|Tina Smith||A common problem for growers during the spring season is overgrown plants. Some growers lower greenhouse night temperatures to save on energy costs, while daytime temperatures are warm due to sunny days. Cool night temperatures along with warm sunny days often result in plant stretch. The greater the difference between day and night greenhouse temperatures, the more stretch occurs.
Bedding plants may become too tall if they are started too soon, are spaced too closely, are shaded by overhead hanging baskets, or are grown in low light greenhouses caused by old plastic. Fast growing herbs such as basil, chives and dill and vegetable plants easily become overgrown, especially if started too early. Depending on the extent of these adverse factors, growers can sometimes use cultural practices and chemical growth regulators to salvage plants by reducing stretch.
Temperature growth control
The DIF technique of temperature control was developed at Michigan State University during the 1980s. Research showed that the difference (DIF) between day temperature and night temperature could be used to control stem stretch in many plant species. Stem elongation is promoted by warmer days than nights (positive DIF) and inhibited by warmer nights than days (negative DIF). Plants grow taller when DIF becomes more positive and plants remain short as DIF becomes smaller or more negative.
A different, but easier approach to using DIF with similar results is to reduce the greenhouse temperature 5°F-10°F lower than the night temperature for two to three hours at dawn. Called a “cool morning pulse”, this technique reduces plant height as much as a negative DIF.
DIF 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.
Average daily temperature
Temperature also affects the rate of plant development to flowering or marketing time. Growth rate is a function of the average daily temperature. When using DIF, the average daily temperature should also be calculated and used with DIF. As the average daily temperature increases the rate of plant development increases. As the average daily temperature decreases the rate of plant growth decreases.
Average daily temperature can be calculated by adding the night temperature times the length of the night period (in hours) to the day temperature times the length of the day period (in hours), and then divide this total by 24 (the number of hours in a day).
A DIF treatment that raises the average daily temperature speeds up crop development. A treatment that lowers the average daily temperature slows crop development.
Maximize light levels
One of the easiest ways to reduce stretch is to maximize the amount of light plants receive. Providing adequate spacing, reducing the number of overhead baskets/containers and installing new or cleaning the existing glazing can help to prevent overgrown plants. Dirty glazing can reduce light levels by 20 percent.
Nutrition, water control
Reducing the application of fertilizer or water can also prevent stretch. Some growers try to hold back plants using lower temperatures in combination with nutrient and/or water stress.
Low fertility or mild water stress can prevent stretch if carefully controlled. However, there are risks including the development of nutrient deficiency symptoms which can be difficult to correct or plant damage due to water stress. The nutrients which have the most effect on plant size are nitrogen and phosphorus. Limiting nutrition and water will not likely help plants that are already overgrown.
Plant growth regulators
Plant growth regulators are another common method that growers use as a control treatment. There are no growth regulators labelled for use on herbs. The growth regulator Sumagic (uniconazole) is now labelled for use as a foliar spray on several vegetable transplants grown in greenhouses.
Growth regulators are treated as pesticides and have re-entry intervals. These products should be used wisely and not as substitutes for good cultural practices.
Growth regulators can be used to hold plants in check once they reach their final size. This is helpful when weather conditions dictate that plants be held longer than anticipated. In these situations, a spray or drench application of one of the root-active compounds, including A-Rest, Bonzi, Sumagic or Topflor, can be used. Spray applications are preferred since high concentrations are required to hold the plants at this later development stage. Sprays work faster but have a shorter residual effect. Therefore, high concentration sprays are less likely to reduce garden performance than high concentration drenches.
Growers should take precautions to prevent an overdose, which can have an extended residual effect resulting in poor garden performance. To ensure adequate control, growers should consider selecting a rate in the upper half of the normal recommended range for a particular crop or even 50 percent higher than the recommended rate for slowing growth during the grow-out stage. For example, the normal spray rate for A-Rest on ageratum plants in flats during grow-out is 10-15 parts per million, but a rate of 15-26 ppm is recommended to hold this crop.
The same principle would apply for other growth regulators. Cycocel is not recommended for this purpose since the spray rates required would be so high that phytotoxicity would likely occur. Plants can also be treated before being transplanted into larger containers or combination planters to hold them.
Growers without growth regulator application experience to hold plants can start with the high end of the normal recommended rate. A second treatment can be applied if the first one is not enough.
A proper hold rate should provide enough control to hold a mature plant under hot weather for two to three weeks, but low enough that plants recover and perform for consumers. Spray treatments carry less risk of overdose than drench treatments, especially when root-active growth regulators are used.
When using a growth regulator for the first time, treat a small group of plants and keep accurate records of the response and environmental conditions in the house.
When the other control methods are not effective, plants can be cut back. Cutting back is the process of removing one-half or more of a plant to reduce its size. Some species respond well to being cut back by producing abundant new growth, but others do not.
Be aware that pinching some overgrown plants can result in poor branching from hard woody stems below. If the response to cutting back a particular species is unknown, test it on a few plants to determine their ability to recover. Cutting back should be reserved as a last measure.
Pinching can be used if plants are slightly overgrown or need some shaping. Removing the terminal growing point and one or two uppermost leaves is a soft pinch. A soft pinch allows the dormant buds below the pinch to grow.
A soft pinch can be done by using your thumb and forefinger or by cutting off the growth mechanically with a knife, scissors or clippers. Many growers make one or more soft pinches to hanging basket plants to control their overall size and shape, to increase flower number and to create full, thick growth.
When plants are overgrown, beyond using a soft pinch, growers may remove the terminal growing point and two to four leaves. This is considered a hard pinch and delays flowering more than a soft pinch. A hard pinch may also result in undesirable branching from the plant if not enough nodes are left on the stems.
Plants can be pruned and shaped at any time to reduce stretch and to improve aesthetics. This treatment will delay flowering or re-flowering by two or more weeks. Pinching is labor intensive.
Some growers will make subsequent pinches at three- to six-week intervals, depending on the plant species and growing conditions. Additional pinches result in fuller growth and add to the quality, but also add significantly to total production time.
Tina Smith is extension floriculture specialist, University of Massachusetts, (413)545-5306; firstname.lastname@example.org