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Winter protection of container-grown nursery stock as affected by container design, hardening-off time and solar exposure
Scientia Hortieulturae, 20 (1983) 275--280 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
275
WINTER PROTECTION OF CONTAINER-GROWN NURSERY STOCK AS AFFECTED BY CONTAINER DESIGN, HARDENING-OFF TIME AND SOLAR EXPOSURE
H.M. ROHSLER and H. V A N DE WERKEN
Department o f Ornamental Horticulture and Landscape Design, University of Tennessee, Knoxville, TN 37901 (U.S.A.) (Accepted for publication 7 May 1982)
ABSTRACT Rohsler, H.M. and van de Werken, H., 1983. Winter protection of container-grown nursery stock as affected by container design, hardening-off time and solar exposure. Scientia Hortic., 20: 275--280. The effects of container type (round vs. square), solar exposure (100% vs. 50%) and hardening-off time (0, 3 or 6 weeks) on winter damage to llex crenata Thumb. 'Hetzii' (Hetz Japanese holly) and Buxus sempervirens L. (common boxwood) were studied. Container type and hardening-off time had significant effects on plant viability of Ilex crenata 'Hetzii' after exposure to freezing temperatures. Plants in square containers achieved substantially higher viability ratings than plants in round containers after freezing. Buxus sempervirens was significantly affected by hardening-off time only. Plant viability increased greatly with 3 or 6 weeks of hardening-off as compared to no hardening-off. Common boxwood plants in square containers did not achieve the same level of growth as plants in round containers during the 1980 growing-season (before freezing). Optimum growth for Buxus sempervirens occurred under full solar exposure. Keywords: Buxus sempervirens L. ; container type; hardening; llex crenata Thumb.; solar exposure; temperature. INTRODUCTION
With the increasing shift to container culture, growers in northern regions have realized certain problems associated with growing nursery plants in containers. One of the most serious of these problems has been that of winter damage to container root systems (Steponkus et al., 1976; Self, 1977). Root versus shoot hardiness of woody plants differs considerably, with roots being less hardy (Mityga and Lanphear, 1971; Pellett, 1971). These findings explain why the roots of a container-grown plant are much more susceptible to cold damage than those of a field-grown plant which are protected by the internal natural heat of the earth. Critical root-killing temperatures for many ornamental nursery plants have been determined (Havis, 1976; Studer et al., 1978). These temperatures are very useful to nurserymen from the stand-point of providing adequate winter protection for container plants. Knowledge about cold 0304-4238/83/$03.00
© 1983 Elsevier Science Publishers B.V.
276
tolerance of roots has resulted in greater efficiency of winter protection methods. However, with increasing fuel costs, new methods of winter protection which use little or no energy are needed. The objectives of this research were to determine the effects of container t y p e (round vs. square) on temperature changes in the medium (as influenced by low ambient air temperatures), and to determine the effects of container type, hardening-off time and solar exposure levels on postfreezing plant performance. MATERIALS AND METHODS
The study was initiated in the spring of 1980 and terminated approximately one year later. Species selected for the experiment were 2-year-old bed-grown liners of Ilex crenata 'Hetzii' and Buxus sempervirens. A total of 192 plants was used for each species. One half (96) of each species was grown under full sun, while the other half was grown under 50% shade cloth. Within each exposure treatment, one half (48) of each species was planted in plastic round containers, while the other half was planted in square containers constructed from roofing-felt material. The grade of roofing-felt selected had a rigidity close to that of the walls of the plastic containers. The square containers had straight side walls to facilitate tight blocking. Both container types had approximately 2.84 1 capacity. Each container-type was placed in a separate block. Finally, each container block was randomly split into 3 groups to establish the hardening-off treatments scheduled for late autumn after growth cessation. Each hardening-off group consisted of 3 replications of 4 plants each. Additionally, every replication of the 3 hardening-off groups contained 4 randomly distributed plants for fresh and dry t o p weight analyses, which were done prior to the hardeningoff treatments. All plants were grown for one full growing-season and then remained in the field until mid-autumn, where they were allowed to achieve their first stage of cold acclimation naturally. After removal of fresh and dry top weight samples, the remaining plants were taken from the field and subjected to additional hardening-off for 0, 3 or 6 weeks in a refrigerator unit set at --1.1 to 1.7°C. These temperatures were intended to initiate the second and final stage of cold acclimation in plants w i t h o u t causing damage to r o o t or shoot tissues (Weiser, 1970; Nyholm, 1975). An 8-h light period was provided once every 24 h in the refrigerator unit to help prevent defoliation of plant tops. The light source was 60 watt incandescent bulbs. With 8 light bulbs and aluminum reflectors on the walls, a light intensity of 40-60 footcandles was obtained. At the end of each hardening-off period, the plants of the appropriate treatments were transferred to a freezer unit where they were exposed to a 41-h freezing-period. Plants were blocked in the freezer by container type, and container medium temperatures were monitored by glass bulb
