Energy-Efficient Annuals: Perfecting Temps & Light

Energy-Efficient Annuals: Perfecting Temps & Light

Bedding and garden plants are the largest category of floriculture crops in the United States with a wholesale value of $1.76 billion in 2007. Scheduling these crops in flower for specific market dates, at different times of the year, can be a challenge. In addition, most bedding plants are produced when energy for heating a greenhouse is a large production cost, particularly in northern climates. To optimize the greenhouse environment and produce crops as energy efficiently as possible, more information is needed on how bedding plants respond to temperature and light.

Figure 1. The effects of average daily temperature on time to flower and
number of flower buds in petunia. Plants were grown under a 16-hour
photoperiod and an average daily light integral of 20 mol∙m−2∙d−1. The
photograph was taken four weeks after transplant from a 288-cell plug tray.

During the past several years, we have performed greenhouse experiments at Michigan State University to study the effects of temperature and light on crop timing and plant quality of many popular seed-propagated annuals. In this 12-part series, we will present research-based information so that these crops can be scheduled in a more energy-efficient, predictive manner. We will also use computer software (Virtual Grower) to predict how different scheduling options influence greenhouse energy consumption. This first article reviews how temperature and light influence crop timing and plant quality

Average Daily Temperature

The rate of plant development (time to flower) is controlled by the 24-hour average daily temperature (ADT). Many greenhouse crops develop a certain number of leaves before flowering. Therefore, how fast or slow a crop develops can be controlled by raising or lowering the ADT. For example, petunia ‘Dreams Red’ grown at 79°F (26°C) flowered 19 days earlier than plants grown at 59°F (15°C) (Figure 1). Some growers have lowered the night temperature in an attempt to lower energy costs for greenhouse heating. However, this practice delays development and increases production time unless the day temperature is increased so that the ADT is the same. The net result can be that plants grown at cool temperatures can use the same or more heating fuel (spread out over more time) and fewer crop turns are possible.

Figure 2. Conceptual diagram of the rate of plant development (such as
leaf unfolding) in relation to the average daily temperature. The
shape of the curve varies from crop to crop.

The rate of plant development stops when the ADT is near the base temperature and increases linearly as temperature increases until some optimum temperature is reached (Figure 2). With further increases in temperature, the rate of plant development begins to decrease and thus delay flowering. The base and optimum temperature vary among species, and until now, little information on bedding plants has been available. In our greenhouse experiments, plants were grown at four or five different ADTs between 57 and 79°F (14 and 26°C) and flowering time was recorded.

Plants grown at cooler temperatures often have more flowers at first flowering than plants grown at warmer temperatures. For example, petunia ‘Dreams Red’ grown at 59 or 64°F had 15 more flower buds at flower than plants grown at 79°F (Figure 1). This is especially a concern under light-limiting conditions. Higher plant quality at a cooler temperature can occur because plants are in the greenhouse longer and have more time to harvest light for photosynthesis. Therefore, there can be a trade-off between producing a high-quality crop and short crop timing. We will present our research information on how individual crops respond to temperature and light, and the impacts on energy consumption, in future articles of this series.

Figure 3. Effects of average daily temperature and daily light integral (DLI)
on time to flower in marigold ‘Moonstruck Orange.’ Plants were grown under a
16-hour photoperiod. Photograph was taken eight weeks after transplant
from a 288-cell plug tray.

Daily Light Integral

Flowering time and plant quality can also be influenced by the total amount of photosynthetic light (daily light integral, or DLI) that a plant receives. DLI is the cumulative amount of light received during a 24-hour period and is expressed as moles per square meter per day (mol∙m¯²âˆ™d¯¹). In the north (above 35°N latitude) and during the winter, the DLI inside a greenhouse without supplemental lighting can be less than 5 mol∙m¯²âˆ™d¯¹. In late spring, the greenhouse DLI can reach 25 to 30 mol∙m¯²âˆ™d¯¹ before shading is used to prevent unwanted high temperatures.

Many crops grown under a high DLI flower faster than those grown under a low DLI. For example, marigold ‘Moonstruck Orange’ grown at 63°F and under 12 mol∙m¯²âˆ™d¯¹ flowered 8 days earlier than plants grown at the same temperature, but under 5 mol∙m¯²âˆ™d¯¹ (Figure 3). The acceleration of flowering under a high DLI can be related to various factors, including: 1) greater photosynthesis; 2) formation of fewer leaves before flower initiation; and 3) warmer plant temperature. Species that produce fewer leaves before flowering when grown under a high DLI are described as having a facultative irradiance response. As with temperature, there is a saturation value at which any further increase in DLI has little or no effect on flowering time.

A higher DLI can also improve crop quality. Plants grown under a high DLI typically have smaller and thicker leaves, thicker stems, shorter internodes, increased rooting and more lateral branches and flowers. This is why plants finished in late spring are generally of higher quality than those produced earlier. Plants in our experiments were grown under DLIs ranging from 4 to 20 mol∙m¯²âˆ™d¯¹ to determine the influence of DLI on flowering. DLI responses for different annual crops will be discussed in future articles.

Photoperiod (day length) can also influence crop timing because many plants flower in response to short days (for example, poinsettia) or long days (for example, petunia). Other plants are day neutral (not affected by photoperiod). For energy-efficient greenhouse production, photoperiod-sensitive plants should be grown under a photoperiod that promotes flowering. In our greenhouse experiments, most of the annuals studied were all long-day crops, so plants were grown under a 16-hour photoperiod.

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