Subirrigating Seed Geraniums with Bonzi

August 8, 2003 - 07:46

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

A number of new and innovative methods of applying plant
growth regulators (PGRs) have gained EPA approval in recent years. PGRs may be
applied by "sprench" (combination of spray and drench applied as a
high volume, low concentration spray to the media surface), media spray before
planting (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, practical
recommendations are needed for growers on how to apply PGRs by these new

The Bonzi (paclobutrazol) label has been expanded 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 has not been widely researched
despite the existence of permissive labeling.

Why Subirrigate

As greenhouse labor becomes increasingly scarce and
expensive, growers are looking for methods to more quickly and efficiently
accomplish time-consuming and repetitive tasks like watering, fertilizing and
applying PGRs. 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 PGRs while also 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
Á 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 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.

This article reports the results of a study, funded by a
grant from the New England Greenhouse Conference, that compared the response of
4-inch seed geraniums (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 calculated the effect of stock tank dilution
on the growth-inhibiting effect of Bonzi.

How the Plants Were Grown

Seeds of 'Ringo 2000 Red' geraniums (S&G Flowers) were
sown in plug trays. Seedlings were transplanted into 4-inch pots of Fafard 3B
containing superphosphate fertilizer January 23 and February 21. Plants were
grown using standard commercial practices and watered and fertilized with a
20-0-20 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 studied
by making one or multiple treatments. Bonzi treatments began March 25 when
plants were about 2-3 inches in diameter. Bonzi solutions were applied at 3.4
fl.oz. per 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 subirrigated once with either 0.11, 0.22 or
0.33 ppm Bonzi. Other plants were subirrigated 11 times over four weeks with
fertilizer solutions that contained Bonzi concentrations with 10 percent less
active ingredient than that applied to the plants getting one treatment. Both
of these treatments were called "full strength" referring to the fact
that the solution in the stock tank was not adjusted (diluted) to the original
volume to replace what was used to treat the plants, and thus, the levels of PGRs remained constant, at
full strength, over the four-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 four-week
treatment period by the addition of fresh fertilizer solution to the stock
tanks to maintain volume. This approach 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 percent
of full strength.

The study was concluded May 15, one month after the 11th
application of Bonzi. Time to flowering was recorded at the opening of the
first floret. Plant height, plant diameter, pedicel (flower stalk) length, 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 to flowering was
not affected by Bonzi treatments. On average, the first floret of the control
plants and the Bonzi-treated plants opened within 93-95 days after sowing.

Plant growth, however, was significantly affected by
subirrigation with Bonzi. Bonzi-treated plants were shorter and smaller in
diameter and had shorter pedicels than the control plants (See Figure 1, page
32). 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 larger leaves and greater shoot fresh weight
than plants subirrigated once. Overall, the size of Ringo 2000 Red geraniums
was effectively suppressed by all levels of Bonzi, whether applied once or 11
times over four 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 and measured its affect on the degree of growth control by
Bonzi. It's notable that, regardless of the degree of dilution, Bonzi
treatments were effective in suppressing the growth of Ringo 2000 Red geraniums
compared to the control (See Figure 2, above). 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 60
percent of full strength had the greatest suppressive effects on plant growth.
Plants subirrigated with Bonzi solutions gradually diluted to 40 and 20 percent
of full strength were taller and larger in diameter and had bigger leaves and
greater shoot fresh weight than plants receiving the full strength 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 this study show that the growth of seed
geraniums can be successfully controlled by subirrigating from saucers with 3.4
fl.oz. per pot of Bonzi solutions at 0.11, 0.22 or 0.33 ppm once when the
plants were 2-3 inches in diameter. Subirrigating with lower levels (10 percent
of the active ingredient applied to the plants getting one subirrigation
treatment) of Bonzi 11 times over a four-week 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
growth suppression from Bonzi solutions. Over the four-week trial, plants grew
larger when they were subirrigated with very dilute Bonzi solutions (40 or 20
percent of full strength) compared to plants that received less dilute
solutions (80 or 60 percent of full strength) or a solution not diluted (full

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

Based on the results of this study, commercial growers
subirrigating with Bonzi should not allow their stock tank solution to become
diluted to a level more than about 60 percent 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

About The Author

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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