October 2021 - 42

technically speaking
BY ERIK RUNKLE
Strategies to Shorten Crop
Production Time
In the production of floriculture crops, a common goal is
to produce high-quality and uniform crops in a short period of
time. There are a number of factors that influence crop timing of
flowering plants, including the species and cultivar (genetics),
maturity of the young plant at transplant, plant culture and
environmental conditions. This article briefly summarizes how
these factors can shorten, or in some cases unintentionally
lengthen, the production time of crops grown in greenhouses.
Genetics. Numerous cultivars are available for virtually all of
the major greenhouse-grown species. There is usually
some variation in plant vigor and flowering time from one
cultivar or series to another. When growers have flexibility
in the specific cultivars they grow, determine whether
options exist with shorter production times.
Maturity of starting material. The finish time of crops
depends partly on the maturity of the plug or liner at
transplant. The number of leaves on the main stem(s)
is usually a better indicator of plant age than cell size.
To shorten crop production time, start with a larger and
more mature transplant. For example, plants grown in
larger cells for an additional week or two often finish a
week or two quicker than transplants grown in smaller
cells for a shorter period of time. Taken to an extreme
can be problematic though, since transplanting is more
difficult with root-bound plugs and liners, and grow-out
into the final container can be delayed.
Raise the temperature. The time it takes for a plant to
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.
develop a leaf or flower is strongly influenced by the average daily
temperature. Oftentimes, the crops with the longest greenhouse
production periods are cold-sensitive crops, which we have
defined as crops that do not begin to grow until the temperature is
at least 46° F. Examples of cold-sensitive crops are angelonia, blue
salvia, browallia celosia, gomphrena, hibiscus, pentas, torenia,
vinca and many ornamental grasses. An average temperature of
less than 70° F usually delays production time, so for these and
similar warm-season crops, increase the temperature (especially
during the day) to accelerate crop development. However, in some
cases, high temperature can delay flowering. Examples include
poinsettia and chrysanthemum, in which a night temperature
higher than about 73° F delays flowering.
Regulate the photoperiod. Many common annual and
herbaceous perennial crops flower earlier when the days are long
(the nights are short). What constitutes a long day depends on the
species and sometimes cultivar; some long-day plants may flower
under an 11-hour day (13-hour night), whereas others may not
flower until the days are 14 or 15 hours long. However, for many
long-day floriculture crops, the days are naturally long beginning
in late March or early April. Before then, delivering at least a low
intensity of light (at least 1 to 2 µmol×m-2
×s-1), either at the
end of the day or during the middle of the night for 4 hours, can
accelerate flowering.
42 OCTOBER 2021 GPNMAG.COM
Figure 1. Production time of ornamentals can often be decreased
by growing cultivars with short flowering times, starting with more
mature transplants, increasing the average daily temperature and
light integral, and delivering inductive photoperiods.
There are a handful of floriculture crops that flower earlier when
the days are short (the nights are long). Common examples of
short-day crops are African marigold, celosia, chrysanthemum,
cosmos, poinsettia and zinnia. For these crops, shortening the day
using blackout curtains in the morning and/or evening, from March
through September, can accelerate flowering and shorten crop
production time.
Increase light levels. Except for some shade-tolerant plants,
the production time of ornamental crops decreases as the average
daily light integral (DLI) inside greenhouses increases. This is
particularly noticeable during the winter and early spring when
the DLI is less than 8 to 10 mol×m-2
×s-1. Adding supplemental
lighting during these low-light periods can decrease production
time by one to two weeks. In addition, plants grown under higher
light are typically of higher quality (more compact, more roots,
more branches, and more flowers) than plants grown without
supplemental lighting.
Plant culture. While good growing practices do not shorten
production time, suboptimal conditions can delay production. For
example, a root zone with an excessively low or high pH or low
or high fertility can cause nutrient deficiency or toxicity, which
decreases growth rate and plant vigor. The presence of insect pests
or pathogens also hinder growth and can delay the marketability
of plants. Improper use of plant growth regulators can also delay
production time, especially when too high of a concentration is
applied late in the production cycle. These are just some examples
that illustrate the importance of good crop management. Closely
monitoring crops on a routine (e.g., weekly) basis, such as
checking sticky cards for insect pests and measuring the substrate
pH and EC, followed by proactive corrective actions, can identify
and resolve problems before they manifest themselves and create
much larger challenges and production delays.
http://www.GPNMAG.COM
October 2021

Table of Contents for the Digital Edition of October 2021
