Producing Vegetative Petunias and Calibrachoa

March 1, 2002 - 12:21

Vigor is victory for the home garden, but tangled plants are tiresome in the greenhouse. New NC State research shows that achieving highly branched, compact vegetatively propagated petunias and calibrachoas is possible with a few tweaks to your production

Two stars of the booming vegetatively propagated bedding
plant market are petunias and calibrachoa. These closely related species offer
consumers rapid growth, a wide range of colors and durability. The vigor of
petunias and calibrachoa, while great for consumers, presents a challenge to
growers — too much of a good thing can make for tangled plants and shipping problems. In the following article we will discuss how to cost-effectively grow petunias and calibrachoas with an emphasis on the areas we have been researching: propagation and growth (vine) control.

Cultivars

Most vegetatively propagated petunias are hybrids between
the bedding petunia (Petunia x hybrida) and Petunia axillaris. Of course, the
original seed-propagated bedding petunia is a hybrid between P. axillaris and
P. intergrifolia. However, the reintroduction of P. axillaris genetics brought
a greater emphasis on vigorous, trailing growth and postproduction durability.
The first of the new cultivars were released in the early 1990s with the introductions of ‘Supertunias’. Supertunias are vegetatively propagated because this group produces few if any seed. The seed-propagated ‘Wave’ series was also introduced and has similar characteristics, but a more restricted color range. Since then several series of great vegetatively
propagated petunias have been released, expanding the range of growth habits,
colors and degrees of doubleness offered.

The calibrachoa (Calibrachoa hybrids) is considered a new
species in the floriculture trade, but actually has been in cultivation since
the early 19th Century. A renewed focus on the Calibrachoa genus has resulted
in the introduction of several new cultivar series in the last few years.
Calibrachoa have a more limited color range than petunias but finer-textured
foliage, a more pronounced trailing habit and abundant, small, single flowers.

Propagation

Our research goal was to develop high-quality, highly
branched, compact vegetatively propagated petunia and calibrachoa plugs. A
number of factors were examined that influence flowering and branching,
including photoperiod, plug flat density, Florel application, light intensity,
light quality and fertilizer rate. Petunia cultivars examined included
‘Cascadia Charme’, ‘Cascadia Improved Charlie’,
‘Petitunia Bright Dream’, ‘Doubloon Blue Star’,
‘Marco Polo Odyssey’ and ‘Pocket Petunia’; calibrachoas
tested included ‘Liricashower Rose’ and ‘Colorburst
Violet’.

HID lighting improved rooting during low-light times of the
year, especially for slow-rooting cultivars such as Cascadia Charme, and will
reduce time to transplanting by 7-10 days. Without HID lighting, cultivars that
normally root rapidly will be ready to transplant in 10-14 days; slow-rooting
cultivars will be ready to transplant in 17-24 days. Plugs grown under HID
lighting and short days were the shortest. Light quality from either metal
halide or high-pressure sodium lighting apparently had little effect,
indicating that both types of lights are acceptable. Ambient lighting and long
days increased plug height and final plant height for some cultivars. Long-day
photoperiods increased plug height and final plant height for some cultivars
and decreased final plant diameter for all cultivars. High-density (288) flats
increased the percentage of cuttings lost to bacterial diseases and delayed
flowering by an average of 2-4 days for all cultivars compared to low-density
(105) flats.

Cuttings should be rooted at 70° F minimum temperature
with bottom heat. Use propagation tents and avoid overmisting, which slows
rooting and increases the likelihood of Botrytis and bacterial diseases. Some
propagators recommend using rooting hormones with 2,500 IBA, especially on
slow-to-root cultivars such as Cascadia Charme.

Flowering Control

Typical seed-propagated petunias are facultative long-day
plants. Flowering will occur under LD or SD; however, plants will flower
earlier with longer photoperiods and higher light intensities. Most vegetative
petunias are also facultative long-day plants; however, a few cultivars act as
short-day or day-neutral plants. Apparently, high light intensity can overcome
photoperiodism, as some cultivars are long-day plants under ambient light, but
are day-neutral under HID lighting. All calibrachoa cultivars tested to date
have been facultative long-day plants.

What does this mean for the grower? Long-day lighting should
be given to both plugs during propagation and finished plants during the
short-day time of the year, from October to March, to decrease time to
flowering. Long-day lighting applied only during propagation may decrease crop
time for photoperiod-responsive cultivars, such as Petitunia Bright Dream, by
up to two weeks. Generally, however, long-day lighting during propagation will
decrease crop time by a week or less.

Production

Temperature. Grow at
60-65° F night temperature until rooted cuttings or plugs are established
in the final container. Continue growing petunias at 60-65° F nights and
60-75° F days for most rapid growth; lower temperatures of 55-60° F can
be used, but will slow growth. Calibrachoa can be grown at a wider temperature
range from 50° F night to 85° F day.

Light. Grow plants
under a minimum of 5,000-6,000 footcandles. HID lighting of 500 footcandles or
more improves rooting during low-light periods, especially for slow-to-root
cultivars. HID lighting can also increase the number of branches, but only for
plants grown under long days.

