Greenhouse - Growth Regulators and Growing Media Interactions with Drenches

The following is information on the effect of growth media on the effectiveness of plant growth regulators when used as drenches in the greenhouse.

Growing medium composition directly affects the efficacy of drench applications of certain PGRs. The effectiveness of A-Rest® (ancymidol), Bonzi® (paclobutrazol), and Concise® and Sumagic® (uniconazole) decreases as the pine bark component of a medium increases. In general, as organic matter content of a growing medium increases, the effectiveness of a PGR drench decreases. As the inorganic content of the medium increases (for example, the amount of sand, soil, rockwool, or perlite), the effectiveness of a PGR drench increases. Growing medium composition may indirectly influence plant response to PGRs by affecting plant vigor. Plants growing in media that reduce, retard or stunt growth (i.e., cause stress) are especially sensitive to PGRs. A slow growing plant is most sensitive to a PGR application, and takes a longer time to resume normal growth. Uniform application of a PGR drench is critical for uniform height control when multiple cuttings are growing in a single container such as a basket or pot. If most of the PGR is applied to only one side of the pot, then the plants on that side will be more affected than the plants on the opposite side.

Reprinted from the 2009-2010 New England Greenhouse Floriculture Guide http://www.umass.edu/umext/floriculture/pest_management/ne_pest_manage_guide.html