277 m e r cu r y t h e r m o m e t e r s and by t h e r m o c o u p l e s attached t o a recording t e l e t h e r m o m e t e r . The t e m p e r a t u r e in the freezer was started at 4.4°C and stepped down 2.4°C/h until a m i n i m u m range o f - - 1 2 . 2 t o --17.7°C was attained. B o t t o m heat of --1.1 to 1.7°C was supplied during freezing by heatingcables set 7.5 cm below the sand and gravel surface on which t he containers were placed. This was done to simulate o u t d o o r soil heat flux. Container m e d i u m temperatures attained during the last 4 h of freezing are given in Table I. TABLE I Minimum container medium temperatures during freezing at 5 cm from container sidewall Depth (cm)
Square container (°c)
Round container (°c)
5 10 15
--2.2 to--1.6 0 0.5 to 1.1
--8.3 --6.7 --5 to --4.4
After each freezing-period, plants were transferred to a greenhouse and held at 18.3--29.4°C t o stimulate new growth. After 3 weeks in the greenhouse, a plant viability rating-system of 0--5 was used to rate plant performance: 0 = dead; 1 = only lower part of main stem alive with a small basal shoot an d /o r one or m or e breaking buds; 2 = stem alive and 10--30% lateral branches with green leaves a n d / o r a new shoot; 3 = a p p r o x i m a t e l y 30--50% of the top alive with a few growing shoots; 4 = 60--90% of the t op alive and producing new shoots; 5 = the entire t op alive and producing new shoots (no more than 3 small shoots with freeze damage to the leaves). Plant viability ratings were based on stem and foliage condi t i on and on q u a ntity and quality of new t o p growth after freezing. Vigor of new t o p growth was believed to be a good indication of r o o t system performance. An analysis of variance p r o c e d u r e was used t o determine significant differences in fresh and dry t o p weight and plant viability rating data. Regression analysis was used to determine the effect of hardening-off time on plant viability after freezing for each species. RESULTS AND DISCUSSION Fresh and d r y w e i g h t data (pre-freezing). -- Container t y p e and solar exposure did n o t have a significant effect on either fresh or dry t op weight of Ilex crenata 'Hetzii', but did significantly affect the fresh and dry t o p weight of B u x u s sempervirens. B o x w o o d plants in conventional r o u n d containers had substantially greater t o p weights than those plants grown in
278 s q u a r e c o n t a i n e r s ( T a b l e I I ) . O p t i m u m g r o w t h o f B u x u s s e m p e r v i r e n s in both container types was achieved under full solar exposure (Table II).
Plant viability rating data (post-freezing).
-- Hardening-off time at --1.1 t o 1.7°C h a d a s i g n i f i c a n t e f f e c t o n p l a n t v i a b i l i t y r a t i n g s o f b o t h s p e c i e s . Major cold acclimation (2nd stage) was induced by 3 weeks of exposure to TABLE II Mean fresh and dry top weights of Buxus sempervirens grown in square or round containers, or in full sun vs. 50% shade. Means in columns not followed by the same letter are significantly different at the 5% level
Round Square Full sun 50% shade
5
I
'
Fresh wt. (g)
Dry wt. (g)
27.21 18.21 24.55 20.87
11.47 7.57 10.36 8.95
a b a b
~
~
a b a b
'
Buxus 4
llex
$~_~
=~i~
~EEKS OF HARDENING-OFF TIME Fig. 1. The effect of hardening-off time on plant viability after freezing for llex crenata 'Hetzii' and Buxus sempervirens regardless of container type or solar exposure level.
279
the temperature range used in the experiment (Fig. 1). Buxus sempervirens displayed the most dramatic increase in plant viability after freezing when subjected to hardening-off treatment. Container type had a significant effect on plant viability of Ilex crenata 'Hetzii'. Ilex plants in square containers averaged a viability rating of 4.3, and those in round containers of 1.7 (significantly less at 5%) after being exposed to freezing-temperatures. Visual inspection of square and round container root systems during the greenhouse phase of the experiment revealed that freeze damage to secondary (mature) and tertiary (young) roots in round containers was the cause of poor plant viability (Fig. 2A and B). Differences in root mortality between square and round containers were a direct result of temperature levels found in the 2 container types during freezing (see Table I and Fig. 2).