Irrigation. While
petunias and calibrachoas are drought-tolerant in the landscape, keep plants
moist during production for optimum growth.

Nutrition. Both
petunia and calibrachoa are considered to be relatively heavy feeders with
recommended rates from suppliers of 250-350 ppm nitrogen for petunia and
200-300 ppm nitrogen for calibrachoa. However, we found that 150 ppm nitrogen
was sufficient for good growth; higher fertilizer rates either had no effect or
delayed flowering. Not surprisingly, however, foliage color was darker green
for plants irrigated with 300 ppm nitrogen (bottom right). If lower fertilizer
rates near 150 ppm nitrogen are used, plants can be greened up by using higher
rates for a couple of weeks just prior to shipping. Phosphorus rates should be
1/4-1/3 of nitrogen and potassium rates should be 2/3 of nitrogen. Calibrachoa
and petunias have a high requirement for iron; be sure to incorporate
additional iron into the nutritional program. Iron chelates can be used as
sprays or drenches.

Growing Media.
Petunia and calibrachoa are adaptable plants that thrive in a variety of
commercial growing media. pH should be monitored and maintained at 5.5-6.0.
Calibrachoa prefer low pH and can readily tolerate pHs lower than 5.5.

Growth Control

Both petunia and calibrachoa grow rapidly and can become
leggy. Environmental manipulation is the first control strategy to consider.
Grow plants under high light, cool temperatures and zero to negative DIF to
reduce stretch and promote rapid flowering. Pinching and pruning can also be
used as needed to control growth (see Pinching section for details).

Many vegetative petunia cultivars are particularly vigorous
and plant growth regulator (PGR) applications may be required. The general
cultural information provided by the propagators list Bonzi and Sumagic as
possible PGRs to apply, although no specific rates were provided. Dr. Terri
Starman of Texas A&M University recommends Sumagic foliar sprays of 20 ppm
in the “Tips on Regulating Growth of Floriculture Crops” guide.
Researchers at Michigan State University conducted a number of PGR studies on
vegetatively propagated plants. For vegetative petunias, they used A-Rest and
Sumagic on 6-week-old plugs, while Bonzi, B-Nine and Cycocel were not used.
They recommended a single spray application of A-Rest at 100 ppm or two spray
applications of Sumagic, with the first at 20 ppm, followed by a single application two weeks later at 10 ppm. Please note that their recommended rates were based on a higher spray volume (1 gallon of spray applied over 150 sq. ft.) than the typical volume (1 gallon of spray applied over 200 sq. ft.) listed on most PGR labels.

Research at North Carolina State University also
investigated the use of a wide range of PGR rates on the growth of Cascadia
Improved Charlie vegetative petunia. The PGRs used included: A-Rest (60, 80,
100, 120 or 140 ppm), Bonzi (40, 60, 80, 100 or 120 ppm), Sumagic (5, 10, 20,
30 or 40 ppm) and a tank mix using a single rate of B-Nine (2,500 ppm) plus
increasing rates of Bonzi (40, 60, 80, 100 or 120 ppm). The PGRs were applied 2
weeks after transplanting the cuttings into 6-inch pots. Flowering was not
delayed by any of the PGR applications, with most plants flowering 71 days
after transplanting.

A-Rest at spray rates less than 80 ppm were effective in
producing plants that were 17 percent smaller in diameter than the untreated
control. Only a single application was made and the residual effects appeared
to wear off a few weeks prior to the time when data was taken. This would
suggest that a second application at a lower rate may be beneficial.

Sumagic provided a greater degree of control than A-Rest.
Rates of 5 and 10 ppm Sumagic were too low and did not provide sufficient
control. Foliar spray rates of 20 ppm resulted in plants that were both 23
percent shorter and 20 percent smaller in diameter, as compared to the control.
Plants were 34 percent shorter and 46 percent smaller in diameter when a 30 ppm
spray was used. Sumagic rates between 20-30 Á ppm are recommended,
depending on the degree of control desired.

Bonzi also worked well on vegetative petunia. Foliar spray
rates of 40-60 ppm resulted in plants that were 23-29 percent shorter,
respectively, than the control, and plant diameters that were 28-44 percent
smaller. Therefore, Bonzi rates of 40-60 ppm are recommended.

The tank mix of B-Nine at 2,500 ppm and Bonzi at 40-60 ppm
was also effective. It resulted in plants that were 32 percent shorter and 45
percent smaller in diameter when compared to the control. The tank mix using
Bonzi at 40 or 60 ppm did not provide a synergistic effect because the degree
of control was similar as when only Bonzi was used. Greater synergistic effects
occurred when B-Nine at 2,500 ppm was used with Bonzi rates less than or equal
to 80 ppm (less than 55 percent smaller-diameter plants when compared to the
control), but that degree of control might be considered excessive by many
growers. Based on the similar results of Bonzi alone at 40 or 60 ppm or in
combination with B-Nine, growers may find it easier to use only one chemical
(Bonzi).

