The height of greenhouse plants can be controlled by a number of non-chemical cultural methods. Interest in these techniques has grown because of the tighter controls placed on the use of agricultural chemicals and the public's negative perception of chemicals in general. The Worker Protection Standards recently developed by the USEPA control the use of plant growth regulators (PGRs) and limits have been set on how soon workers may reenter greenhouse areas treated with PGRs. Reentry intervals (REIs) for PGRs range from 12 to 48 hours. What follows is an outline of other methods for controlling plant growth which may be effective alone or in combination with low levels of PGRs.
Scheduling and Cultivars
Plants started too soon often need to be "held back" by PGRs. Some growers start plants early to spread out transplanting to match the availability of labor and space or cutting production from stock plants. The quality of the earliest plants may suffer from efforts made to hold them back and/or the plants may be past maturity at marketing time. Buying-in cuttings or plugs rather than trying to grow-your-own may help keep crop production on the proper schedule.
Cultivar selection is also important. Some bedding plants are available in tall, medium, and short types which look very similar except for height. Unless customers are fussy about height, using smaller cultivars is an easy way to "control" height.
A review of the basic cultural recommendations and schedules for a crop may be an useful and "eye-opening" exercise. For example, the PGR requirements for most of today's poinsettias are rather minimal compared to the Heggs assuming the /proper crop timing, light, and temperature requirements are met. But when the cuttings are potted and pinched early or if the plants are crowded the number of PGR applications generally increases.
One of the eaiest ways to reduce height and the need for PGR treatment is to maximize the amount of light plants receive to reduce "stretch." This means adequate spacing, clean glass, and fresh plastic covering. For some plants supplemental HID lighting may be feasible. One of the most common reasons for frequent use of PGRs is that the grower is not taking full advantage of all the available natural light.
DIF Temperature Control By now most growers have heard of the DIF technique of temperature control developed by Dr. Royal Heins and colleagues at Michigan State University. In fact, some growers in our area are using some form of DIF on a regular basis. DIF is defined as the difference between day temperature (DT) and night temperature (NT). Stem elongation is promoted by warmer days than nights (+ DIF) and inhibited by warmer nights than days (- DIF). Plants become taller as DIF becomes more positive and plants become shorter as DIF becomes smaller or more negative.
Significant height control and reduction in PGR use is possible by reducing the difference between DT and NT as much as possible. Another approach to using DIF is the "cool morning pulse." A cool morning pulse is created by reducing the greenhouse temperature 5-10 F lower than the NT for 2- 3 hours at dawn. This approach reduces plant height as much as a negative DIF and may be the most easy DIF treatment to make.
DIF seems to be effective on most greenhouse plants, but research continues with many different species. Some responsive and non-responsive species are listed in Table 1. DIF, like a PGR, has its greatest effect on height during the period of most rapid stem elongation. DIF does not have to be applied continuously throughout a crop cycle to be effective, but rather only during the period of most active vegetative growth.
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A note of caution: DIF treatments affect the rate of crop development as well as stem elongation. Growers using DIF should determine the effect of their DIF treatment on the average daily temperature. A DIF treatment raising the average daily temperature would speed crop development, while a treatment lowering the average daily temperature would slow crop development.
One of the oldest and most common ways of attempting to prevent stretching is to withhold fertilizer or water. Some growers try to hold back plants using low temperature in combination with nutrient and/or water stress. Low fertility or mild water stress can be successful if carefully controlled. However, there are risks - too much growth inhibition, development of nutrient deficiency symptoms which are unsightly and hard to correct, or damage to the plants from water stress.
The nutrients which have the most effect on the size of greenhouse plants are nitrogen (N) and phosphorus (P). The biggest effect of withholding a water-soluble fertilizer is N deficiency. Unfortunately if N deficiency conditions go on too long the plants will be too small and also very yellowed. A P deficiency is somewhat more difficult to create than a N deficiency. However, if carefully managed a mild to moderate P deficiency will result in a desirable reduction in growth and no foliar symptoms. In fact, a mild P deficiency actually makes many plants appear greener! A well-known fertilizer company promotes the "phosphorus starvation" technique for growth control and markets two water-soluble fertilizers, 20-1- 20 and 20-2-20, for this purpose. Routine use of these fertilizer supposedly results in shorter, stockier plants than fertilizers with higher P analysis (e.g., 15-16-17, 20-10-20). Growers should be warned that low P is said to reduce bract diameter of poinsettias, so low P fertilizers should not be used on this crop.
