Due to the current COVID-19 outbreak and the restrictions imposed by state governments to minimize spread of the virus, growers may struggle to get plants to market in an environment with reduced demand. This may force wholesale growers to keep their crops in greenhouses beyond their scheduled shipping dates, and retail growers to hold plants in the greenhouse for significantly longer than planned.
When holding crops in the greenhouse, the primary goals are to slow down plant growth and maintain the quality of the plants. Both cultural practices and chemical methods can be useful for controlling plant growth and preserving plant quality.
Cultural practices to reduce plant growth may include the following:
- Reduced watering: Mild water stress can stimulate reductions in plant growth rates. This can be implemented by consistently allowing the root zone to dry down, by withholding irrigation until early symptoms of wilting are observed.
- Reduced fertilizer application: Reducing nitrogen fertilization will slow overall plant growth. However, proceed with caution because if nitrogen becomes deficient plants can end up stunted and yellow. Regarding phosphorus, a mild or moderate phosphorus reduction can result in a more measured growth rate and enhance green color of foliage. Unfortunately, a phosphorus deficiency is more difficult to create than a nitrogen deficiency.
- Temperature manipulation:
- Decreasing greenhouse temperature is among the best best strategies to slow plant growth, if outside temperatures allow. For cold tolerant plants such as petunia, snapdragon, nemesia, dianthus and marigold, greenhouse temperature can be reduced to 40-45 degrees F. For cold sensitive plants such as celosia, portulaca, torenia, angelonia and vinca, greenhouse temperature can be reduced to 50-55 degrees F. With reduced greenhouse temperature, be sure to keep relative humidity low because high humidity reduces water uptake and increases incidence of diseases such as botrytis.
- The difference between day and night temperature (DIF) can also be used to control plant growth. DIF affects stem elongation and plant height. Negative DIF (day time temperature less than night temperature) reduces stem elongation and plant growth and is an effective way of reducing growth rates.
- Pinching: Pinching involves removal of the terminal growing point and young leaves. Pinching eliminates apical dominance and allows the dormant buds below the pinch to grow. Pinching is used to increase branching and improve plant shape, and also helps to reduce height and to slow plant growth.
- Spacing: The distance between containers affects the amount of light, water and nutrients available to individual plants. Closely spaced plants grow taller with smaller diameter stems. Widely spaced plants grow shorter with larger diameter stems. If possible space plants strategically to reduce stem length.
Chemical methods to reduce plant growth:
- Applying plant growth regulators (PGRs) is another tool for slowing down plant growth. There are several PGRs available in a variety of formulations, and it is very important to take special care in selecting the right PGRs for this objective. Performance of PGRs can also vary depending on application method. When the goal is holding plants temporarily sprench applications are preferable to sprays or drenches, as a sprench lasts longer than a spray. Drench application is not advised soon before shipping or selling plants because the effect of the PGR may last longer than desired.
For more information refer to the links below:
Runkle, E. and H. Lindberg, 2020. Holding greenhouse crops. Michigan State University Extension Fact Sheet. https://www.canr.msu.edu/news/holding-greenhouse-crops
Currey, C. J. and R.G. Lopez, 2010. Applying plant growth retardants for height control. Purdue University Extension Fact sheet. https://www.extension.purdue.edu/extmedia/ho/ho-248-w.pdf
Cox D., 2007. Controlling plant height without chemicals. UMass Extension Fact Sheet.
- Geoffrey Njue, UMass Extension Greenhouse Crops and Floriculture Program