DECISION SUPPORT FOR DESIGN AND CONTROL OF PLANT GROWTH SYSTEMS

Knowledge Area (KA)	Subject of Investigation (SOI)	Field of Science (FOS)	Percent
205	2120	1060	25%
205	2122	1060	25%
205	2123	1060	50%

Progress 10/01/98 to 09/30/03

Outputs
Research in this project contributed to the publication of 20 refereed journal articles and 13 grower trade publications since 1998. These articles focused on quantifying plant response to the greenhouse environment. Three major thrusts were developed. The first was the development of energy-balance models used to predict shoot-tip temperature. These models were developed to better understand how plant temperature in a greenhouse differs from air temperature. Understanding plant shoot-tip temperature is important because this is the temperature that controls rate of plant development. The second thrust modeled plant development rate of several floricultural plants to plant shoot-tip temperature. These models allowed prediction of plant base temperature and thermal time to flower, as well as time to flower over a broad range of temperatures. The third thrust focused on relating plant quality in greenhouses to the ratio of thermal energy (degree days) to radiant energy (daily light integral) using the term photothermal ratio. Plant quality was found to be closely related to this ratio.

Impacts
This project has developed models that can be used by commercial greenhouse growers to more reliably time the production and flowering of their crops for specific market dates and market specifications.

Publications

No publications reported this period

Progress 01/01/02 to 12/31/02

Outputs
Although Phalaenopsis orchids are now the second most valuable flowering potted plant according to 2001 USDA statistics, little specific quantitative information is available on the plant relating plant development to the environment. The objective of this investigation was to quantify the effects of temperature on time from spike emergence to flowering. Vegetative Phalaenopsis BL. were induced to flower then were placed into growth compartments at different constant temperatures. An initial calibration experiment was performed with P. Taisuco Smile, followed by validation experiments using several cultivars. Flower, node, and bud development rate were modeled as a linear function of development rate. Average time to flower increased from 10 to 26 weeks as temperature decreased from 26 to 14C. At any constant temperature for any given cultivar, time to visible bud was about 60% of the total time from spiking to flower. Plants grown at 29C failed to develop to anthesis, and most buds aborted soon after they were visible. The thermal time from appearance of the flower spike to anthesis was 769 degree days with, a calculated base temperature of 10.8C. Node and bud development rates also increased linearly as temperatures increased. Temperatures from 17 to 26C did not affect the number of nodes and flowers, spike height, or flower size.

Impacts
This project has quantified plant responses to the environment, and then developed decision support tools for commercial greenhouse producers. The most widely used tool is the graphical tracking technique for managing height control and timing of crops.

Publications

Liu, B., Heins, R. 2002. Photothermal ratio affects plant quality in 'Freedom' poinsettia. J. Amer. Soc. Hort. Sci. 127(1):20-26.
Niu, G., Heins, R., Carlson, W. 2002. Using Paclobutrazol to control height of poinsettia 'Freedom'. HortTechnology 12(2):232-236.
Niu, G., Heins, R., Cameron, A., Carlson, W. 2002. Prevernalization Daily Light Integral and Vernalization Temperature Influences Flowering of Herbaceous Perennials. HortScience 37(7):1028-1031.
Runkle, E., Heins, R. 2002. Stem extension and subsequent flowering of seedlings grown under a film creating a far-red deficient environment. Scientia Horticulturae 96:257-265.
Kim, H., Heins, R., Carlson, W. 2002. Development and flowering of petunia grown in a far-red deficient light environment. Acta Horticulturae 580:127-135.
Runkle, E., Heins, R., Jaster, P., Thill, C. 2002. Plant responses under an experimental near infra-red reflecting greenhouse film. Acta Horticulturae 580:137-143.
Runkle, E., Heins, R., Jaster, P., Thill, C. 2002. Environmental conditions under an experimental near infra-red reflecting greenhouse film. Acta Horticulturae 578:181-185.
Shimizu, H., Heins, R. 2002. Prediction of plant shoot-tip temperature on a Penman-Monteith model. Acta Horticulturae 580:169-176.
Runkle, E.,Shimizu, H., Heins, R. 2002. How low can you go? GrowerTalks 65(10) 63-68.
Enfield, A., Runkle, E., Heins, R., Cameron, A., Carlson, W. 2002. Herbaceous Perennials: Quick-cropping Part I. Greenhouse Grower 20(1):80-86.
Enfield, A., Runkle, E., Heins, R., Cameron, A., Carlson, W. 2002. Herbaceous Perennials: Quick-cropping Part II. Greenhouse Grower 20(3)40-48.
Enfield, A., Runkle, E., Heins, R., Cameron, A., Carlson, W. 2002. Herbaceous Perennials: Quick-cropping Part III. Greenhouse Grower 20(5):66-75.
Fausey, B., Runkle, E., Heins, R., Cameron, A., Carlson, W. 2002. Herbaceous Perennials: Agastache. Greenhouse Grower 20(9):74-82.
Faust, J., Heins, R. 2002. Late-season bonzi drenches: northern and southern perspectives. Greenhouse Product News 12(9):46-51.

