Forcing Containerized Roses in a Retractable Roof
Greenhouse and Outdoors in a Semi-Arid Climate
Ursula K. Schuch
Plant Sciences Department
University of Arizona, Tucson, AZ 85721
Abstract
Sales of containerized roses have increased dramatically in recent years and
producing flowering plants in containers in a timely manner is important to the
nursery industry. An experiment was conducted to determine whether forcing
containerized roses will be faster in a retractable roof greenhouse compared to
outdoors. Results suggest that forcing roses in a retractable roof greenhouse can
shorten the production time and increase quality of finished plants, however,
cultivar, time of harvesting, and time in cold storage also affect these
parameters.
Introduction
Forcing containerized bare root roses has become an important industry in the United States in the last decade.
Previously roses were marketed bare root or with packaged roots, but more recently flowering roses in containers
have become a major industry. The majority of the approximately 50 million rose plants produced in the field
annually in the U.S. are produced in California and Arizona. Rose plants are generally dug in the field between
November and February and are kept in cold storage until shipping and planting. The duration of cold storage can
last from a few days to several weeks, depending on when plants are scheduled to be marketable and ready to
bloom. The objective of this study was to determine how different cultivars of containerized bare root roses
performed with regard to vegetative and reproductive development in a retractable roof greenhouse and outdoors in
full sun.
Materials and Methods
Rose cultivars ‘Oregold’ and ‘Mr. Lincoln’ were dug in the field on Jan. 16, 2003 and Feb. 28, 2003 at a
commercial nursery in Litchfield Park, Arizona. Bare root plants were packed in cardboard boxes lined with plastic
sheets and were transported to Tucson, Arizona. Plants were kept in cold storage at 2°C. Once a week boxes were
opened and plants were sprinkled with water to maintain high humidity and prevent desiccation of roses. ‘Oregold
and ‘Mr. Lincoln’ plants were kept in cold storage for 0, 4, or 8 weeks.
Plants were potted in 4.5 L black polyethylene containers in a media consisting of 80% compost and 20% sand.
Plants were well watered in and moved to the RRGH where a white tarp was placed over the potted plants for one to
two weeks until buds started to break. Pots were watered as needed to maintain adequate soil moisture. Once buds
started to grow, plants were moved either outside in full sun or in a flat roof RRGH to an area that provided 35%
shade (RC 98 uncoated). Plants that were potted on Jan. 22, 2003 were moved outside of the RRGH on February
10, 2003. Each pot was amended with complete controlled release fertilizer and micronutrients at the recommended
rates within two weeks after transplanting.
_________________________________
This is a part of the University of Arizona College of Agriculture 2004 Turfgrass and Ornamental Research Report, index at:
http://cals.arizona.edu/pubs/crops/az1359/
The roof of the RRGH opened in the morning when light levels reached 300 µmol ·sec-1·m-2 and was closed at 10:00
HR every morning except for a 1.5 to 2 m ventilation gap every 10 m. Side wall curtains of all four walls were
raised simultaneously to allow cross ventilation throughout the structure. At 17:00 HR the roof was fully opened
and side walls were closed. When light levels dropped below 300 µmol ·sec-1·m-2, roof and side wall curtains were
fully closed.
Temperature probes (HOBO H8 outdoor logger) collected air temperature within the plant canopy and 4 cm below
the media surface towards the middle of the container. Additional environmental data was collected from a weather
station installed at approximately 0.6 m above the roof at the east end of the structure and within the bay adjacent to
the rose experiment within the retractable shade structure.
The number of days from potting to first leaf, first flower bud, and anthesis were recorded for each plant. When a
plant had at least one flower at anthesis, the number of flowering shoots (shoots with flowers at anthesis or flower
buds), the number of blind shoots, percent dieback, and an overall quality rating (1-4 with 1=worst and 4=best) was
recorded. On June 26, 2003, ‘Oregold’ and ‘Mr. Lincoln’ plants that were dug on Jan. 17, 2003 and stored for 8
weeks, and plants dug on Feb. 28, 2003 and stored for 0 or 8 weeks were selected. New growth since potting was
removed, and stem and leaves were separated. Flowers or flower buds were discarded. Leaf area was measured
(LI-3100, Licor, Lincoln, NE) and leaf and stem dry weight were determined after drying for 5 days at 60°C.
The experiment was conducted at the Campus Agriculture Center at the University of Arizona in Tucson, Arizona.
Plants were arranged in a completely randomized block design with two replications in five blocks each in the two
environments. Data was analyzed with analysis of variance (SAS Institute) and means were compared when
P<0.05.
