Perennial production

Using Plant Growth Regulators on Containerized Herbaceous Perennials
By Joyce G. Latimer, Extension Specialist, Greenhouse Crops, Virginia Tech and Holly Scoggins, Associate Professor, Herbaceous Perennials Program, Virginia Tech

In production settings, as well as in retail locations, herbaceous perennials grown in pots tend to stretch and become leggy or simply overgrow their pots before their scheduled market date. These plants are less marketable and
harder to maintain. Many growers resort to pruning, which is not only costly in terms of labor, but also
delays plant production for two to four weeks.

Handle PGRs as production tools -- like water or feed.

PGRs are chemicals that are designed to affect plant growth and/or development. They are applied for specific purposes to bring about specific plant responses. Although there is much scientific information on using PGRs in the greenhouse, it is not an exact science. Achieving the best results with PGRs is a combination of art and science — science tempered with a lot of trial and error and a good understanding of plant growth and development.

For best results, PGRs should be handled as production tools — like water and fertilizer. They should not be used as crutches for poor management of other cultural practices. PGRs should be an integrated part of your crop production cycle. The selection of PGRs and their application rates will be affected by how your crop is grown. Especially with very vigorous plants, as many of the herbaceous perennials are, higher fertility and irrigation levels will increase the amounts of growth regulator required to prevent excessive growth. Shading, lower light levels, or tight plant spacing — especially under higher growing temperatures — will also increase plant stretch and reduce lateral branching. For the highest-quality plants, the use of PGRs must be integrated into your production plan. Plants to be treated with PGRs should be healthy, turgid, and unstressed — never wilted. The label will identify the target tissue for that PGR.

PGRs are most effective when applied at the appropriate times to regulate plant growth or development. In other words, growth retardants cannot shrink an overgrown plant. They must be applied before the plant is overgrown to avoid plant stretch. When planning PGRs in your production schedule, consider what you want to accomplish with the treatment.
• Do you want to regulate shoot growth of the plant, resulting in a sturdier, more compact plant with improved color? If so, you probably want a growth retardant.
• Do you want to increase plant branching for enhanced cutting production or for a bushier potted plant or hanging basket? If so, you probably want to use a branching agent or “chemical pincher.”
• Do you want to enhance flower initiation or synchronize flowering? If so, you probably want to use
chlormequat chloride or gibberellic acid.
• Do you want to remove flowers from stock plants to increase the number of vegetative cuttings? If so, you probably want to use an ethylene generating compound.

Regulating Shoot Growth
Most PGRs used in the greenhouse or nursery are used to regulate shoot growth of containerized crops. These PGRs are referred to as “growth retardants.” Typical growth retardants are daminozide, chlormequat chloride, ancymidol, flurprimidol, paclobutrazol, and uniconazole.
These PGRs control plant height by inhibiting the production of gibberellins — the primary plant hormones responsible for cell elongation. Therefore, these growth-retardant effects are primarily seen in stem, petiole, and flower stalk tissues. Lesser effects are seen in reductions of leaf expansion, resulting in thicker leaves with a darker green color.
Other benefits of using these PGRs in plant production include improved plant appearance by maintaining plant size and shape in proportion with the pot. Plant growth retardants also increase the tolerance of plants to the stresses of shipping and handling, as well as drought or temperature stresses common during retail marketing, thereby improving shelf life and extending plant marketability.
Generally, growth-retarding PGRs should be applied just prior to rapid shoot growth. This is usually one to two weeks after transplanting a plug/liner, after the roots are established and as the plant resumes active growth. On pinched plants, it is after the new shoots are visible and starting to elongate.
This is where the art of plant growth regulation is most important. You must learn how your crop grows and when to intervene to obtain the desired results.
Remember to note details of crop development in your records of PGR treatments. For example, due to weather conditions, next year you may need to treat at seven days after transplanting instead of at 10 days after transplanting, which you used this year. You must gauge when rapid elongation will likely occur and treat to counter it.

Enhancing Lateral Branching
Another group of PGRs used in floricultural crops are those that enhance branching, including ethephon, benzyladenine, dikegulac sodium, and methyl esters. These PGRs are frequently called “chemical pinchers” because they generally inhibit the growth of the terminal shoots or enhance the growth of lateral buds, thereby increasing the development of lateral branches. They can be used to replace mechanical pinching of many crops like vinca vine, verbena, lantana, and English ivy (Hedera). Often, this increased branching will also reduce the overall height of the plant.
The ethylene released inside the plant by ethephon also inhibits internode elongation, keeping treated plants more compact than untreated plants. Ethephon also affects flowering.
Configure is a synthetic cytokinin (6-benzyladenine), which is a plant hormone that stimulates lateral branching.
If you are looking for enhanced branching, you must have sufficient growth on the plant to allow for sites of lateral development. They cannot enhance lateral branching if there are no laterals on the plant.
Timing of the application is critical to a good branching response.
Again, read the label for details of when to apply for optimum response. Always consider the side effects of treatments.

Read the Label!
Plant growth regulators are classified as pesticides. Therefore, they are subject to all of the same U.S.
Department of Agriculture (USDA) recordkeeping and Worker Protection Standard (WPS) rules as all of your other pesticides. Their use is governed by the manufacturer’s label, as with other pesticides. The label not only contains information on restrictions, but it also has a great deal of information on how to use the product effectively. Before going to the time and expense of applying PGRs to your crop, answer these questions:
• Is the chemical labeled for the crop you wish to treat?
Most PGR labels have undergone revisions that apply to a broad range of similar crops not specifically listed on the label, with the user taking responsibility for determining appropriate rates. This provides label permission to use the compound on these crops without the manufacturer accepting the responsibility for the rate selection.
• Is the chemical labeled for the area you wish to treat?
Many PGRs are only labeled for use inside a greenhouse or other growing structure.
• Are there any potential side effects, such as phytotoxicity?
Note that you may need to look elsewhere for this information for your specific crop.
• Are there label warnings regarding the PGR’s effect on plant flowering? For example, many branching enhancers delay flowering. Ethephon causes flower bud abscission prior to enhancing branching; therefore, is not recommended within six to eight weeks of marketing. Side effects are frequently affected by the timing of the application, e.g., late applications of growth retardants may delay flowering.

Read more here:
http://pubs.ext.vt.edu/430/430-103/430-103_pdf.pdf