Plant growth regulators

Continuing the series of teach-and-learn, this 3rd post covers the use of plant growth regulators in floriculture. I just realized that my approach to studying combines what I love to do (blogging) with what I have to do (studying for my exam). It's the best of both worlds. The information for this part is taken largely from "Selecting and Using Plant Growth Regulators of Floricultural Crops" by Joyce G. Latimer, Extension Specialist in Greenhouse Crops at Virginia Tech.

Part 1-Plant growth medium
Part 2-Nutritional deficiencies
Part 4-Post-harvest longevity of flowers
Part 5-Legal protection of cultivar propagation

Plant growth regulators (PGRs) are useful tools to grow crops with the necessary characteristics for optimal marketing. The choice of PGR depends on what needs to be accomplished.
1. To reduce the growth rate of plants, improve color, and make the plant tough, use a growth retardant (B-nine, Cycocel, Bonzi)
2. To increase branching or make a bushy plant, use a chemical pincher such as Florel^®
3. To initiate or synchronize flowering use a gibberellin such as Cycocel or Florel^®
which causes release of ethylene in the plant.

Because PGRs are classified as pesticides, they can only be applied like someone with the appropriate training (I had to be certified to apply PGRs for a class), and only on the crops they are labeled for. When applying PGRs it is crucial not to apply an overdose, because of toxicity to the plant. Also, some plants may be more or less sensitive to a particular PGR, so you need to look that up before you start.

1. Growth retardants
These are compounds that inhibit the production of gibberellins, the plant hormone that causes cell elongation, and therefore affect the stems, and flower stalks. Shorter plants often look better in pots and are less likely to damage during shipping. Apply these PGRs early, because they may be slower in opening of flowers. B-nine and Cycocel are usually applied as early sprays (sometimes mixed together in the same tank). Bonzi can be applied later in the season as a drench (directly poured onto the moist medium).

2. Chemical pinchers
These PGRs inhibit growth of terminal shoots, causing development of lateral branches. This results in a more bushy appearance (broad as opposed to tall), and a shorter plant. They work by causing the release of ethylene inside the plant. Ethylene is a naturally occurring gaseous plant hormone that (among other things) reduces internode elongation. Be sure to apply these only if there are enough nodes on the plant to allow for lateral branching.

3. Flowering
Flowering can be enhanced by plant hormones called gibberellins, and can in some cases even be use instead of the cold temperature-requirement to break dormancy of buds for certain plants (azalea's for example). Flowers can be removed by using Florel^®. This can be necessary to maintain stock plants used for vegetative propagation, or it can synchronize flowering. In the latter case, it's important to leave enough time after the treatment for flowers to develop. Peter Konjoian uses the rule of thumb of a cut-off date of mid-March for plants to bloom by Mother's Day. At least he's got his priorities straight.

There is a lot more on this topic, but these basics are all I have time for right now.