Fig. 2. A. The effects of freezing on roots and tops of l l e x crenata 'Hetzii' in round containers was evidenced by darkened dead roots, leaf drop and dehydrated shoots. B. Healthy white roots and hydrated tops of Ilex crenata 'Hetzii' in square containers provide visual evidence of winter protection achieved by minimizing heat loss from container side-walls. CONCLUSIONS
Square containers packed tightly together have a protective effect, preventing severe freeze damage to root systems of some container-grown
280 species. For sensitive species such as Ilex crenata 'Hetzii', this protective effect becomes quite obvious. The fresh and dry top weight data suggest that warm air circulation is required by certain species during the growing-season to maximize growth. This is possibly due to higher root-zone temperatures, which apparently stimulate growth activity. Square containers might be separated during the summer months and then condensed into block formation in late autumn to provide for the desired winter root protection. Pre-cooling may be used by nurserymen from the standpoint of hardeningoff a container crop during autumn to prevent freeze damage to susceptible root systems during winter.
REFERENCES Havis, J.R., 1976. Root hardiness of w o o d y ornamentals. HortScience, 11: 385--386. Mityga, H.G. and Lanphear, F.O., 1971. Factors influencing the cold hardiness of Taxus cuspidata roots. J. Am. Soc. Hortic. Sci., 96: 83--86. Nyholm, I., 1975. Cold storage of plants. Acta Hortic., 54: 143--145. PeIIett, H., 1971. Comparison of cold hardiness levels of root and stern tissue. Can. J. Plant Sci., 51: 193--195. Self, R.L., 1977. Winter protection of nursery plants: summary. Int. Plant Propag. Soc., Comb. Proc., 27: 303--307. Steponkus, P.L., Good, G.L. and Wiest, S.C., 1976. Root hardiness of w o o d y plants. Am. Nurseryman, C X L I V (6) 16, 76--79. Studer, E.J., Steponkus, P.L., Good, G.L. and Wiest, S.C., 1978. Root hardiness of container grown ornamentals. HortScience, 13: 172--174. Weiser, C.J., 1970. Cold resistance and injury in w o o d y plants. Science, 169: 1269-1278.
275
WINTER PROTECTION OF CONTAINER-GROWN NURSERY STOCK AS AFFECTED BY CONTAINER DESIGN, HARDENING-OFF TIME AND SOLAR EXPOSURE
H.M. ROHSLER and H. V A N DE WERKEN
Department o f Ornamental Horticulture and Landscape Design, University of Tennessee, Knoxville, TN 37901 (U.S.A.) (Accepted for publication 7 May 1982)
ABSTRACT Rohsler, H.M. and van de Werken, H., 1983. Winter protection of container-grown nursery stock as affected by container design, hardening-off time and solar exposure. Scientia Hortic., 20: 275--280. The effects of container type (round vs. square), solar exposure (100% vs. 50%) and hardening-off time (0, 3 or 6 weeks) on winter damage to llex crenata Thumb. 'Hetzii' (Hetz Japanese holly) and Buxus sempervirens L. (common boxwood) were studied. Container type and hardening-off time had significant effects on plant viability of Ilex crenata 'Hetzii' after exposure to freezing temperatures. Plants in square containers achieved substantially higher viability ratings than plants in round containers after freezing. Buxus sempervirens was significantly affected by hardening-off time only. Plant viability increased greatly with 3 or 6 weeks of hardening-off as compared to no hardening-off. Common boxwood plants in square containers did not achieve the same level of growth as plants in round containers during the 1980 growing-season (before freezing). Optimum growth for Buxus sempervirens occurred under full solar exposure. Keywords: Buxus sempervirens L. ; container type; hardening; llex crenata Thumb.; solar exposure; temperature. INTRODUCTION
With the increasing shift to container culture, growers in northern regions have realized certain problems associated with growing nursery plants in containers. One of the most serious of these problems has been that of winter damage to container root systems (Steponkus et al., 1976; Self, 1977). Root versus shoot hardiness of woody plants differs considerably, with roots being less hardy (Mityga and Lanphear, 1971; Pellett, 1971). These findings explain why the roots of a container-grown plant are much more susceptible to cold damage than those of a field-grown plant which are protected by the internal natural heat of the earth. Critical root-killing temperatures for many ornamental nursery plants have been determined (Havis, 1976; Studer et al., 1978). These temperatures are very useful to nurserymen from the stand-point of providing adequate winter protection for container plants. Knowledge about cold 0304-4238/83/$03.00
© 1983 Elsevier Science Publishers B.V.
