter (1.0 tbsp/gal).
At 6 weeks, cuttings were harvested and data recorded.
Data included, percent rooting, number
of
primary roots
~
1
mm (0.04 in) in length, and individual root lengths. All data
except rooting percentages were based on the actual number
of
cuttings that rooted (at least one primary root). Data were
subjected to analysis
of
variance and regression analysis.
Where appropriate, means were separated using least sig-
nificant difference (LSD) procedures at P = 0.05.
Results and Discussion
Stem cuttings rooted in high percentages at all growth
stages. Overall percent rooting was 85%, 86%, and 96% for
softwood, semi-hardwood, and hardwood cuttings, respec-
tively. Rooting occurred rapidly, with root emergence vis-
ible 3 to 4 weeks following treatment
of
cuttings and inser-
tion into the rooting medium.
For
the semi-hardwood growth stage, percent rooting var-
ied with IBA treatment and cutting type (Tables 1 and 2).
Percent rooting
of
softwood and hardwood cuttings was un-
affected by cutting type
or
IBA treatment (data not presented).
Although
no
interaction
occurred
at
the
semi-hardwood
growth stage, both IBA and cutting type affected indepen-
I dently percent rooting. Semi-hardwood lateral cuttings rooted
I
I', in higher percentages than terminal cuttings when averaged
I over all IBA treatments (92% vs. 79%). This is consistent
with reports that lateral cuttings taken from more lignified
tissue (semi-hardwood) tend to root better than terminal cut-
tings (5). Rooting response to IBA treatment was quadratic,
with the greatest rooting (92%) occurring after treatment with
3000 (0.3%)
or
6000 (0.6%) ppm IBA (Table 1).
During this study, semi-hardwood and hardwood cuttings
were subjected to bottom heat although it does not appear
necessary for successful rooting
of
'Green Giant' arborvi-
tae. For many propagators, bottom heat is a routine practice,
particularly when rooting hardwood cuttings, and has been
recommended when rooting species
of
Thuja (3, 8). In unre-
lated studies with 'Green Giant', the authors observed that
rooting occurred more slowly without bottom heat although
overall percent rooting remained high.
1.
Environ. Hort. 16(4):212-214. December 1998
Table
1.
Effect
of
IRA
concentration
on
percent
rooting
and
mean
root
number
of
semi-hardwood
cuttings
of
'Green
Giant'
arborvitae.
IRA
concn.
(ppm)
Rooting(%)
Mean
root
no.
0 73.6 6.0
3000 91.7 13.8
6000 91.7 17.4
9000 86.1 18.0
Linear NS
**
Quadratic * *
NS,
*,
**
Nonsignificant or significant at
P:S;
0.05 or 0.01, respectively.
Table 2. Effect
of
cutting
type
on
percent
rooting
and
mean
root
num-
ber
of
semi-hardwood
cuttings
of
'Green
Giant'
arborvitae.
Cutting
type
Rooting
(%)
Mean
root
no.
Terminal 79a' 12aY
Lateral 92b 16b
'Mean separation within column by LSDo.o, =9, n =24.
yMean separation within column by LSDo.
,=
2.5, n =24.
o
Table 3.
Effect
of
IRA
concentration
by
cutting
type
on
mean
root
number
of
softwood
cuttings
of
'Green
Giant'
arborvitae.
Cutting
type
IRA
concn.
(ppm)
Terminal
Lateral
o
6.9a'
8.7a
1500 I
1.3
a 12.4a
3000 14.8a 18.5a
6000 13.2a 27.4b
9000 24.0a 27.3a
Linear
**
**
Quadratic
NS
NS
'Mean
separation within rows by LSDo.o, =6.21, n =
5.
NS,
**
Nonsignificant or significant at P
:s;
0.01, respectively.
Table
4.
Effect
of
IBA
concentration
by
cutting
type
on
mean
root
length
(mm)
of
hardwood
cuttings
of
'Green
Giant'
arborvi·
t,ae.
Cutting
type
IRA
concn.
(ppm)
Terminal
Lateral
0 17.9a' 31.2b
3000 32.7a 32.0a
6000 33.6a 28.8a
9000 29.8a 27.3a
Linear * NS
Quadratic
**
NS
'Mean
separation within rows by LSDo.
