One of the many challenges faced by greenhouse growers is keeping the plants height short and controlled. We nurture our plants providing the proper amounts of food, light, water, and temperatures and, even with all that, the plants stretch and get leggy anyway. Are there ways to control plant height and keep them from stretching?
There are three methods of control: biological, physical and chemical. Lets look at these methods separately.
Gardening Tips: Controlling Plant Height In the Greenhouse
Biological Method: thoroughly research the cultivars you want to grow. Many bedding plants have varying maturity heights; short, medium and tall. Select the cultivars that best suit the area in the greenhouse where it will grow. It is best to grow varieties that stay short, as this is an easier way to control the plants height. The plant takes care of itself.
Schedule your start times so the plants will mature on time. Starting seeds too soon and allowing them too much time to grow creates a need to “hold back” the plant. Trying to hold a plant back from maturing is very hard to do and in the efforts made to try and accomplish this, the quality of the plant can diminish. I remember starting tomato seeds in late February for a May planting. I was a little over anxious. The seeds germinated right on time and continued to grow rapidly. There was no holding them back and I wound up with tomato trees. I lost many of the plants along the way as I was trying to maintain them. The following season I adjusted the start times, what a difference a few weeks made.
Physical Method: this control method encompasses many aspects of the plants growth from the growing environment to the cultural practices used.
1. Light intensity is one of the easier ways to keep plant height controlled. Give the plant ample room to grow. Crowding the growing area creates competition for all available light giving less to each plant. Grow only the amount of plants that you can feasibly fit in your greenhouse. Once a plant senses the others presence next to it, the plant will start to grow upward. Keep the canopy open by limiting the amount of hanging baskets grown. The plants below will be shaded too much for proper growth. I ignored these suggestions when I grew for retail purposes. I would try to squeeze out as many plants as I could from the greenhouse and indeed some plants did suffer; growing tall and leggy and ultimately not surviving the season due to the stresses they were put under.
2. If your greenhouse is made of glass, make sure it is kept clean as much as possible. The amount of light that is able to come through a dirty glass panel is diminished considerably. Plastic coverings that are old tend to turn yellow. This too causes less light to come through. Replace the covering when it is showing signs of aging. Poor lighting conditions cause the plants to stretch and grow leggy. Provide as much light as you can.
3. Container size helps to control plant height as well. Using containers that are too small, creating restricted root systems, reduces the stretching of bedding plants.
4. Watering a plant less often, allowing it to wilt slightly between waterings, gives a shorter plant, but you risk poor quality if wilting is allowed to occur too often. Plants can tolerate a certain amount of wilting. Be careful to not allow the plant to wilt to the point of no return.
5. Fertilizing less is an old time favorite way to control plant height. This method can be quite successful, so long as it is controlled. The nutrients that effect plant size are nitrogen and phosphorous. Nitrogen withheld has the biggest effect on plant height; however, allowing too much deprivation of the N nutrient for too long a period can cause yellowing of the leaves and overall poor quality. There are special fertilizers sold at garden centers that have predetermined nutrient combinations, for example, regular use of the water- soluble 20-1-20 or 20-2-20, has been shown to produce shorter, stockier plants.
6. Temperature control, using a method known in the industry as DIF, has been shown to be a useful method in controlling plant height. Developed by researchers from Michigan State University back in the 1980’s, their research basically showed that the average temperature (the average day plus night temp) affects a plants growth rate with higher averages resulting in more rapid growth and development.
DIF, the difference between day temps and night temps affects stem elongation and height. Stem elongation is the distance of the stem from one node to the next. DIF is calculated as the day temp minus the night temp and can be either positive DIF (day temp is higher than night), zero DIF (day temp = night) or negative DIF (day temp is less than night).
Trying to keep a greenhouse in negative DIF is a difficult task for most. Another way was found that accomplished reducing a plants height and is the easiest DIF treatment to use, it is called “cool morning pulse”. By reducing the greenhouse temperature 5 to 10 degrees F lower than the night temperature for 2 to 3 hours, starting 30 minutes before dawn, reduced plant height as effectively as negative DIF and was easier to do. Here is an example: night temp of 68 -degreesF, two hour drop to 60 -degreesF (30 minutes before dawn), and then 65 -degreesF maintained during the day. For warmer plants: 72 -degreesF at night; 64 -degreesF pre-dawn for 2 to 3 hours; 65-degreesF day.
There are thermostatically controlled devices that you can set up to automatically change these temperatures at the correct times. Salvia, Rose, Snapdragon and Fuchsia had very good responses to this DIF control compared to Aster, French Marigold, Tulip and Squash which showed little or no response.
Mechanical Methods are used and have been known for a long time to be a good control. By brushing, shaking or bending the plant on a regular basis caused the plants to stay short. A mechanical device was created for use in commercial greenhouses that grow vegetable seedlings. A bar is drawn across the tops of the seedlings once or twice a day. The bar is low enough to make contact with the plant without breaking the tops. A 40 percent reduction in height was recorded. Other systems using vibrations, periodic shaking and blowing air movement are also good methods of height control.
Chemical Method is really the last resort that should be taken. Improperly applying these chemicals can cause extensive damage to the plant and can be unhealthy for anyone entering the greenhouse. Chemical growth regulators are not approved for use on vegetable seedlings. A hormone called gibberellins is responsible for a plants cellular growth and elongation. Growth regulators and retardants are anti-gibberellins that inhibit gibberellin’s synthesis thus keeping the plants from growing tall. Common growth regulators and retardants are A-Rest, B-Nine, Bonzi, Cycocel, Florel and Sumagic, each used on different plants and performing different functions.
These chemical controls are considered pesticides and are best left to the professional. For the hobby greenhouse grower the non-chemical methods of control are best.
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