276
tolerance of roots has resulted in greater efficiency of winter protection methods. However, with increasing fuel costs, new methods of winter protection which use little or no energy are needed. The objectives of this research were to determine the effects of container t y p e (round vs. square) on temperature changes in the medium (as influenced by low ambient air temperatures), and to determine the effects of container type, hardening-off time and solar exposure levels on postfreezing plant performance. MATERIALS AND METHODS
The study was initiated in the spring of 1980 and terminated approximately one year later. Species selected for the experiment were 2-year-old bed-grown liners of Ilex crenata 'Hetzii' and Buxus sempervirens. A total of 192 plants was used for each species. One half (96) of each species was grown under full sun, while the other half was grown under 50% shade cloth. Within each exposure treatment, one half (48) of each species was planted in plastic round containers, while the other half was planted in square containers constructed from roofing-felt material. The grade of roofing-felt selected had a rigidity close to that of the walls of the plastic containers. The square containers had straight side walls to facilitate tight blocking. Both container types had approximately 2.84 1 capacity. Each container-type was placed in a separate block. Finally, each container block was randomly split into 3 groups to establish the hardening-off treatments scheduled for late autumn after growth cessation. Each hardening-off group consisted of 3 replications of 4 plants each. Additionally, every replication of the 3 hardening-off groups contained 4 randomly distributed plants for fresh and dry t o p weight analyses, which were done prior to the hardeningoff treatments. All plants were grown for one full growing-season and then remained in the field until mid-autumn, where they were allowed to achieve their first stage of cold acclimation naturally. After removal of fresh and dry top weight samples, the remaining plants were taken from the field and subjected to additional hardening-off for 0, 3 or 6 weeks in a refrigerator unit set at --1.1 to 1.7°C. These temperatures were intended to initiate the second and final stage of cold acclimation in plants w i t h o u t causing damage to r o o t or shoot tissues (Weiser, 1970; Nyholm, 1975). An 8-h light period was provided once every 24 h in the refrigerator unit to help prevent defoliation of plant tops. The light source was 60 watt incandescent bulbs. With 8 light bulbs and aluminum reflectors on the walls, a light intensity of 40-60 footcandles was obtained. At the end of each hardening-off period, the plants of the appropriate treatments were transferred to a freezer unit where they were exposed to a 41-h freezing-period. Plants were blocked in the freezer by container type, and container medium temperatures were monitored by glass bulb
277 m e r cu r y t h e r m o m e t e r s and by t h e r m o c o u p l e s attached t o a recording t e l e t h e r m o m e t e r . The t e m p e r a t u r e in the freezer was started at 4.4°C and stepped down 2.4°C/h until a m i n i m u m range o f - - 1 2 . 2 t o --17.7°C was attained. B o t t o m heat of --1.1 to 1.7°C was supplied during freezing by heatingcables set 7.5 cm below the sand and gravel surface on which t he containers were placed. This was done to simulate o u t d o o r soil heat flux. Container m e d i u m temperatures attained during the last 4 h of freezing are given in Table I. TABLE I Minimum container medium temperatures during freezing at 5 cm from container sidewall Depth (cm)
Square container (°c)
Round container (°c)
5 10 15
--2.2 to--1.6 0 0.5 to 1.1
--8.3 --6.7 --5 to --4.4
After each freezing-period, plants were transferred to a greenhouse and held at 18.3--29.4°C t o stimulate new growth. After 3 weeks in the greenhouse, a plant viability rating-system of 0--5 was used to rate plant performance: 0 = dead; 1 = only lower part of main stem alive with a small basal shoot an d /o r one or m or e breaking buds; 2 = stem alive and 10--30% lateral branches with green leaves a n d / o r a new shoot; 3 = a p p r o x i m a t e l y 30--50% of the top alive with a few growing shoots; 4 = 60--90% of the t op alive and producing new shoots; 5 = the entire t op alive and producing new shoots (no more than 3 small shoots with freeze damage to the leaves). Plant viability ratings were based on stem and foliage condi t i on and on q u a ntity and quality of new t o p growth after freezing. Vigor of new t o p growth was believed to be a good indication of r o o t system performance. An analysis of variance p r o c e d u r e was used t o determine significant differences in fresh and dry t o p weight and plant viability rating data. Regression analysis was used to determine the effect of hardening-off time on plant viability after freezing for each species. RESULTS AND DISCUSSION Fresh and d r y w e i g h t data (pre-freezing). -- Container t y p e and solar exposure did n o t have a significant effect on either fresh or dry t op weight of Ilex crenata 'Hetzii', but did significantly affect the fresh and dry t o p weight of B u x u s sempervirens. B o x w o o d plants in conventional r o u n d containers had substantially greater t o p weights than those plants grown in
278 s q u a r e c o n t a i n e r s ( T a b l e I I ) . O p t i m u m g r o w t h o f B u x u s s e m p e r v i r e n s in both container types was achieved under full solar exposure (Table II).