,=
7.6, n =
6.
o
NS,
*,
**
Nonsignificant
or
significant at
P:S;
0.05 or 0.01, respectively.
213
At the different growth stages, root number varied as a
function
of
cutting type and IBA treatment. At the softwood
growth stage an interaction occurred between IBA treatment
and cutting type for root number (Table 3). Increasing IBA
concentration increased the number
of
roots per rooted soft-
wood cutting with lateral cuttings treated with 6000 ppm
(0.6%) having a greater response than terminal cuttings.
When in the semi-hardwood growth stage, both IBA treat-
ment and cutting type affected root number with no interac-
tion (Tables 1 and 2). The main effect
of
IBA treatment was
quadratic with a maximum number
of
roots (18 roots per
rooted cutting) occurring following treatment with 9000
ppm
(0.9%) IBA. Lateral cuttings at the semi-hardwood growth
stage produced more roots than terminal cuttings (16 vs. 12,
respectively). Root number
of
hardwood cuttings responded
only to IBA treatment. Response to increasing IBA concen-
tration was quadratic with 0, 3000, 6000, or 9000 ppm (0%,
0.3%,0.6%,
or 0.9%) resulting in 7, 10, 14, or 14 roots per
rooted cutting, respectively. Developing roots at all growth
stages had radial symmetry around the cutting base which
may
explain
why
trees
are
reportedly
not
subject
to
windthrow.
Mean root length
of
softwood cuttings was unaffected by
either IBA concentration
or
cutting type and averaged
18
mm (0.7 in) per root. A quadratic response to IBA was ob-
served for mean root length
of
semi-hardwood cuttings with
the optimum concentration
of
6000 ppm (0.6%) producing
roots>
25
mm (1.0 in) in length. For hardwood cuttings,
mean root length
of
lateral cuttings was unaffected by IBA
concentration, whereas root lengths
of
terminal cuttings ex-
hibited a quadratic response with an optimal IBA concentra-
tion
of
6000 ppm (Table 4).
Results herein demonstrate that asexual propagation
of
Thuja x 'Green Giant' by stem cuttings
is
rapid and effi-
cient. Moreover, cuttings root in high percentages through-
out the year, reducing the importance
of
growth stage as a
factor in successful rooting. In addition, hardwood cuttings,
which are less perishable and inexpensive to handle (5), con-
sistently
rooted>
95% regardless
of
cutting type or auxin
concentration.
Literature Cited
1.
Alexander,
1.
1994. New plant forum. Proc. Intern. Plant Prop. Soc.
44:549-556.
2.
Bitner R.L. 1997. Thuja 'Green Giant'. The Green Scene 26(2):7-8.
3.
Chong,
c.,
a.B.
Allen, and H.w. Barnes. 1992. Comparative rooting
of
stem cuttings of selected woody landscape shrub and tree taxa
to
varying
concentrations
of
rnA
in talc, ethanol and glycol carriers.
J.
Environ. Hort.
10:245-250.
4. Dirr, M.A. and C.W. Heuser,
Jr.
1987. The Reference Manual
of
Woody Plant Propagation: From Seed to Tissue Culture. Varsity Press,
Athens,GA.
5.
Hartmann, H.T., D.E. Kester, ET. Davies, Jr., and R.L. Geneve. 1997.
Plant Propagation: Principles and Practices. 6
th
ed. Prentice Hall, Inc., Upper
Saddle River, NJ.
6.
Lahiri, A.K. 1979. Vegetative propagation
of
forest trees. Indian Forest
105:
101-105.
7.
Martin,
S.
and
K.
Tripp. 1997. The tale ofThuja 'Green Giant'. Amer.
Conifer Soc. Bu!. 14:153-155.
8.
Ticknor, R.L. 1981. A comparison
of
several hormone formulations
for rooting cuttings. Proc. Intern. Plant Prop. Soc.
31:
109-112.
9.
Tripp,
K.
1995. Hardy New Englanders. Amer. Nurseryman
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1.
Environ. Hort. 16(4):212-214. December 1998 214
