A spike in grocery shopping and stocking up amid the COVID-19, the infectious disease caused by novel coronavirus, pandemic has posed large challenges to retailers who struggle to stock their shelves with essential items. The supply chain issues for grocery retailers and box stores has also created shipping challenges for some greenhouse growers. Some are not able to secure the trucking that they need to supply their product to stores or have the retail space in the store typically dedicated to floral products. In addition, demand for some floral products appears to be reduced as people instead focus on buying paper products, cleaning supplies, meats and other essential food items. The demand has also shrunk in response to job layoffs and reduced income for many families in the service industries or those who work for non-essential businesses. This has forced some wholesale growers to keep crops in their greenhouses beyond their scheduled shipping dates, and retail growers to keep their crops indefinitely.
Photo by Erik Runkle, MSU
When crops need to be held, Michigan State University Extension suggests two fundamental things commercial greenhouse growers can do to slow down growth and maintain crop quality: lowering the temperature and applying PGR sprays and sprenches. There are other strategies, such as cutting back plants or increasing plant spacing, but those are often not viable options.
Lower the temperature
In most cases, decreasing the greenhouse temperature is the best strategy to slow down plant development. For cold-sensitive plants (for example, angelonia, celosia, hibiscus, pentas, portulaca, torenia and vinca), the greenhouse temperature can be reduced to around 50–55 degrees Fahrenheit. For cold-tolerant plants (for example, dianthus, marigold, nemesia, petunia, snapdragon and stock), the greenhouse can be reduced as low as 40–45 F. Obviously, the outdoor weather may prevent greenhouses from getting this low, but lowering both the day and night setpoints will slow down growth and flower development.
If the greenhouse temperature is lowered, but sure to watch humidity levels. A high humidity reduces water uptake and also increases the potential for several plant pathogens such as botrytis. Therefore, keep plants on the dry side, especially on a cloudy day, and water in the afternoon only when necessary, so that the foliage is dry by sunset.
Apply PGR sprays and sprenches
Plant growth retardants (PGRs) can be applied to ornamental crops to inhibit extension growth and maintain compact growth. As plants near their marketing stage, their spacing becomes increasingly tight. In response, extension growth increases as plants compete for available light.
One way to slow down extension growth of ornamentals is to apply a PGR as a foliar spray (2 to 3 quarts per 100 ft2) or sprench. A sprench is a hybrid of a spray and a drench, where the foliage gets wetted with the PGR solution, and some of that solution reaches the top layer of the growing substrate. A PGR sprench typically lasts longer than a spray but not as long as a drench. With a couple of exceptions, PGRs cannot be applied to edibles, such as vegetables or herbs.
PGR drenches are usually not advised soon before shipping, except for crops that will stay in their final container. For plants intended to be transplanted (plugs, liners and flats), the effects of a drench can last longer than desired, and crops may not grow out sufficiently for the grower or the consumer.