You gotta regulate

The number of nodes on the stem and the internode length determine plant height.

Plant Growth Regulators (PGRs) can be one of the most important tools available to growers. They allow growers to produce uniform plants that can hold up to the rigors of shipping, and they generally have a better shelf life than plants not treated with PGRs during production.

PGRs are an excellent tool to produce hardier plants and control plant height, which affects the overall perceived quality in ornamental production. Growers have a toolbox of PGRs to meet their growth control needs for various crops. With planning and review of appropriate uses, a high quality uniform crop can be achieved.

Understand PGRs
PGRs are chemical compounds that interact with natural growth hormones produced by plants. They can mimic or interfere with naturally occurring compounds by altering plant height, influencing branching, interfering with dormancy, initiating flowering or flower abortion, promoting rooting, and prolonging flower and plant life.

PGRs can also provide benefits by producing greener plants due to increased chlorophyll production; reducing abscisic acid production, resulting in reduced water stress; and providing disease suppression resulting from sterol suppression in fungal cells (this activity is seen with paclobutrazol, the active ingredient in Bonzi).

Know how they affect plant height
The two factors that determine plant height are the number of nodes on the stem and the length of the stem between these nodes (internode length). PGRs control plant height by causing the plant to produce smaller and more compact cells. Simply stated, PGRs restrict cell elongation. But each PGR inhibits gibberellins (natural plant hormones responsible for cell elongation) in slightly different ways. PGRs must be applied before internode elongation. They will not shrink cells that have elongated; therefore, careful planning should take place before use.

In general, plant height affects the overall perceived quality in ornamental plant production. Wholesale production growth standards for total height or canopy height control are typically 20 to 35 percent shorter plants when compared to untreated plants. 

For example, if the goal for poinsettia height is 16 inches from pot rim to top of canopy, and the non-PGR treated height is 20 inches, the ratio produces a plant that is 80 percent of the untreated plant, so a 20 percent shorter plant is achieved. Growers wishing to create shorter plants need to focus on the 30 to 35 percent shorter plant standard. The 20 to 25 percent height standard may be for growers that wish to slow but not halt growth. Plant diameter wholesale standards normally range from 15 to 25 percent smaller plants, when compared to untreated plants.

Know the application methods
Gibberellin inhibitors are absorbed by the plant in two ways: root/shoot or leaf/shoot. Plant growth regulators absorbed by the root/shoot are Bonzi (paclobutrazol), Sumagic (uniconazole), Topflor (flurprimidol) and A-Rest (Ancymidol)

These materials are absorbed by the stems and roots and move up the xylem. Drench applications are taken up by the root system and move up the stems into the leaves. Stem applications are absorbed and move upward into the leaves. There is no movement out of the leaf since there is very little movement in the phloem. These materials are applied by sprays to the plant stems or to roots using drenches, media sprays, dips, bulb soak or sub-irrigation.

Plant growth regulators taken in by the leaf/shoot are B-Nine (Daminozide) and Cycocel (Chlormequat). Cycocel moves within the leaf but cannot move out of the leaf due to the xylem uptake. B-Nine is the only material in this group that can move within the leaves and throughout the plant. These materials are applied by sprays to the plant leaves, or by pre-plant dips.

Factor other considerations
Environmental conditions to be considered when applying PGRs include:

• Time of day application is made — hot, dry, and windy conditions (make applications early in the morning when evaporation rates are lower)
• Summer versus winter — concentration should be adjusted accordingly
• Growing media — organic content in the growing substrate organic content in the growing substrate (can reduce efficacy of paclobutrazol (Bonzi), uniconazole (Sumagic), ancymidol (A-Rest), and flurprimidol (Topflor)
• Plant water status — water stress at application (absorption is greater if plants are turgid)

Know the possible problems
Phytotoxicity is generally not a problem for Bonzi, Sumagic, Topflor or A-Rest, but over-application can result in stunting. These materials are rapidly absorbed into plant tissues and move through the xylem. Plants can be irrigated 60 minutes after application.

B-Nine and Cycocel are absorbed much more slowly into the plant, and absorption only occurs as long as the leaf is wet, so it is best to wait 24 hours before irrigation. Phytotoxicity is generally not an issue unless application is made within seven days of the application of a copper-based material.

It is best not to apply in direct sun. Phytotoxicity is generally not an issue unless application is made within seven days of the application of a copper-based material. Phytotoxicity with Cycocel can be a problem when rates over 1,500 ppm are used, but when tank-mixed with B-Nine, growers see less phytotoxicity issues. Another observation is improved height control as a combination spray versus the effects from the compound alone.

Ensure a uniform crop
Obtaining a uniform crop is your goal, and to do that, you have to follow some steps.

To obtain uniform results, calculate the correct dose and volume for your region. Also, use a known volume of spray on a known area, regardless of the number of plants. Your drench volumes should be consistent from pot to pot and use the correct volume for the size of pot (Bonzi example in Table 1, above). Lastly, drench applications should be uniformly applied to the media surface.
 

Marla Faver is a technical services representative for Syngenta Plant Protection.