Greenhouse - Growth Regulator Use During Grow-out

Controlling growth in greenhouse plants is essential to produce high quality plants in the greenhouse. Using growth regulators during the grow-out stage where plants are grown to their finished size can help greatly. The following is an article on the subject from the New England Greenhouse Update.

During the spring crop production season growers often need to use plant growth regulators (PGRs) for a variety of reasons. Most often the purpose to control plant vigor is during the grow-out stage.

During the grow-out stage the objective it to slow stem stretch while allowing the plant to grow. Low rates of PGRs are recommended for this purpose. With low rate applications, the PGRs can be reapplied as needed to control growth. Any of the commercial PGR products can be used at this stage but of course not all species respond the same to any one material. Therefore, growers are advised to have several different classes of compounds on hand. For instance, make sure you have a triazol class chemical (Bonzi, Sumagic or Topflor) plus either Cycocel or B-Nine. Between the two, you will be able to control most of the plant species you grow.

For growers starting with plugs or rooted cuttings, growth regulation should start at or soon after transplant. If you are starting your own seed, treatments can start within 24-hours of germination.

With transplants in small containers (flats & 4" pots) materials such as B-Nine and Cycocel can be sprayed at weekly intervals. Apply the first spray as soon as new growth is evident. Use a rate at the low end of the recommended range for that species. Repeat at 7-14 day intervals as needed. Note: if you are using Florel to prevent flowering and to stimulate branching, then a growth retardant will not be needed during the early stage of development.

Triazol-class materials (Bonzi, Sumagic etc) offer more options during this stage. These materials can be applied as a soil surface spray prior to transplant (Pre-plant Soil-surface Spray or PSS), or as a Media Spray (MS) applied immediately after transplant. Note that MS applications treat both the soil surface and the plant. Triazol-class materials can also be used as traditional drenches or spray applications once the plants are established and the leaves expand to fill in the allotted space in the pot or flat.

With both PSS & MS applications, the concentration or rate applied is higher than for a drench but less than for a typical spray. For example, the spray recommendation for Sumagic on petunia is 25-50 ppm and the drench rate is 1-2 ppm but the PSS rate is 5-7.5 ppm.
In both PSS & MS, a spray volume of 2-quarts per 100 square feet is used but because the soil surface is exposed and the chemicals are highly root-active, these applications have a mild drench-effect. This spray coverage (2-qt/100 sq ft), typically referred to as spray to glisten, is not enough to soak deep into the soil so the total dosage delivered to the root zone is less then with a traditional drench even though the concentration used is relatively high.

Reprinted from "Controlling Plant Growth During the Grow-out Stage" by Richard McAvoy, University of Connecticut, in the March, 2007 edition of the New England Greenhouse Update http://www.negreenhouseupdate.info/greenhouse_update/?p=2473