Please keep in mind that these recommended rates are based
on North Carolina growing conditions. Growers in other locations will need to
modify the rates, with growers in more northern locations using slightly lower
rates, and more southern growers using slightly higher rates. All the foliar
sprays were applied with a spray volume of 1 gallon over 200 sq. ft.

Spacing. With these
rapidly growing crops, there is never enough space. Petunias in particular can
easily intertwine, making shipping and marketing difficult. Watch the crop and
apply growth retardants or cut back as needed to prevent tangled plants. Space
plants as far apart as possible.

Pinching. Pinching
is not required; however, pinching can be used on strongly trailing petunia
cultivars to encourage axillary branching and bushier plants. Plants can be
pruned and shaped at any time to reduce stretch and improve aesthetics;
however, flowering or reflowering will be delayed by two or more weeks. Most of
the propagators suggest pinching the plants when they are transplanted, and
then repeatedly pinching at 3- to 4-week intervals as needed.

We found Florel at 500 ppm to be effective in increasing
branching on some cultivars, but it greatly delayed flowering. A positive side
effect of Florel use was that it decreased plant diameter for some cultivars.
Unfortunately, for other cultivars the Florel-treated plants were larger at the
time of flowering than the untreated plants because the delay in flowering
allowed the plants to grow larger. If Florel is used as a substitute for
pinching, apply a 500 ppm spray when the roots reach the edge of the pot.

Overgrown, lanky plants in the garden can be sheared back to
allow for vigorous regrowth and renewed flowering later in the season.

Schedule/Timing.
Considering the rapid growth of petunias and Á calibrachoas, crop time
can be short. Four to 6-inch pots of petunias will finish in 5-7 weeks without
a pinch; add 2 or more weeks when pinching. Use one rooted cutting per 4- to
6-inch pot; shorter crop time can be obtained with two cuttings per 6-inch pot.

Plant three to four cuttings per hanging basket, which will
finish in 6-11 weeks. Late winter production will take 1-2 weeks longer than
late spring due to lower temperatures and lower light levels. Calibrachoa will
take 1-2 weeks longer than petunias.

Maintaining a healthy crop

Insects. Petunias
and calibrachoa are more insect-resistant than many bedding plants, but
whiteflies, thrips, aphids and fungus gnats can be a problem. Caterpillars can
be particularly damaging in the spring when the vents are open or when plants
are outdoors in the landscape. Overgrown plants in the retail setting or stock
plants will be difficult to treat adequately.

Diseases. Botrytis
is a common problem on open flowers and can quickly eliminate summer floral
displays after a rain or periods of high humidity. Be sure to provide adequate
air movement and keep humidity below 70 percent to reduce Botrytis. Powdery
mildew and crown rot can also be problems.

Physiological Disorders. Chlorosis can occur with iron deficiency (low pH), nitrogen
deficiency, root rot or low temperatures.

Postharvest. Petunia
flowers are very sensitive to ethylene and exposure results in rapid wilting,
but they will respond to 0.2-0.5 mM silver thosulfate (STS) sprays, which is
not labeled for use on petunias. Ethylbloc is labeled for use on potted plants,
but its efficacy on vegetatively propagated petunias and calibrachoa is not
known.

Consumer Care

Both petunias and calibrachoa perform best in sunny
locations and well-drained soil. As with most bedding species, regular watering
and fertilizer applications produce superior results; however, unlike many
bedding species, once plants are established, they can handle just about
anything including heat and drought. Some of petunia cultivars, especially
those with double or semi-double flowers can be sensitive to Botrytis in humid
climates.

Most vegetatively propagated petunias and calibrachoa
cultivars are cold-tolerant and can survive temperatures well below freezing.
The calibrachoa is a perennial and is listed as hardy in Zones 9B-11, but can
take temperatures down to 15° F in some cases. The cold hardiness allows
consumers to plant petunias and calibrachoas early in the spring for quick
color. In the fall, flowering tends to slow considerably and many plantings may
be predominantly green. 

Petunias and calibrachoas tend to get overgrown and may look
bedraggled by midseason. Rejuvenate established plantings in the garden by
cutting them back hard, fertilizing and irrigating regularly if dry. Excessive
growth in the garden can be prevented by first watering and fertilizing newly
planted plants to allow them to root out into the soil and then cutting back on
water and fertilizer to prevent excessive growth.

Petunias and calibrachoa make great container plants,
producing cascades of color from hanging baskets, mixed containers and window
boxes. The frequent irrigation typically required by containers can rapidly
leach them of nutrients, so be sure to keep plantings fertilized to prevent
yellowing and lack of flowering. Petunias are heavy feeders and periodic
fertilizer applications maintain lush growth.

About The Author

John Dole is an associate professor, Brian E. Whipker is assistant professor in floriculture and Paul V. Nelson is professor of horticultural science at North Carolina State University, Raleigh, N.C. They may be reached by phone at (919) 515-3537 or via E-mail at john_dole@ncsu.edu.

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