Results and Discussion
Vegetative development from planting to emergence of the first leaf and from planting to anthesis was significantly
affected by a three-way interaction of cultivar by environment by the time plants were dug in the field and the length
of cold storage. The number of days from planting to first leaf were reduced for plants dug in January and stored for
8 weeks compared to 0 or 4 weeks of storage by 9 and 13 days, respectively, for Mr. Lincoln and Oregold plants
(Table 1). Plants harvested in February did not break buds sooner when placed in cold storage after digging (Table
1). The increased time to first leaf emergence after 4 weeks of cold storage may be due to another factor. Time of
digging and the period of cold storage had a greater effect than environment or cultivar on vegetative or
reproductive development. The greatest difference in forcing between the two environments was found for Oregold
plants that were dug in February and stored for 4 weeks and developed 7 days faster for leaf emergence and anthesis
in the RRGH compared to full sun (Table 1). For all other treatments, the difference in vegetative and reproductive
rate of development between the two environments was three days or less. Plants in both environments were well
watered and fertilized according to standard nursery practices. This provided plants with an environment free of
stress, and is probably one of the reasons why few differences in finishing were observed in this experiment. It is
likely that if plants had been forced in both environments earlier than February 10, greater differences in finishing
time may have been found, because cooler outdoor temperatures in January would slow growth outside compared to
inside the structure.
Media temperatures in containers and air temperatures within the canopy of rose plants growing in the RRGH or in
full sun are shown in Fig. 1. Daily media temperatures in containers in full sun fluctuated more, always exceeded
minimum temperatures, and with few exceptions exceeded maximum temperature of media in containers inside the
structure. Greatest discrepancies occurred during March. During early March, media in containers outdoors
reached the freezing point, while containers in the RRGH were protected and media temperature was maintained
3.8°C to 5°C higher. Media temperatures in containers in the RRGH maintained favorable conditions for growth
throughout most of the study, except when media in containers exceeded 38°C, above which root functioning is
impaired for many species.
Air temperatures within the RRGH reached generally greater maximum temperatures and outside lower minimum
temperatures (Fig. 1). Daytime highs during April were generally around 10°C higher within the structure than
outside, while much smaller differences between the daytime highs were found during most of the remainder of the
study. It appears that during that time the RRGH can be effectively used to trap energy which will raise both air and
media temperature compared to outside conditions. Should temperatures become too high, lowering the side walls
to provide more ventilation can be an effective strategy for passive cooling. From mid May until mid June when
outdoor temperatures reached around 40°C, few differences in air temperatures between the two environments were
found.
New shoot production was affected by an interaction between environment and cultivar and between environment
and harvest date and cold storage. Greatest total biomass and leaf dry weight were produced by plants that were dug
in January, stored for 8 weeks in the cooler, and were forced in full sun (Table 2). The lowest biomass was
produced in either environment from plants dug in February and maintained for 8 weeks in cold storage. Greatest
leaf area was produced either in full sun or in the RRGH, but from different harvest dates (Table 2). Mr. Lincoln
produced more leaf area and greater stem dry weight in the RRGH, while Oregold produced more leaf area and
greater stem dry weight in full sun (Table 3). Leaf dry weight and total dry weight of new growth was significantly
greater for Mr. Lincoln plants than for Oregold (42.2g versus 29.3 g total dry weight).
Number of flowering shoots when plants had reached anthesis was greatest for Oregold dug in January and stored
for 8 weeks. Mr. Lincoln plants produced on average only about half the number of flowers than Oregold, and much
fewer differences between time of digging and cold storage were found. Oregold had a greater number of flowering
shoots in the RRGH compared to full sun (8.1 versus 7.4), while the opposite was found for Mr. Lincoln plants for
which 4.9 flowering shoots were recorded in full sun and 4.5 in the RRGH environment.
Number of blind shoots was not affected by environment, but was greater for Mr. Lincoln, 8.9 versus 5.5 for
Oregold. The greatest percent dieback of shoots with 10.5% was recorded for Oregold plants dug in February and
stored for 4 weeks, followed by the same cultivar dug in February and stored for 8 weeks (5.8% dieback). Plants
from all other treatments and cultivars had less than 3.3% dieback which can be considered negligible.
Final performance evaluation was significantly affected by the environment, dig date and cold storage, and by
cultivar. Overall performance of Oregold plants was higher than that of Mr. Lincoln plants (3.8 versus 3.0).
Performance in the RRGH was better than in full sun (3.5 versus 3.2), and plants that were dug in January and
planted immediately had the highest performance rating (3.8), while those dug in February and stored for 4 weeks
had the lowest rating (2.9).
Conclusions
• The environment in the RRGH shortened the time from planting to first leaf and from planting to anthesis by
seven days for Oregold roses compared to plants forced in full sun for one out of the six planting dates based
on different harvesting dates and time in cold storage. However, Mr. Lincoln plants were not affected by
environment. It is likely that cultural practices and the growing environment inside the RRGH and in full sun
were equally favorable to yield such similar results.