Plant viability rating data (post-freezing).
-- Hardening-off time at --1.1 t o 1.7°C h a d a s i g n i f i c a n t e f f e c t o n p l a n t v i a b i l i t y r a t i n g s o f b o t h s p e c i e s . Major cold acclimation (2nd stage) was induced by 3 weeks of exposure to TABLE II Mean fresh and dry top weights of Buxus sempervirens grown in square or round containers, or in full sun vs. 50% shade. Means in columns not followed by the same letter are significantly different at the 5% level
Round Square Full sun 50% shade
5
I
'
Fresh wt. (g)
Dry wt. (g)
27.21 18.21 24.55 20.87
11.47 7.57 10.36 8.95
a b a b
~
~
a b a b
'
Buxus 4
llex
$~_~
=~i~
~EEKS OF HARDENING-OFF TIME Fig. 1. The effect of hardening-off time on plant viability after freezing for llex crenata 'Hetzii' and Buxus sempervirens regardless of container type or solar exposure level.
279
the temperature range used in the experiment (Fig. 1). Buxus sempervirens displayed the most dramatic increase in plant viability after freezing when subjected to hardening-off treatment. Container type had a significant effect on plant viability of Ilex crenata 'Hetzii'. Ilex plants in square containers averaged a viability rating of 4.3, and those in round containers of 1.7 (significantly less at 5%) after being exposed to freezing-temperatures. Visual inspection of square and round container root systems during the greenhouse phase of the experiment revealed that freeze damage to secondary (mature) and tertiary (young) roots in round containers was the cause of poor plant viability (Fig. 2A and B). Differences in root mortality between square and round containers were a direct result of temperature levels found in the 2 container types during freezing (see Table I and Fig. 2).
Fig. 2. A. The effects of freezing on roots and tops of l l e x crenata 'Hetzii' in round containers was evidenced by darkened dead roots, leaf drop and dehydrated shoots. B. Healthy white roots and hydrated tops of Ilex crenata 'Hetzii' in square containers provide visual evidence of winter protection achieved by minimizing heat loss from container side-walls. CONCLUSIONS
Square containers packed tightly together have a protective effect, preventing severe freeze damage to root systems of some container-grown
280 species. For sensitive species such as Ilex crenata 'Hetzii', this protective effect becomes quite obvious. The fresh and dry top weight data suggest that warm air circulation is required by certain species during the growing-season to maximize growth. This is possibly due to higher root-zone temperatures, which apparently stimulate growth activity. Square containers might be separated during the summer months and then condensed into block formation in late autumn to provide for the desired winter root protection. Pre-cooling may be used by nurserymen from the standpoint of hardeningoff a container crop during autumn to prevent freeze damage to susceptible root systems during winter.
REFERENCES Havis, J.R., 1976. Root hardiness of w o o d y ornamentals. HortScience, 11: 385--386. Mityga, H.G. and Lanphear, F.O., 1971. Factors influencing the cold hardiness of Taxus cuspidata roots. J. Am. Soc. Hortic. Sci., 96: 83--86. Nyholm, I., 1975. Cold storage of plants. Acta Hortic., 54: 143--145. PeIIett, H., 1971. Comparison of cold hardiness levels of root and stern tissue. Can. J. Plant Sci., 51: 193--195. Self, R.L., 1977. Winter protection of nursery plants: summary. Int. Plant Propag. Soc., Comb. Proc., 27: 303--307. Steponkus, P.L., Good, G.L. and Wiest, S.C., 1976. Root hardiness of w o o d y plants. Am. Nurseryman, C X L I V (6) 16, 76--79. Studer, E.J., Steponkus, P.L., Good, G.L. and Wiest, S.C., 1978. Root hardiness of container grown ornamentals. HortScience, 13: 172--174. Weiser, C.J., 1970. Cold resistance and injury in w o o d y plants. Science, 169: 1269-1278.