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Subirrigating Seed Geraniums with Bonzi

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A number of new and innovative methods of applying plant growth regulators (PGR's) have gained official label approval in recent years. PGRs may be applied by "sprench" (combination of spray and drench applied as a high volume spray), media spray before planting, controlled residue (PGR is applied to the inside of the container before planting), watering in as plants are irrigated, or subirrigation. However, practical recommendations are needed for growers on how to apply PGRs by these new methods.

The Bonzi (paclobutrazol) label has been expaned to include a number of "chemigation" methods including injection through overhead sprinklers, fog systems, spaghetti tube systems and watering in with dosing equipment or application by ebb and flow subirrigation systems or saucers. Application of PGRs by subirrigation, the subject of this article, has not been widely researched despite the existence of permissive labeling.

As greenhouse labor becomes increasingly scarce and expensive, growers are looking for means of accomplishing time-consuming and repetitive tasks like watering, fertilizing, and PGR application more quickly and efficiently. Growers of all sizes are installing subirrigation and other automated irrigation systems in their greenhouses which can potentially do all three tasks at once. While Bonzi and other PGRs may be applied by chemigation it is an option that has not been extensively evaluated for practical use. The advantages of being able to treat large numbers of similar plants at one time with PGR while watering and fertilizing are obvious.

Much less active ingredient is required when subirrigation is used to apply PGRs. According to tests at the University of Florida, subirrigation appears to be a more efficient way to apply PGRs than surface applications because chemicals applied to the surface slowly leach down to the root zone while PGRs applied by subirrigation get to the root zone faster.

Application of PGRs by subirrigation seems to be a method that promotes greater uniformity of plant response and, since less active ingredient is required, results in a savings in chemical costs can be realized and reduces the risk of over application. However, an important factor which has not been studied is how the effectiveness of PGRs applied by subirrigation changes as the stock tank solution becomes more dilute when water or fertilizer solution is added to replace what is absorbed by the plants.

The article reports the results of a study, funded by a grant from the New England Greenhouse Conference, which compared the response of 4-inch seed geranium (a plant sensitive to Bonzi) to several levels of Bonzi applied by subirrigation in a single application or in repeat applications at low levels of active ingredient and the effect of stock tank dilution on the the growth-inhibiting effect of Bonzi.

How the plants were grown

Seeds of 'Ringo 2000' red geranium were sown in plug trays January 23, and February 21 the seedlings were transplanted to 4-inch pots of Fafard 3B containing superphosphate fertilizer. Plants were grown using standard commercial practices and were watered and fertilized with 20-0-20 fertilizer at 200 ppm N by subirrigation from potting to finish. Pots were subirrigated from 5-inch saucers.

Plant response to Bonzi applied by subirrigation was studied by making one treatment or multiple treatments. Bonzi treatments began March 25 when the plants were about 2-3 inches in diameter. Bonzi solutions were applied at 3.4 fl.oz./pot (100 ml) in all treatments; in every instance this amount of solution was completely absorbed by the growth medium when the saucers were filled.

Control plants were subirrigated but received no Bonzi in the fertilizer solution. Some plants were subirrigation once with either 0.11, 0.22, or 0.33 ppm Bonzi.

Other plants were subirrigated 11 times over 4 weeks with fertilizer solutions containing Bonzi at concentration 10% of the active ingredient (a.i.) applied to the plants getting one subirrigation treatment. These treatments were called "full strength" referring to the fact that the solution in the stock tank was not adjusted (diluted) to volume to replace what was used to treat the plants and thus the levels of PGR remained constant at "full strength" over the 4 week treatment period.

Another set of plants was treated 11 times with Bonzi, but the PGR levels were gradually diluted from "full strength" over the 4 week treatment period by the addition of fresh fertilizer solution to the stock tanks to maintain volume. This approoach simulated the normal operation of a greenhouse subirrigation system where water or fertilizer solution is added to the stock tank to replace what is absorbed by the plants. Four dilution treatments were tested resulting in a final dilution at the 11th (last) application of 80%, 60%, 40%, or 20% of "full strength".

The study was concluded May 15 one month after the 11th application of Bonzi. Time to flowering (number of days from sowing) was recorded at the opening of the first floret. Plant height, plant diameter, pedicel (flower stalk) lenght, leaf size (area of the leaf at the first flowering node) and shoot fresh weight were recorded on May 15.


This study included 19 different treatments when all possible treatment combinations are taken into account; for the sake of simplicity I have chosen to report the major effects of Bonzi level, application method, and stock tank dilution in this article.

Bonzi level and method of application. Time of flowering was not affected by Bonzi treatments. On average the first floret of the control plants and the Bonzi-treated plants opened within a preiod 93-95 days after sowing.