Progress 01/01/01 to 12/31/01

Outputs
The effects of temperature on flower size and number of flower buds of Campanula carpatica Jacq. 'Blue Clips' and 'Deep Blue Clips' and Campanula 'Birch Hybrid' were investigated in four temperature and light-transfer experiments. In Expt. 1, 'Blue Clips' and 'Birch Hybrid' plants were grown initially at 20C and then transferred at visible flower bud (VB) to 14, 17, 20, 23, or 26C until flower (Expt. 1). In Expt. 2, 'Blue Clips' and 'Birch Hybrid' plants were transferred from 14 to 26C or from 26 to 14C at various intervals after flower induction. Flower size of both species was negatively correlated with average daily temperature (ADT) after VB; flowers on plants grown at 14C were 35 percent larger than those on plants grown at 26C. In contrast, temperature before VB had only a small effect on final flower size in both species, although flowers of 'Birch Hybrid' plants were 20% smaller when initially grown at 26C before VB than when grown at 20C and then transferred to 26C. For both species, the longer the exposure to high temperature after VB, the smaller the flowers. Number of flower buds at flower in 'Birch Hybrid' decreased as ADT after VB increased. In year 2, 'Deep Blue Clips' plants were grown at constant 20C under high or low daily light integral (DLI, 17 or 5.7 mol per square meter and day) until VB and then transferred to 14, 17, 20, 23, or 26C under high or low DLI (Expt. 3). In Expt. 4, 'Deep Blue Clips' plants were grown at 14, 17, 20, 23, or 26C until VB and then transferred to constant 20C under high or low DLI until flower. Flower size (petal length) was negatively correlated with ADT both before and after VB, while flower bud number was negatively correlated with the ADT only after VB, regardless of DLI. In both experiments, petal length decreased by 0.3 to 0.5 mm per 1C increase in ADT before or after VB. Flowers were larger and more numerous under high than under low DLIs after VB, regardless of the DLI before VB.

Impacts
This project has quantified plant responses to the environment, and then developed decision support tools for commercial greenhouse producers. The most widely used tool is the graphical tracking technique for managing height control and timing of crops.

Publications

Niu, G., Heins, R.D., Cameron, A. and Carlson, W. 2001. Temperature and daily light integral influence plant quality and flower development of Campanula carpatica 'Blue Clips', 'Deep Blue Clips', and Campanula 'Birch Hybrid'. HortScience 36(4):664-668.
Niu, G., Heins, R.D., Cameron, A.C. and Carlson, W.H. 2001. Day and night temperatures, daily light integral, and CO2 enrichment affect growth and flower development of Campanula carpatica 'Blue Clips'. Scientia Horticulturae (87):93-105.
Runkle, E., Heins, R. 2001. Timing Spring Crops. Greenhouse Grower 19(4):64-66.
Niu, G., Heins, R. and Carlson, W. 2001. Keeping Freedom under control. Greenhouse Grower 19(10)88-96.

Progress 01/01/00 to 12/31/00

Outputs
The effects of photoperiod and the difference between day temperature (DT) and night temperature (NT) (DIF) on stem elongation in Verbena bonariensis L. (tall verbena) were investigated. Plants were exposed to nine treatment combinations of -10, 0, or +10C DIF and 8-, 12-, or 16-hour photoperiods. Stem elongation was measured and analyzed by a noncontact computer-vision-based system. Total daily elongation increased as DIF increased; it also increased as photoperiod increased under positive DIF (DT > NT) and zero DIF (DT = NT), but not under negative DIF. Under positive DIF, daily elongation was 90% greater under the 16-hour photoperiod than under the 8-hour photoperiod. DIF affected elongation rate during the daily light span but not during the daily dark span. Total light-span elongation increased as DIF or photoperiod increased. Total dark-span elongation was not influenced by DIF or photoperiod. Elongation rates per hour in the light and dark were not significantly affected by photoperiod but increased in the light as DIF increased. Therefore, for a particular DIF, total elongation during 16-hour photoperiods (long days) was greater than that under 8-hour photoperiods (short days) because there were more hours of light under long days.

