August 2020 - 50

technically speaking
BY ERIK RUNKLE
Height Control of Perennials:
What Works and What Doesn't
Containerized herbaceous perennials are commonly
produced in flower for spring and summer sales. While some
plants require a cold treatment to induce flowering, others
primarily flower in response to day length.
Regardless of the flowering stimulus, a common challenge is
producing plants that are proportional to their containers. Based
on research performed at Michigan State University, this article
discusses height control techniques that generally do (R) or
don't (Q) work for herbaceous perennials forced into flower.
R Provide short days. A long day (short night)
accelerates, or is required for, flowering of many
herbaceous perennials. Low-intensity photoperiodic
lighting at night induces flowering of long-day crops,
and can be turned off in April, when the days become
naturally long for most species. Regardless of a
plant's flowering response to day length, height often
increases with photoperiod. Therefore, height of dayneutral
plants can be controlled by not delivering
long days. In other words, only provide long-day
lighting to crops that need it to flower (or to delay
flowering of short-day plants).
Once long-day plants are induced to flower, the
vast majority of perennials will continue to flower
regardless of the photoperiod. Plant height can be
suppressed if photoperiodic lighting is turned off
once the first flower buds are visible. This technique
is sometimes called " limited induction photoperiod " .
Erik Runkle is
professor and
floriculture
Extension specialist
in the department
of horticulture at
Michigan State
University. He can be
reached at runkleer@
msu.edu.
R Water stress. As with many bedding plants, limiting water
can suppress extension growth of perennials. However,
some crops are more sensitive to this technique than others,
and some crops like astilbe don't tolerate water stress well.
Common perennials that can tolerate at least moderate
drought stress include achillea, coreopsis, echinacea,
hibiscus, lavender, leucanthemum, perovskia, rudbeckia
and salvia.
Q Limited nutrition. Extension growth of some crops is
suppressed when nitrogen and/or phosphorus is limited.
However, this strategy has relatively little effect on
perennials - and some species actually become taller
when fertility is low. In addition, when a low fertilizer rate
suppresses extension growth, it may also decrease overall
plant size and floriferousness. Therefore, this is one of the
least effective height control techniques for perennials.
R High light. Shade avoidance refers to a set of responses
that occur when a plant perceives shade. This response
typically includes larger leaves, longer stems and suppressed
branching. One of the signals that triggers shade avoidance
is low light. Therefore, at least for full-sun perennials, plant
height can be suppressed when supplemental (high-intensity)
lighting is delivered during low-light conditions (Figure 1).
LED fixtures that emit at least a moderate intensity of blue (or
50 AUGUST 2020 GPNMAG.COM
Figure 1. When the daily light integral is low, supplemental lighting
from high-pressure sodium (HPS) or especially light-emitting diode
(LED) fixtures can inhibit extension growth of perennials. R=red,
W=white, and FR=far red. (Photo: Roberto Lopez, Michigan State
University)
white) light, and no far red, are especially effective.
R Wide spacing. The other signal that triggers shade avoidance
is the perception of neighboring plants, which causes a
change in light quality. Plants grown at a high density
perceive their neighbors earlier, and in response, elongate
to compete for the light available. Although wide spacing in
greenhouses is usually a luxury, it does suppress elongation
and increase plant quality.
Q Decrease the temperature. Plants develop progressively
slower as the temperature decreases. Therefore, plants are
shorter when grown cool, but that's simply because of the
delay in development. In actuality, the average temperature
has little to no effect on final plant height. Thus, decreasing
the average temperature is not an effective height control
strategy.
R Negative DIF. The difference (DIF) between the day and night
temperature influences extension growth of most crops.
Delivering a cooler day than night temperature (creating a -
DIF) is an effective way to inhibit extension growth. However,
implementation is only possible when the air outside is
sufficiently cool to achieve a low day temperature inside
the greenhouse.
R Plant growth retardants (PGRs). The last, and usually most
effective, height control technique on herbaceous perennials
is to apply a spray, sprench or drench of a PGR. Products
that contain uniconazole (e.g., Sumagic and Concise) are
especially effective. Applications are best applied just
as plants begin to elongate, and sprays often need to be
repeated for the desired level of control. Avoid late drench
applications because the retarding effects can carry over
after transplant into the landscape.
http://www.GPNMAG.COM
August 2020

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