Maximizing Chemical Growth Retardants

March 18, 2003 - 12:47

Getting the most out of your growth regulators requires familiarity with the chemical, appropriate application methods and in-house experimentation.

This is the season when every grower starts worrying about
how to control plant growth, whether plugs, bedding flats, pots or baskets. All
growers should be doing whatever they can to control temperature, provide more
light, reduce humidity, practice good moisture management and use fertilizers
lower in NH4 and phosphorus to help control shoot growth and improve root
growth. However, even these methods have their limitations. So growers need to
use chemical growth retardants to produce quality plants that can be held in
the greenhouse if needed, shipped properly, look appealing in the stores and
grow out well for the end consumer.

Chemical Growth Retardants

The types of chemical growth retardants we use are
anti-gibberellic acid (GA) compounds, meaning they work against production of
GA in the plant, which causes stretch. The only growth retardant not classified
as an anti-GA compound is Florel, which is an ethylene-generating compound and
works differently in the plant. I extensively covered Florel in the October
2002 issue of GPN and will not review it here, but if you have questions about
its use, please refer back to this issue. In the United States, familiar growth
retardants include B-Nine, Cycocel, A-Rest, Bonzi and Sumagic. These compounds
work on different parts in the production of GA and can be utilized in tank
mixes to get synergistic effects.

B-Nine is the most common and easiest to use of all growth
retardants. It works on a wide range of plants, is easy to spray (spray to
run-off) and has no long-lasting effects. B-Nine works better in cooler
weather, as plants tend to quickly outgrow its control in warmer weather.
B-Nine gets into the plant through leaves and stems but has no activity in the
media through the roots. However, this chemical has a few quirks you need to
know about. First, B-Nine takes a long time to get into the plant, and leaves
should stay wet with the chemical for 3-4 hours to get most of it in. Second,
B-Nine should not be applied to plants seven days before or after spraying
copper fungicides, as a phytotoxicity problem may show up. Third, multiple
applications of B-Nine at high rates may delay flowering and make the first
flower size smaller than desired. B-Nine can be tank-mixed with other growth
retardants, thereby reducing the flowering delay (See Figure 1, below).

Cycocel also has a broad label for use on many plants. This
chemical is most effective as a spray, getting in through the stems and leaves.
Cycocel does have some activity in the media through the roots, but rates for
this purpose are high and not cost-effective. Cycocel should be sprayed to
glistening, as too much chemical could cause more leaf yellowing, known as
haloing. However, plants grow out of this problem and cover up the damaged
leaves. Just like B-Nine, Cycocel should stay wet on the plant 3-4 hours.
Cycocel works best when tank-mixed with B-Nine (See Figure 1, below).

A-Rest is a stronger chemical than B-Nine or Cycocel and is
labeled for a wide range of crops. Used most commonly in plug production,
A-Rest also works well on finished crops such as pansies, vinca, salvia,
dianthus, lilies and snaps. This chemical gets in through all parts of the
plant, making it very useful as a drench. A-Rest gets in very quickly, within
30 minutes, so plants do not have to stay wet for a long time as with B-Nine
and Cycocel. The main complaint with A-Rest is the cost, but it Á is
safer to use than Bonzi or Sumagic, which makes A-Rest a good choice for
growers who want more control than B-Nine or Cycocel but are afraid of using
Bonzi or Sumagic. A-Rest can also be tank-mixed with B-Nine, thereby reducing
the cost per application (See Figure 1, below).

Bonzi and Sumagic work the same way in plants. Both
chemicals work on a wide range of plants, get in quickly like A-Rest but have
longer-lasting effects on plant growth. They get in through stems and roots
only, not leaves. This means growers have to watch their spray applications
closely, making sure they get thorough coverage of stems, but also making sure
not to get too much into the soil. Bonzi and Sumagic are very effective in the
media through the roots and can have the most control this way, making them
good choices for drench applications. Growers should be aware of cool
temperatures around the time of application, as more growth control than
desired could occur. These two chemicals are the hardest for growers to apply,
making it necessary to standardize the methods between applicators and to conduct
your own trials. Both Bonzi and Sumagic can be tank-mixed with B-Nine to avoid
overdosing plants (See Figure 1, page 8).

Application Methods

There are three main ways of applying chemical growth
retardants: sprays, sprenches (or media sprays) and drenches. Sprays are the
most commonly used method. All of the above chemicals can be sprayed on a wide
range of crops. However, Bonzi and Sumagic take more practice by growers, as
stem coverage is essential. Because these two chemicals are so active in the
plant, it is very easy to get non-uniform coverage, overlap or drift onto crops
you did not want to spray, such as begonias. B-Nine and Cycocel should be
applied at day's end, to allow plants to stay wet long enough to get most of
the chemical inside. Incorrect rates or application with Cycocel can result in
considerable leaf haloing, making a crop unsaleable until those damaged leaves
are covered.

Growers familiar with using Florel understand the importance
of water pH and chemical activity. However, water pH is also important for the
other chemical growth retardants. Make sure your water pH in the spray tank is
5.5-6.5 for best effectiveness. Rinse spray tanks thoroughly after using,
particularly when using Bonzi or Sumagic. Some growers keep separate spray
tanks for Bonzi and Sumagic.