Impacts
This project has quantified plant responses to the environment, and then developed decision-support tools for commercial greenhouse producers. The most widely used tool is the graphical tracking technique for managing height control and timing of crops.

Publications

Shimizu, H., Heins, R.D. 2000. Photoperiod and the difference between day and night temperature influence stem elongation kinetics in verbena bonariensis. J. Amer. Soc. Hort. Sci. 125(5):576-580.
Shimizu, H., Heins, R.D. 2000. Development for measuring method for cuticle resistance of plant shoot tip. J. of High Technology in Agriculture 12(3):155-159 (in Japanese).
Shimizu, H., Heins, R.D. 2000. Estimating cuticle resistance of seedling shoot tips based on the Penman-Monteith model. Transplant Production in the 21st Century, 59-62pp (printed in the Netherlands).

Progress 01/01/99 to 12/31/99

Outputs
Photothermal ratio (PTR) is defined as the ratio of radiant energy (light) to thermal energy (temperature). The effect of PTR during the vegetative (PTRv) and reproductive phase (PTRr) on finished plant quality of Freedom poinsettia was determined. In one experiment, plants were grown under 27 combinations of three temperatures, three daily light integrals (DLI) and three plant spacings from pinch to the onset of short day flower induction and then moved to a common PTR until anthesis. In a second experiment, plants were grown under a common PTR during the vegetative stage and then assigned to nine combinations of one temperature, three DLIs, and three plant spacings after the onset of short day flower induction. Both PTRr and PTRv affected final plant dry weight. All components of dry weight (total, stem, green leaf, and bract) responded in a linear way to PTRr and in a quadratic way to PTRv. Stem strength was more dependent on PTRv than PTRr. When PTRv increased from 0.02 to 0.06 mol per degree-day per plant, stem diameter increased about 24% while stem strength increased 75%. The size of bracts and cyathia was linearly correlated to PTRr, but not affected by PTRv. When PTRr increased from 0.02 to 0.06 mol per degree-day per plant, bract area, inflorescence diameter, and cyathia diameter increased 45%, 23%, and 44%, respectively.

Impacts
(N/A)

Publications

Liu, B., Heins, R.D. 1999. Effect of photothermal ratio on poinsettia plant quality at anthesis. HortScience 34:545.
Runkle, E.S., Heins, R.D. 1999. Photoselective filters affect flowering and stem extension in Viola x wittrockiana and Pisum sativum. HortScience 34:544.
Heins, R.D. 1999. The Production Revolution. Greenhouse Grower, The History of U.S. Floriculture Fall 1999 Commemorative Edition, 64-69pp.
Faust, J.E., Liu, B., Heins, R.D., Korczynski, P.C. 1999. Shedding some light on poinsettia stem breakage. Greenhouse Product News 9(10):48-51.

Progress 01/01/98 to 12/31/98

Outputs
An energy-balance model was developed to predict vinca (Catharanthus roseus L.) shoot-tip temperature using four environmental measurements: solar radiation, dry bulb, wet bulb, and glazing material temperature. The time and magnitude of the differences between shoot-tip and air temperature were determined in greenhouses maintained at 15, 20, 25, 30, or 35C. At night, shoot-tip temperature was always below air temperature. Shoot-tip temperature decreased from 0.5 to 5C below air temperature as greenhouse glass temperature decreased from 2 to 15C below air temperature. During the photoperiod under low vapor-pressure deficit and low air temperature conditions, shoot-tip temperature increased linearly as solar radiation increased. Under high vapor-pressure deficit and high air temperature conditions, shoot-tip temperature initially decreased at sunrise, then increased later in the morning as solar radiation increased. The model can be used to simulate shoot-tip temperatures under different vapor-pressure deficit, solar radiation, and air temperature conditions.