• Mr. Lincoln plants in the RRGH produced more leaf area and stem dry weight than Oregold. However, leaf
dry weight and total dry weight of new shoots were also affected by environment, dig date, and time in cold
storage, with greatest dry weight produced in full sun.
• Oregold produced 9% more flowering shoots in the RRGH, while Mr. Lincoln produced 9% less flowering
shoots in the RRGH compared to the full sun environment. Overall quality rating was slightly higher for
plants grown in the RRGH, although plants in both environments produced marketable quality.
• Media temperature in containers in the RRGH had less daily fluctuations than in containers outside, but with
few exceptions root zone temperatures stayed within a favorable range for growth. Media temperatures in the
RRGH were 3.8°C to 5°C higher when outside media temperature was at 0°C. Greatest differences in air
temperatures between the two environments were found in April when daily maximum temperatures in the
RRGH were up to 10°C higher than in full sun.
Table 1. Number of days from planting to emergence of the first leaf, and from planting to first open flower
(anthesis) for cultivars Mr. Lincoln and Oregold forced in two environments, dug in the field at two dates, and
stored for 0, 4, or 8 weeks at 2°C.
Cultivar
Dig date
Cold storage
in 2003
(weeks)
Planting to first leaf
Full sun
Retractable
Planting to anthesis
Full sun
shade
Retractable
shade
(Number of days)
Mr. Lincoln
Jan. 16
Feb. 28
Oregold
Jan. 16
Feb. 28
0
26.3
27.1
72.4
71.7
4
25.3
22.9
59.3
60.5
8
17.7
17.5
47.5
48.0
0
23.8
21.0
55.5
54.5
4
31.6
33.0
55.7
59.4
8
23.9
21.3
47.3
45.3
0
28.7
30.4
66.3
67.2
4
25.1
23.4
57.8
54.4
8
16.4
17.2
42.7
42.9
0
17.2
18.1
42.9
46.8
4
37.0
30.3
58.3
51.9
8
21.9
20.3
41.4
39.9
LSD0.05=7.4
LSD0.05=9.6
Table 2. Leaf area, leaf dry weight, and total dry weight of new growth from selected plants after transplanting of
containerized rose plants. Means are averaged over cultivars Mr. Lincoln and Oregold.
Environment
Full sun
Retractable shade
Dig date
Cold storage
Leaf area
2
Leaf dry wt.
Total dry wt. (g)
(weeks)
(cm )
(g)
Jan. 16, 2003
8
2435 a*
27.8 a
45.6 a
Feb. 28, 2003
0
2042 b
23.8 ab
40.5 ab
8
1909 b
14.1 c
21.3 c
Jan. 16, 2003
8
2077 b
21.0 b
39.7 b
Feb. 28, 2003
0
2055 b
19.7 bc
38.8 b
8
2386 a
17.0 bc
29.0 bc
* Means within a column followed by different letters are significantly different at P<0.05.
Table 3. Effect of environment and cultivar on leaf area and stem dry weight. Means are averaged over dig date
and cold storage treatments.
Environment
Cultivar
Leaf area
Stem dry weight
2
(cm )
Full sun
Retractable shade
(g)
Mr. Lincoln
2467 ab*
16.4 ab
Oregold
1791 b
11.3 b
Mr. Lincoln
2735 a
21.1 a
Oregold
1610 b
12.1 b
* Means within a column followed by different letters are significantly different at P<0.05.
6/
14
/2
3
3
3
3
3
3
3
00
3
03
00
00
00
00
03
00
20
/2
/2
/2
/2
7/
31
24
17
10
6/
5/
5/
5/
5/
00
20
3
3
03
00
/2
/2
/2
3/
26
19
12
5/
4/
4/
4/
00
00
20
/2
/2
3
03
00
20
3
3
03
00
00
20
/2
5/
29
4/
3/
22
15
8/
1/
/2
/2
RRGH
3/
3/
3/
3/
22
15
6/14/2003
6/7/2003
5/31/2003
5/24/2003
5/17/2003
5/10/2003
5/3/2003
4/26/2003
4/19/2003
4/12/2003
4/5/2003
3/29/2003
3/22/2003
3/15/2003
3/8/2003
3/1/2003
2/22/2003
2/15/2003
0
2/
2/
Air Temperature (C)
Media Temperature (C)
50
RRGH
Full Sun
40
30
20
10
Date
50
Full Sun
40
30
20
10
0
Date
Fig. 1. Media temperature in containers and air temperatures in the retractable roof greenhouse and in full sun.