Residue problems occur with A-Rest, Bonzi and Sumagic. This
is due mainly to high spray rates getting onto the plastic trays, benches and
concrete floors. The residue can stay there a long time and is not easily
washed off. Growing a sensitive crop such as begonias right after growing
petunias that have been sprayed with Bonzi in the same area can result in some
stunting. To effectively remove residues, soak containers or surface areas for
one hour with Greenshield solution and rinse off. Many growers will not reuse
flats or pots that may have residue problems for crops that are sensitive to
those chemicals. A similar situation occurs with growers using their own
compost. Bonzi and Sumagic stay in plants a long time and will still be active
in compost.

A relatively new technique for growth retardant application
is a "sprench." This involves spraying the chemical at volumes higher
than those used for plant sprays and spraying the soil surface when plants are
very small. Normally, growth retardants are sprayed at 2 quarts per 100 sq.ft.
of bench space. With sprenches, chemicals such as A-Rest, Bonzi and Sumagic are
sprayed at 4 quarts per 100 sq.ft. or slightly higher. Rates used are lower
than spray rates, as the goal is to get the chemical into the soil to be active
through early rooting, either in plugs or finished containers. In plug
production, sprenches keep crops from stretching very quickly during
germination and can reduce the number of subsequent sprays needed for height
control. Sprench Á rates for A-Rest and Bonzi used on plugs, which
should be applied before germination is completed, range from 1⁄2-5 ppm,
and half as much for Sumagic. For sprenching finished flats right after
transplanting, double the above rates. A "media spray" can be done
either right before or after transplanting, using normal spray rates and
volumes. Remember, a sprench uses a greater volume, so reduce the ppm

More and more growers are learning how to drench finished bedding
plants. A drench uses a higher volume but lower ppm compared to a sprench.
Typically, a drench is applied at 8 quarts per 100 sq.ft., with rates half as
much as a sprench. Drenches are used with A-Rest, Bonzi or Sumagic, all of
which are active in the media through the roots. Drenches can be applied to
plugs to hold them for transplanting, to vigorous Á

vegetative liners before potting into combos, to finished
bedding plants when up to saleable size and starting to flower, and to pot
crops to slow down growth and hold them for sale. Growers can apply drenches by
hand through a portable injector, through a drip system or through
ebb-and-flood systems. It is important to calibrate how much active ingredient
you are putting into the container and standardize the method between growers.

There are some important facts to remember when applying
drenches: 1) Make sure containers have been watered the day previously, as dry
soil ties up the chemical; 2) work out your volume and ppm used through trials
and duplicate the method each time; 3) apply drenches when plants are up to
saleable size and starting to flower; 4) make sure plants grow out in 2-3
weeks; and 5) pine bark in the media will tie up some of the chemical. Plants
can be sprayed effectively before flowering, but Bonzi and Sumagic can delay
flowering if buds are showing when sprayed. Drenches do not delay flowering.

The goal of a drench is to hold the crop for 2-3 weeks, then
let plants grow out of the chemical. You can reapply to hold them longer. This
technique is very effective when spring sales are late. Increase ppm used by
50-100 percent when pine bark is in the media, depending on the amount of bark
in the mix and how composted it is.

When applying a drench to bedding flats and 4-inch pots not
spaced, use a portable injector and drench the plants like you were feeding
them. You will use less chemical if the crop was watered previously and media
is moist before drenching. Reduce the ppm used, as you are putting on more
volume than a standard drench where you know how much volume was applied and
what ppm was used. When drenching larger pots or hanging baskets, you should
add a known amount of solution to each and every pot. When drenching through a
drip system, calibrate how much solution comes out at the beginning and end of
the lines. The longer the drip line, the more variation you will see. When
drenching with ebb-and-flood systems, cut the rates to 1⁄4 to 1⁄2
of normal top-down drench rates. This is due to more chemical getting to the
roots from the bottom up. If using water saucers, consider rates to be closer
to flood floors.

Vigorous vegetative liners can be drenched (or dipped) with
A-Rest, Bonzi or Sumagic before they are planted into combos. This technique
will hold back growth for 2-3 weeks, allowing the slower plants in the combo to
start growing. Once the vigorous plants come out of the growth regulator, they
will fill in the combo without overgrowing the slower crops. Vigorous liners
include vegetative petunia, sweet potato vine, verbena and some calibrachoas
and helichrysum. Use 2-5 ppm of Bonzi or Sumagic, higher for A-Rest.

Starting Points

To get started with spraying bedding plants, consider the
following  rates and adjust
according to reaction, variety, weather conditions, growing techniques and
spray methods:

If spraying plug crops, reduce starting rates by one half.
Growers in the North should start with low end of rates. Figure 2, page 10,
gives you crop groupings for spraying Bonzi and Sumagic. It is important to
understand that you cannot use the same ppm for all crops when spraying Bonzi
or Sumagic. How you spray a crop (volume used) will also determine the rate

To get started with drenching finished bedding plants,
consider the starting points for rates on different crops in Figure 3, pages
11-13. These rates are more for mid-South growers in peat-based media, so
Northern growers will need to cut the rates by half, while far-South growers
may need to increase the rates. Remember, you will naturally need to increase
your drench rates as the temperature gets warmer and plants grow faster. Or
increase rates if using pine bark in your media.

When learning how to drench crops, you must know not only
your technique and the type of growth retardant used but also the vigor of the
varieties, growing environment, fertilizer used and how much time before
shipping. It is imperative you do some trials to work out your technique and to
see how long it takes the plants to grow out. Remember, you want the plants to
grow out of the drench in 2-3 weeks. 

About The Author

Dr. Roger Styer is president of Styer's Horticultural Consulting, Inc., Batavia, Ill. He can be reached by phone at (630) 208-0542 or E-mail at

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