I conducted research on the potential of nutrition to control growth with support of the Massachusetts Flower Growers Association and the New England Greenhouse Conference. The main impetus for this project is that growers have no recommendations to follow if they try to use low fertility as a growth control method. The objective is to develop methods which reduce plant height while avoiding too much stunting, deficiency symptoms, and undesirable delays in crop development. There are many different practical approaches which could be tried, but I have had a chance to try only a couple.
In one experiment I grew 'Red Elite' geranium in a commercial mix amended with 20% superphosphate in a range of 0 to 32 oz./cu. yd.. The fresh weight of the tops at the end of the experiment followed a typical fertilizer rate response pattern (Fig. 1). Plants grown with no superphosphate or 2 and 4 oz./cu. yd. were darker green in color, but only slightly shorter than the higher P treatments. Perhaps more growth control did not occur because there was enough P in the starter charge to carry the plants during the early, active stage of growth.
In another experiment I grew 'First Lady' marigold and withheld fertilizer for 10 days from a different group of plants periodically as the plants grew. The biggest effect on growth measured at the end of the experiment occurred when fertilizer was withheld during periods 10-20, 20-30, or 30- 40 days after transplanting the seedlings (Fig. 2). The effects of no fertilizer during these periods was apparent mainly in leaf size, stem thickness, and branch development rather than height. These two experiments demonstrate that using mild or moderate nutrient deficiency to control growth is not as straightforward as might be expected. I plan more trials beginning this fall.
Future Height Control Techniques
Mechanical Conditioning. It has been known for a long time that mechanical stresses such as repeated brushing, shaking, or bending caused by air movement or contact with animate or inanimate objects can reduce plant growth. Recent research conducted by Dr. Joyce Latimer at the University of Georgia has demonstrated the commercial potential of this technique for controlling the height of vegetable transplants, particularly tomato. This work was stimulated, in part, by the fact that B- Nine is no longer registered for use on edible crops. One system of mechanical conditioning adapted to commercial greenhouses involves drawing a bar across the
Figure 2. Growth of marigold is affected by when fertilizer is withheld for short periods at different times during the crop.
tops of the plants once or twice a day. The bar is set low enough to contact the plants, but not so low that the plants are injured or uprooted. Thirty to 40% reductions in height have been reported with this system. Other systems involve periodic shaking, blowing air treatments, or water sprays. For this to become useful to flower growers research is needed to determine the response of flower crops.
Light filters. Plant physiologists have found that changing the ratio of red to far-red light can influence stem elongation and branching. Red light inhibits stem elongation compared to far-red light which promotes stem elongation. Red light also promotes branching by stimulating lateral bud growth. In nature there are daily and seasonal changes in the red:far-red ratio. Natural light in the middle of the day and in the summer has a higher proportion of red than sunrise and sunset and the winter. The shading effect of plant canopies also changes the ratio increasing the proportion of far-red light. This is an important factor in why plants stretch when they are spaced too closely. Current research is being directed at developing greenhouse coverings which alter the red:far-red balance to control plant height and branching.
- Heins, R. and J. Erwin. 1990. Understanding and applying DIF. Grnhse. Grower. 8(2):73- 78. (February issue).
- Latimer, J.G. 1991. Mechanical conditioning for control of growth and quality of vegetable transplants. HortScience. 26(12):1456-1461.
- McAvoy, R.L. and R.J. Shaw. 1994. Growth regulators. In New England Grnhse. Flor. Crop Pest Man. Growth Reg. Guide. pp. 81- 103.
- McMahon, M. and J. Metzger. 1995. Cultural alternatives to chemical growth regulators. Bull. Ohio Flor. Assoc. 788:1, 10-12. (June issue).
- Rajapakse, N.C. and J.W. Kelly. 1992. Regulation of chrysanthemum growth by spectral filters. J. Amer Soc. Hort. Sci. 117(3):481-485.
Plant, Soil and Insect Sciences
University of Massachusetts Amherst