Plant growth regulators (PGRs) are chemicals that are designed to affect plant growth and/or development. They are applied for specific purposes to elicit 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.
Read all about it in this publication from Virginia Cooperative Extension.