Ask Us: About Plant Growth Regulators

April 24, 2007 - 06:49


How can PGRs be used to reduce pruning and plant spacing of summer-planted
perennials during the summer bulking-up period?


First, except for extremely vigorous crops, you probably will not want to use a liner dip prior to planting. The goal during the first growing season is to fill the pot and maximize root growth. Although PGRs have less effect on root growth than on shoot growth, you can still see some effects with higher rates or actual root applications. If you can use your irrigation system to apply a soil-active PGR, consider the watering-in or drench type application through the irrigation system when the plants are approaching the edge of the pot, but most perennial growers are using sprayers to apply their PGRs.

If you are using a soil-active PGR, consider using a sprench application (a high-volume spray) to give you more residual PGR activity with the drench effect of the higher volume. The goal here is to reduce shoot growth to limit the number of times you have to prune or space the crop. Once the crop is established, you can hit the plant with higher rates to control shoot growth. Plan on applying multiple applications. Be careful with your rates of application, especially as you change the volume applied. In a limited number of cases we have seen carryover effects of summer applications at high rates on growth the following spring. Some growers have reduced their pruning down to once a month with a weekly PGR spray and documented labor savings with the process.


I am trying to use PGR drench applications more, but they seem less effective on
crops in a bark mix. What are the effects of media on drenches?


Pine bark, an important component of growing media for many growers, reduces the activity of all the PGR products applied as a drench. This includes A-Rest (ancymidol, SePRO Corporation); Sumagic (uniconazole-p, Valent USA Corporation); the Paclo. (paclobutrazol) products Bonzi (Syngenta Professional Products), Paczol (Chemtura Corporation) and Piccolo (Fine Americas, Inc.); and the new material Topflor (flurprimidol, SePRO Corporation).

When a drench is applied, the active ingredient is absorbed by the waxes on the surfaces of the bark particles, which reduces the amount of chemical in the soil and in contact with plant roots. The absorbed PGR is not deactivated and slowly diffuses out into the soil. If this media is composted and reused for another crop, the PGR will still be there and possibly reduce the growth of the second crop.

Because the microbial breakdown of pine bark ties up nutrients — particularly nitrogen — the higher quality barks’ components are those that are more decomposed. However, it is the opposite for bark’s affect on PGRs. The more decomposed it is and the smaller the particle size, the greater the surface area for absorbing the PGR. Therefore, the better quality barks often have the greatest effect on drench activity.

This absorption is not a cation-exchange-cite (CEC) type binding of positive nutrient ions, such as calcium and potassium, so inorganic components with CEC, such as vermiculite, do not reduce the activity of PGRs. Inorganic components such as perlite, sand and poly beads also do not absorb the PGR. The use of coir is growing, and it acts like peat, which does bind some of the PGR but not nearly as much as pine bark. Other barks, wood chips and other composted wood products do not seem to cause a problem.

PGR rate recommendations generally are based on peat mixes. Due to the variations in barks and percentages in different mixes, it is impossible to give precise rates for drenches to bark mixes. Growers with a bark mix need to conduct trials to determine optimum PGR rates for their individual situations using a starting point about twice the recommended rate for peat mixes. Strategies to improve height control in bark mixes are: 1) make the drench application earlier in the crop, 2) plan to use multiple drench applications, and/or 3) apply PGRs to liners prior to transplanting into the bark mix.

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