Subirrigating Seed Geraniums with Bonzi By Douglas Cox

Subirrigation of PGRs on seed crops gives growers a new way to control growth.

A number of new and innovative methods of applying plantgrowth regulators (PGRs) have gained EPA approval in recent years. PGRs may beapplied by “sprench” (combination of spray and drench applied as ahigh volume, low concentration spray to the media surface), media spray beforeplanting (a higher concentration, lower volume spray applied to the media),controlled residue (applied to the inside of the container before planting),watering in as plants are irrigated or subirrigation. However, practicalrecommendations are needed for growers on how to apply PGRs by these newmethods.

The Bonzi (paclobutrazol) label has been expanded to includea number of “chemigation” methods including injection throughoverhead sprinklers, fog systems, spaghetti tube systems and watering in withdosing equipment or application by ebb and flow subirrigation systems orsaucers. Application of PGRs by subirrigation has not been widely researcheddespite the existence of permissive labeling.

Why Subirrigate

As greenhouse labor becomes increasingly scarce andexpensive, growers are looking for methods to more quickly and efficientlyaccomplish time-consuming and repetitive tasks like watering, fertilizing andapplying PGRs. Growers of all sizes are installing subirrigation and otherautomated irrigation systems in their greenhouses, which can potentially do allthree 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. Theadvantages of being able to treat large numbers of similar plants at one timewith PGRs while also watering and fertilizing are obvious.

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

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

This article reports the results of a study, funded by agrant from the New England Greenhouse Conference, that compared the response of4-inch seed geraniums (a plant sensitive to Bonzi) to several levels of Bonziapplied by subirrigation in a single application or in repeat applications atlow levels of active ingredient and calculated the effect of stock tank dilutionon the growth-inhibiting effect of Bonzi.

How the Plants Were Grown

Seeds of ‘Ringo 2000 Red’ geraniums (S&G Flowers) weresown in plug trays. Seedlings were transplanted into 4-inch pots of Fafard 3Bcontaining superphosphate fertilizer January 23 and February 21. Plants weregrown using standard commercial practices and watered and fertilized with a20-0-20 fertilizer at 200 ppm nitrogen by subirrigation from potting to finish.Pots were subirrigated from 5-inch saucers.

Plant response to Bonzi applied by subirrigation was studiedby making one or multiple treatments. Bonzi treatments began March 25 whenplants were about 2-3 inches in diameter. Bonzi solutions were applied at 3.4fl.oz. per pot (100 ml) in all treatments; in every instance, this amount ofsolution was completely absorbed by the growth medium when the saucers werefilled. Control plants were subirrigated but received no Bonzi in thefertilizer solution.

Some plants were subirrigated once with either 0.11, 0.22 or0.33 ppm Bonzi. Other plants were subirrigated 11 times over four weeks withfertilizer solutions that contained Bonzi concentrations with 10 percent lessactive ingredient than that applied to the plants getting one treatment. Bothof these treatments were called “full strength” referring to the factthat the solution in the stock tank was not adjusted (diluted) to the originalvolume to replace what was used to treat theplants, and thus, the levels of PGRs remained constant, atfull strength, over the four-week treatment period.

Another set of plants was treated 11 times with Bonzi, butthe PGR levels were gradually diluted from full strength over the four-weektreatment period by the addition of fresh fertilizer solution to the stocktanks to maintain volume. This approach simulated the normal operation of agreenhouse subirrigation system where water or fertilizer solution is added tothe stock tank to replace what is absorbed by the plants. Four dilution treatments were tested, resultingin a final dilution at the 11th (last) application of 80, 60, 40 or 20 percentof full strength.

The study was concluded May 15, one month after the 11thapplication of Bonzi. Time to flowering was recorded at the opening of thefirst floret. Plant height, plant diameter, pedicel (flower stalk) length, leafsize (area of the leaf at the first flowering node) and shoot fresh weight wererecorded on May 15.


This study included 19 different treatments when allpossible treatment combinations are taken into account; for the sake ofsimplicity, 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 to flowering wasnot affected by Bonzi treatments. On average, the first floret of the controlplants and the Bonzi-treated plants opened within 93-95 days after sowing.

Plant growth, however, was significantly affected bysubirrigation with Bonzi. Bonzi-treated plants were shorter and smaller indiameter and had shorter pedicels than the control plants (See Figure 1, page32). Bonzi-treated plants also had smaller leaves, and the shoots weighed lessthan 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 subirrigatedonce or 11 times with Bonzi, but plants subirrigated 11 times tended to beslightly taller and had slightly larger leaves and greater shoot fresh weightthan plants subirrigated once. Overall, the size of Ringo 2000 Red geraniumswas effectively suppressed by all levels of Bonzi, whether applied once or 11times over four weeks. The amount of growth suppression, however, was desirableand not excessive.

Stock tank dilution effects. Stock tank dilution treatments simulated the effects of thecommercial greenhouse practice of maintaining a constant volume of solution inthe stock tank and measured its affect on the degree of growth control byBonzi. It’s notable that, regardless of the degree of dilution, Bonzitreatments were effective in suppressing the growth of Ringo 2000 Red geraniumscompared to the control (See Figure 2, above). Plants subirrigated with fullstrength Bonzi solutions (i.e., the stock tank solutions were not diluted byadding fresh fertilizer solution) and Bonzi solutions gradually diluted to 60percent of full strength had the greatest suppressive effects on plant growth.Plants subirrigated with Bonzi solutions gradually diluted to 40 and 20 percentof full strength were taller and larger in diameter and had bigger leaves andgreater shoot fresh weight than plants receiving the full strength solution.These results demonstrate the potential for less growth control with Bonziapplied by subirrigation when the stock tank solutions are significantlydiluted by water or fertilizer solution.


The results of this study show that the growth of seedgeraniums can be successfully controlled by subirrigating from saucers with 3.4fl.oz. per pot of Bonzi solutions at 0.11, 0.22 or 0.33 ppm once when theplants were 2-3 inches in diameter. Subirrigating with lower levels (10 percentof the active ingredient applied to the plants getting one subirrigationtreatment) of Bonzi 11 times over a four-week period was as effective as onesubirrigation treatment for controlling growth.

Stock tank dilution with fresh fertilizer solution toreplace the volume of solution used to treat the plants affected the degree ofgrowth suppression from Bonzi solutions. Over the four-week trial, plants grewlarger when they were subirrigated with very dilute Bonzi solutions (40 or 20percent of full strength) compared to plants that received less dilutesolutions (80 or 60 percent of full strength) or a solution not diluted (fullstrength).

In this study, I created the different levels of dilution byusing stock tanks with different volumes. Stock tanks for the 80- and60-percent treatments had the largest starting volumes, and thus, the solution addedto maintain volume would have less diluting effect on the concentration ofBonzi compared to the 40- and 20-percent treatments, which had much lowerstarting volumes and needed more solution to maintain volume.

Based on the results of this study, commercial growerssubirrigating with Bonzi should not allow their stock tank solution to becomediluted to a level more than about 60 percent of full strength. If the stocktank becomes more dilute, the effectiveness of Bonzi may be reduced.Significant dilution could result in little or no growth control, especiallyfor species less responsive to Bonzi than geranium and at minimal Bonziconcentrations.

Douglas Cox

Douglas Cox is associate professor and extension specialist in the Plant and Soil Sciences Department at the University of Massachusetts, Amherst, Mass. He can be reached by phone at (413) 545-5214 or by E-mail at

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