Impacts
(N/A)

Publications

Faust, J.E., Heins, R.D. 1998. Modeling shoot-tip temperature in the greenhouse environment. J. Amer. Soc. Hort. Sci. 123(2):208-214.
Whitman, C.M., Heins, R.D., Cameron, A.C., Carlson, W.H. 1998. Lamp type and irradiance level for daylength extensions influence flowering of Campanula carpatica blue clips, Coreopsis grandiflora early sunrise, and Coreopsis verticillata moonbeam. J. Amer. Soc. Hort. Sci. 123(5):802-807.
Liu, B., Heins, R.D. 1998. Modeling poinsettia vegetative growth and development: the response to the ratio of radiant to thermal energy. Acta Hort 456:133-142.
Wang, S., Heins, R.D., Carlson, W.H., Cameron, A.C. 1998. Modeling the effect of temperature on flowering of Hibiscus moscheutos. Acta Hort 456:161-169.
Fisher, P.R., Lieth, J.H., Heins, R.D. 1998. Predicting variability in anthesis of Easter lily (Lilium longiflorum Thunb.) populations in response to temperature. Acta Hort 456:117-124.
Fisher, P.R., Heins, R.D., Smith, B., Chong, J. 1998. Tracking geraniums. Greenhouse Grower 16(8):79-80.
Fisher, P.R., Heins, R.D., Liu, B., Bilodeau, L. 1998. Tracking oriental and hybrid Lilies. Greenhouse Grower. 16(1):39-40.
Wang , S., Heins, R.D., Carlson, W., Cameron, A. 1998. Forcing perennials Hibiscus moscheutos Disco Belle Mixed. Greenhouse Grower 16(2):29-32.

Progress 01/01/97 to 12/31/97

Outputs
Graphical-tracking curves were developed for unpinched poinsettia (Euphorbia pulcherrima) plants. Experiments showed elongation of unpinched poinsettia plants was not the same as with pinched plants because unpinched plants develop many more leaves on the stem. The large number of internodes resulted in several weeks of rapid and constant growth rate. The process of flowering eventually led to a slower plateau phase of elongation, which began six weeks after the start of short days. During the plateau phase, plants grew about 2.5 cm. The developed graphical tracking curve consists of two straight lines. The first starts at the time the unpinched plant has elongated 2.5 cm (point A) and ends 6 weeks after the start of short days at a height 2.5 cm below the final desired height (point B). The second line starts at point B and ends at anthesis date at the desired final height (point C). Graphical tracking curves for Asi-florum and Oriental lilies were also developed. Experiments showed elongation of Asi-florum lilies was linear from emergence to flower so a linear graphical tracking curve was developed. Elongation of Oriental lilies was rapid until about 50% of the time from emergence until flower; it then slowed. The graphical tracking curve takes the form of two linear line segments where the first reaches 80% of final height at 50% of time to flower from emergence. The second connects this point to final height at flower.

Impacts
(N/A)

Publications

FISHER, P.R., HEINS, R.D., EHLER, N., LIETH, J.H. 1997. A decision-support system for real- time management of Easter lily (Lilium longiflorum Thunb.) Scheduling and Height - I. System Description. Agricultural Systems 54(1):23-37.
FISHER, P.R., HEINS, R.D., EHLER, N., LIETH, J.H. 1997. A decision-support system for real- time management of Easter lily (Lilium longiflorum Thunb.) Scheduling and height - II. Validation. Agricultural Systems 54(1):39-55.
FAUST, J.E., HEINS, R.D., SHIMIZU, H. 1997. Quantifying the effect of plug-flat color on medium-surface temperatures. HortTechnology 7(4):387-389.
HICKLENTON, P.R., HEINS, R.D. 1997. Chapter 2 - Temperature. Plant Growth Chamber Handbook, N. Central Reg. Res. Pub. No. 340, Feb 97. Iowa Agric. & Home Economics Exp. Sta. Special Report No. 99.
HEINS, R.D., FISHER, P.R. 1997. Graphical Tracking 101. Greenhouse Grower 15(6):81-82.
HEINS, R.D., FISHER, P.R. 1997. Tools for the Tracking. Greenhouse Grower 15(8):131-132.
FISHER, P.R., HEINS, R.D., LIETH, J.H. 1997. Hitting the right curve. Greenhouse Grower 15(9):103-104.
FISHER, P.R., HEINS, R.D. 1997. From pinch to first color. Greenhouse Grower 15(10)81-82.
FISHER, P.R., HEINS, R.D. 1997. From first color to flower. Greenhouse Grower 15(12):27-28.
FISHER, P.R., HEINS, R.D. 1997. Tracking Easter lilies. Greenhouse Grower 15(13):65-66.

