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 Planting to first leaf Planting to anthesis in 2003 (weeks) Full sun Retractable shade Full sun Retractable shade (Number of days) Mr. Lincoln Jan. 16 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 Feb. 28 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 Oregold Jan. 16 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 Feb. 28 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 LSD 0.05 =7.4 LSD 0.05 =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 Dig date Cold storage (weeks) Leaf area (cm 2 ) Leaf dry wt. (g) Total dry wt. (g) Full sun 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 Retractable shade 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 (cm 2 ) Stem dry weight (g) Full sun Mr. Lincoln 2467 ab* 16.4 ab Oregold 1791 b 11.3 b Retractable shade 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.
50 RRGH Full Sun 40 Media Temperature (C) 30 20 10 0 2/15/2003 2/22/2003 3/1/2003 3/8/2003 3/15/2003 3/22/2003 3/29/2003 4/5/2003 4/12/2003 4/19/2003 4/26/2003 5/3/2003 5/10/2003 5/17/2003 5/24/2003 5/31/2003 6/7/2003 6/14/2003 Date 50 RRGH Full Sun 40 Air Temperature (C) 30 20 10 0 2/15/2003 2/22/2003 3/1/2003 3/8/2003 3/15/2003 3/22/2003 3/29/2003 4/5/2003 4/12/2003 4/19/2003 4/26/2003 5/3/2003 5/10/2003 5/17/2003 5/24/2003 5/31/2003 6/7/2003 6/14/2003 Date Fig. 1. Media temperature in containers and air temperatures in the retractable roof greenhouse and in full sun.