Energy-Efficient Annuals: Timing Marigolds
In the first article of this scheduling annuals series, we introduced the concepts of temperature and daily light integral (DLI) and how these factors influence crop timing and plant quality. In the second article, Virtual Grower software was presented as a tool to predict energy costs for greenhouse heating. In this article, we present crop timing data for two species of marigolds, then estimate greenhouse heating costs to produce marigolds at different temperatures and in different locations.
Marigolds are among the top 10 bedding plants produced in the United States. In 2007, the 15 largest floriculture-producing states collectively sold 3.7 million flats at a total wholesale value of $31.7 million. Four species of marigolds are commonly grown commercially and include African or American marigold (Tagetes erecta), French marigold (T. patula), sweet-scented marigold (T. lucida), and signet marigold (T. tenuifolia). Our crop scheduling research focused on African and French marigolds.
Materials and Methods
Seeds of African marigold ‘Antigua Primrose’ and ‘Moonstruck Orange’ and French marigold ‘Janie Flame’ and ‘Bonanza Yellow’ were sown in 288-cell plug trays by C. Raker & Sons, then grown in controlled environmental growth chambers at Michigan State University at 68°F (20°C). The photoperiod was 16 hours and the DLI was 9 to 11 molâˆ™m¯²âˆ™dâˆ’¹.
When plugs were ready for transplant (two to four weeks after seed sow, depending on variety), they were transplanted into 4-inch pots and grown in greenhouses with constant temperature set points of 57, 63, 68, 73 and 79°F (14, 17, 20, 23 and 26°C). At each temperature, plants were grown under a 16-hour photoperiod with two different DLIs provided by a combination of shade curtains and different light intensities from high-pressure sodium lamps.
The experiment was performed twice to obtain average DLIs that ranged from 3.5 to 21 molâˆ™m¯²âˆ™dâˆ’¹. The flowering date was recorded for each plant when an inflorescence with at least 50 percent of the ray petals were fully reflexed. When each plant flowered, plant height, number of leaves and number of flowers and flower buds were recorded.
Crop timing data was used to develop mathematical models to predict flowering time and plant quality under different temperature and DLI conditions. The scheduling models were validated by growing marigolds at three different constant temperatures to compare predicted flowering times with actual times. Temperature responses were similar between cultivars of African and French marigolds, so one crop timing model was used for each species. The Virtual Grower software (free at www.virtualgrower.net) was used to estimate the cost to heat a 21,504 square foot greenhouse (about half an acre) to produce a marigold crop for different finish dates and at different locations in the U.S.
In both African and French marigolds, time to flower decreased as temperature and DLI increased. For example, under an average DLI of 10 molâˆ™m¯²âˆ™dâˆ’¹, time to flower of African marigold decreased by 24 days as temperature increased from 58 to 68°F (Figure 1). French marigold grown under the same DLI flowered eight days earlier at 68°F compared to 58°F (Figure 2). This information can be used to determine the date 288-cell plugs need to be transplanted for two different market dates when grown at different temperatures (Table 1).
As the DLI increased from 4 to 16 molâˆ™m¯²âˆ™dâˆ’¹, time to flower in African and French marigold grown at 63°F decreased by 10 and four days, respectively. The saturation DLI for the shortest time to flower was 12 molâˆ™m¯²âˆ™dâˆ’¹. In other words, increasing the DLI above 12 molâˆ™m¯²âˆ™dâˆ’¹ did not shorten crop time.
In both species, the number of inflorescences decreased as temperature increased and as DLI decreased. For example, in African marigold grown under an average DLI of 10 molâˆ™m¯²âˆ™dâˆ’¹, the number of flowers decreased by nine as temperature increased from 58 to 73°F.
Therefore, there is a trade-off between fast cropping and plant quality. African and French marigolds grown at the warmest temperature (79°F) and under the lowest DLI (4 molâˆ™m¯²âˆ™dâˆ’¹) in our study were of poorest quality (e.g., few flowers and branches), whereas plants grown at 58°F and under 16 molâˆ™m¯²âˆ™dâˆ’¹ were of highest quality.
The growing temperature that had the lowest predicted heating cost to produce a crop of African marigolds varied among locations and market dates (Table 2). For example, to produce a finish crop for April 1, a greenhouse located in San Francisco, Calif., would save 9 cents per square foot per crop in heating costs by growing at 58°F compared to 73°F.
In contrast, heating costs per square foot were 4 to 11 cents cheaper at four of seven locations when the crop was grown at 73°F versus 58°F. In other words, less energy was consumed by transplanting the African marigold crop later and growing warm compared to transplanting earlier and growing cool.
For French marigold, a production temperature of 58°F had the lowest predicted energy cost for both market dates at all locations. In every simulation, the heating cost to produce a crop of French marigolds was at least 50 percent cheaper than the heating costs for African marigolds because crop timing was so much shorter. The different responses of African and French marigold to temperature indicate that at many locations, it would be more energy efficient to grow these crops at different temperature set points.
The cost of energy for heating is just one of the many production expenses for greenhouse crops. Other factors, such as the number of crop turns and overhead costs, should also be considered when choosing the most economical growing temperature.