Progress 01/01/96 to 12/30/96

Outputs
An energy-balance model was developed that predicted vinca (Catharanthus roseus L.) shoot-tip temperature using four environmental measurements: solar radiation, dry bulb, wet bulb, and glazing material temperature. The time and magnitude of the differences between shoot-tip and air temperature were determined in greenhouses maintained at 15, 20, 25, 30, or 35C. At night, shoot-tip temperature was always below air temperature. Shoot-tip temperature decreased from 0.5 to 5C below air temperature as greenhouse glass temperature decreased from 2 to 15C below air temperature. During the day under low VPD and low air temperature conditions, shoot-tip temperature increased linearly as solar radiation increased. Under high VPD and high air temperature conditions, shoot-tip temperature initially decreased at sunrise, then increased later in the morning as solar radiation increased. The 24-h average shoot-tip temperature decreased relative to air temperature as air temperature increased. The model predicted shoot-tip temperatures within 1C for 81% of the observed 1-h average shoot-tip temperatures.

Impacts
(N/A)

Publications

FISHER, P.R., HEINS, R. D., LIETH, J.H. 1996. Quantifying the relationship between phases of stem elongation and flower initiation in poinsettia. J. Amer. Soc. Hort. Sci. 121(4):686-693.
FISHER, P. R., HEINS, R.D. 1996. A graphical control chart for monitoring leaf count of Easter lily to support crop timing decisions. HortTechnology 6(1):68-70.
FISHER, P.R., LIETH, J. H., HEINS, R.D. 1996. Modeling flower bud elongation in Easter lily (Lilium longiflorum Thunb.) in response to temperature. HortScience 31(3):349-352.
SI, Y., HEINS, R.D. 1996. Influence of day and night temperatures on sweet pepper seedling development. J. Amer. Soc. Hort. Sci. 121(4):699-704.
FISHER, P., LIETH, H., HEINS, R. 1996. Easter lilies: hit your target date. Greenhouse Grower 14(1):98-102.

Progress 01/01/95 to 12/30/95

Outputs
A model for stem elongation of poinsettia was developed by using an approach that explicitly models the three phases of a sigmoidal growth curve: 1) an initial lag phase characterized by an exponentially increasing stem length, 2) a phase in which elongation is nearly linear, and 3) a plateau phase in which elongation rate declines as stem length reaches an asymptotic maximum. For each growth phase, suitable mathematical functions were selected for smooth height and slope transitions between phases. These growth phases were linked to developmental events, particularly flower initiation and the first observation of visible bud. The model was fit to a data set of single-stemmed poinsettia grown with 13-, 26-, or 54-day-long vegetative periods, resulting in excellent conformance (R2= 0.99). Poinsettia stem elongation response to a single foliar application of the growth retardant chlormequat was quantified. The dose response function assumed that the initial retarding effect was independent of concentration between 500 and 4000 ppm and that concentration primarily affected the duration of growth-retarding activity. The dose response function was incorporated into the three-phase mathematical function of stem elongation of single-stemmed poinsettia. Validation of the dose response model against an independent data set resulted in an R2 of 0.99, and predicted final stem length was within 12 mm of observed final height.

Impacts
(N/A)

Publications

SHIMIZU, H., HEINS, R.D. 1995. Computer-vision-based system for plant growth analysis. Transactions ASAE 38(3):959-964.
FISHER, P.R., HEINS, R.D. 1995. A process-control approach to poinsettia height control. HortTechnology 5(1):57-63.
FISHER, P., HEINS, R.D. 1995. Computer offers opinion on growing decisions. GrowerTalks 58(9):62-64.
FAUST, J.E., HEINS, R.D. 1995. Take your plants' temperature. Greenhouse Grower 13(5):68-69.
FAUST, J.E., HEINS, R.D., KIEFER, P. 1995. Thermal screens increase plant temperature. Grower Talks 59(2):84-86.
FISHER, P., HEINS, R.D., WALLACE, T.F., JR. 1995. Tracking poinsettias with CARE. Greenhouse Grower 13(9):92-94.
FAUST, J.E., HEINS, R.D. 1995. Shedding light on cloudy days. Greenhouse Grower 13(10:36-40.

