The Case for Space By Erik Runkle

Space is often a limitation when growing floriculture crops in controlled environments. Greenhouse space is expensive to build, maintain, heat and light. In addition, overhead costs are often allocated on a square foot per week basis, so overhead costs increase as the amount of space occupied increases. Therefore, thoughtful utilization of greenhouse space should be a high priority.

Growing as many plants as possible per heated square foot is a common objective. Although this practice could reduce overhead costs per crop, there are trade-offs, such as reduced plant quality and increased stem elongation. This article briefly discusses some of the factors that should be considered for plant spacing.

Factors to Consider

Economics. Overhead costs per container decrease as container density increases. Plants spaced pot-to-pot occupy a relatively small amount of floor space. As plants are spaced farther apart, the number of plants that can be grown within a confined space decreases. Naturally, there is a temptation to jam-pack an area with plants, especially when it’s cold outside.

Plant height. What happens when plants compete for light? The most obvious response is plant elongation. Plants use most of the red light for photosynthesis but transmit or reflect most of the far-red light. Therefore, at a high plant density, the relative amount of red light decreases much more than the amount of far-red light. As a result, the ratio of red to far-red light decreases and stem elongation increases. So, plants closely spaced will be taller and thus may require a higher rate, or more frequent applications, of a plant growth retardant.

Plant quality. As plant density increases, the amount of light that plants can capture decreases. In other words, plants with luxury spacing will capture more light than plants tightly spaced. As a result, the quality of plants decreases as the amount of space a plant occupies decreases, particularly when light is limiting. Crops grown with wider spacing will appear as if they were grown under a higher daily light integral (DLI); those tightly packed will appear as if grown under a lower DLI. This means that tightly-spaced plants will have fewer branches, a less-developed root system and fewer flowers than those with more space.

Pathogens. When plants are spaced close together, there is generally greater potential for the spread of pathogens. Media and foliage take longer to dry when plants are grown at a high density. If plants are overwatered or the humidity is too high within the plant canopy, pathogens such as Botrytis can be more problematic. Thus, watering should be watched very carefully when crops are grown at cool temperatures, low light levels and high plant density.

Finally, consider your market when determining plant spacing. Plants that have more space will be of higher quality — but can you recoup the higher costs involved in producing the higher-quality plants?

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Erik Runkle

Erik Runkle is professor and floriculture extension specialist in the
department of horticulture at Michigan State University. He can be reached at [email protected]

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