Plant growth, however, was significantly affected by subirrigation with Bonzi. Bonzi-treated plants were shorter, smaller in diameter, and had shorted pedicels than the control plants (Table 1). Bonzi-treated plants also had smaller leaves and the shoots weighed less than plants subirrigated without Bonzi.

Plants were smaller as the concentration of Bonzi increased regardless of whether Bonzi was applied once or in 11 subirrigation treatments. There were no large differences in plant growth between plants subirrigated once or 11 times with Bonzi, but plants subirrigated 11 times tended to be slightly taller and had slightly largere leaves and greater shoot fresh weight than plants subirrigated once or 11 times over 4 weeks. The amount of growth suppression, however, was desirable and not excessive.

Stock tank dilution effects. Stock tank dilution treatments simulated the effects of the commercial greenhouse practice of maintaining a constant volume of solution in the stock tank on the degree of growth control by Bonzi. Its notable that, regardless of degree of dilution, Bonzi treatments were effective in suppressing the growth of 'Ringo 2000 Red' geranium compared to the control (Table 2). Plants subirrigated with "full strength" bonzi solutions (i.e., the stock tank solutions were not diluted by adding fresh fertilizer solution) and Bonzi solutions gradually diluted to 80 and 60% of full strength over the four week treatment period had the greatest suppressive effects on plant growth. Plants subirrigated with Bonzi solutions gradually diluted to 40 and 20% of full strength over four weeks were taller, larger in diameter, had bigger leaves, and greater shoot fresh weight than plants receiving the "full strenght" solution. These results demonstrate the potential for less growth control with Bonzi applied by subirrigation when the stock tank solutions are significantly diluted by water or fertilizer solution.


The results of my study show that the growth of seed geraniums can be successfully controlled by subirrigating from saucers with 3.4 fl. oz./pot Bonzi solutions of 0.11, 0.22, or 0.33 ppm once when the plants were 2-3 inches in diameter. Subirrigating with lower levels (10% of the active ingredient (a.i.) applied to the plants getting one subirrigation treatment) of Bonzi 11 times over a 4 weeks period was as effective as one subirrigation treatment for controlling growth.

Stock tank dilution with fresh fertilizer solution to replace the volume of solution used to treat the plants affected the degree of grwoth suppression from Bonzi solutions for 4 weeks. Plants grew larger when they were subirrigated with very dilute Bonzi solutions (40 or 20% of full strength) compared to plants receiving less dilute solutions (80 or 60% of full strength) or a solution not diluted (full strength).

In this study I created the different levels of dilutions by using stock tanks with different volumes. Stock tanks for the 80% and 60% treatments had the largest volumes and, thus, solution added to maintain volume would have less diluting effect on the concentration of Bonzi in the tank compared to the smaller volume tanks for the 40% and 20% treatments.

Based on the results of this study, commercial growers subirrigating with Bonzi should not allow their stock tank solution from becoming diluted to a level more than about 60% of full strength. If the stock tank becomes more dilute the effectiveness of Bonzi may be reduced. Significant dilution could result in little or no growth control especially for species less responsive to Bonzi than geranium and at minimal Bonzi concentrations.

Table 1. The growth of 'Ringo 2000 Red' geranium as affected by Bonzi applied by subirrigation.
Treatment Plant height (cm) Plant diameter (cm) Pedicel length (cm) Leaf area (cm2) Shoot fresh weight (gm)
Control, no Bonzi 18.7 24.7 21.3 74.3 132
Plants subirrigated with Bonzi once
0.11 13.9 21.4 18.1 40.2 115
0.22 11.3 20.9 16.8 35.9 106
0.33 10.4 19.9 16.5 32/8 104
Plants subirrigated with Bonzi 11 times*
0.11 13.7 21.1 16.7 42.6 116
0.22 12.6 20.5 16.3 41.5 112
0.33 11.5 19.0 15.6 40.0 107
Overall effect of application method
One application 11.9 20.7 17.1 36.3 108
Eleven applications 12.6 20.2 16.2 41.3 112

*Average measurements include all stock solution dilution treatments. (1 inch=2.54 cm).

Table 2. Stock tank dilution effects on the growth of 'Ringo 2000 Red' geranium subirrigated with Bonzi.
Treatment Plant height (cm) Plant diameter (cm) Pedicel length (cm) Leaf area (cm2) Shoot fresh weight (gm)
Control, no Bonzi 18.7 24.7 21.3 74.3 132
Full strength 11.6 19.7 15.7 37.9 107
80% full strength 12.3 19.8 16.1 39.4 109
60% 12.0 19.7 16.0 42.2 109
40% 13.3 20.9 16.8 42.9 115
20% 13.7 21.0 16.2 44.4 120

(1 inch = 2.54 cm)

Prepared by:
Dr. Douglas A. Cox
Plant and Soil Sciences
University of Massachusetts


Commercial Horticulture
Commercial Horticulture topics: 
Greenhouse Management