Progress 01/01/94 to 12/30/94

Outputs
A decision-support system (DSS) was developed for recommending night and day temperature setpoints to control the timing and height of Lilium longiflorum. Existing biological models and qualitative rules were combined into an overall model to the response of plant development rate to temperature. A process control algorithm was used to determine appropriate height control actions based on a graphical control chart of plant height over time. A knowledge-based system checked the feasibility of output from the development and height control models and generated a text report. The DSS was validated in three locations (Michigan, California, and Copenhagen, Denmark). Flowering occurred within one day of the specific target date. The effects of 100 micro mol/sq m/s supplemental lighting on Catharanthus roseus plant temperature were quantified. Relative to air temperature, plant shoot-tip temperature depended on the irradiance and vapor-pressure deficit (VPD). Under relatively low VPD conditions (1 kPa), plant temperature was greater than air temperature, while under high VPD conditions (4 to 5 kPa), temperature of both lighted and unlighted plants remained below air temperature. Temperature of lighted plants, however, remained 1 to 2C above that of unlighted plants. The hastened development of plants grown under supplemental lighting can be explained by increased plant temperature rather than any specific photosynthetic effect.

Impacts
(N/A)

Publications

FAUST, J.E., HEINS, R.D. 1994. Modeling inflorescence development of the African violet (Saintpaulia ionantha Wendl.) J. Amer. Soc. Hort. Sci. 119(4):727-734.
FISHER, P., HEINS, R. 1994. Count on your Easter lily crop. Greenhouse Grower, Vol 12, No. 14, 39-42pp.
HEINS, R.D. 1994. Plant growth and development responses to the cultural environment. Proceedings - Greenhouse Systems Inter. Conf., New Brunswick, N.J. NRAES-72, July, 96-101pp.
FAUST, J.E., HEINS, R.D. 1994. African violet flowering: it's all in the timing. Greenhouse Grower, Vol 12, No. 8, 70-72pp.

Progress 01/01/93 to 12/30/93

Outputs
Development of a computerized grower-management decision-support aid to assist with poinsettia (Euphorbia pulcherrima) height-control decisions continue. The program consists of a graphical-height entry system and a decision-support module. Management suggestions are based on a comparison of current plant height and expected plant height, expected growth rate, expected height in three days' time, and the stage of crop development. The program was released for commercial marketing. A predictive model relating flower-bud length of the Oriental lily cultivar Stargazer to temperature and time to flower was developed. Time to flower in days is predicted by (397 - 24.5 * ADT + 0.47 * ADT * ADT) + (-83.5 + 5.1 * ADT - 0.99 * ADT * ADT) * ln (bud length), where ADT is the average daily temperature. A "bud meter" was developed from the model and allows growers to predict days to flower by measuring bud length with the meter. Leaf unfolding rate of vegetative poinsettia was quantified for five cultivars, Annette Hegg Dark Red, V-17 Angelika White, Freedom, Red Sails, and Supjibi. Calculated base temperatures varied from 1.5 to 8.2C and number of degree-days per leaf from 47 to 85, depending on cultivar. Maximum leaf unfolding rate occurred at 27C, and varied from 0.30 leaves per day for 'Supjibi' to 0.38 leaves per day for V-17 'Angelika White.'.

Impacts
(N/A)

Publications

BRONDUM, J.J., HEINS, R.D. 1993. Modeling temperature and photoperiod effects on growth and development of dahlia. J. Amer. Soc. Hort. Sci. 118(1):36-42.
HEINS, R.D. 1993. Sumagic's effect on Easter lilies. Greenhouse Grower 11(2):44-50.
FAUST, J.E., HEINS, R.D. 1993. Stop poor poinsettia branching. Greenhouse Grower 11(8):66-70.
BLOM, T.J., HEINS, R.D. 1993. PPGA recommends international standard for numbering weeks of the year. PPGA News, Vol XXIV, No. 7 (July).
ERWIN, J.E., PIERSON, G., STREFELER, M., HEINS, R. 1993. Research report: temperature manipulation of vegetable stem elongation and flowering. Minnesota Flower Growers Bulletin, Vol 42, No. 2 (March).
ERWIN, J.E., HEINS, R.D. 1993. Temperature effects on bedding plant growth. Minnesota Commercial Flower Growers Association Bulletin, Vol 42, No. 3 (May).