132 J.E. FAUST, J.M. DOLE, AND R.G. LOPEZ
- Photoperiod.Ideally, stock plants partition all of the photosynthates
generated toward vegetative growth that results in cutting production,
because energy spent on flowering is unproductive, and cuttings that
possess flower buds are not desired in the market place. For species with
obligate photoperiod responses, the stock plants can be kept entirely
vegetative by providing the desired photoperiod for the duration of
the stock plant’s production cycle. For example, low-intensity, night-
interruption lighting can be supplied to poinsettias, chrysanthemum,
and kalanchoe throughout the year to create entirely vegetative stock
plants.
Flowering of species with a facultative photoperiodic response can
be inhibited, but not prevented, by creating short days (for facultative
long-day plants) or long days (for facultative short-day plants). Many
annual and perennial species fall into this facultative-response cate-
gory. For example, petunias are facultative long-day plants that flower
earlier under long days than short days, but nonetheless will flower to
some degree throughout the year (Warner 2015). Blackout-curtain sys-
tems are utilized for species like petunia and calibrachoa to provide
long night lengths in order to create more vegetative stock plants.
Stock-plant greenhouses are typically located at latitudes between the
Tropics of Cancer (23.5◦N) and Capricorn (23.5◦S), where the photope-
riod perceived by plants ranges from 11 to 13.75 h d−^1. Thus, stock
plants grown in these locations never experience the 15- to 16-h day
lengths that occur during the summer months in temperate climates.
Thus, the flowering response in the tropics for many facultative long-
day plants, especially for herbaceous perennials, is reduced for most or
all of the year.
Managing the photoperiod for stock plants of herbaceous perenni-
als tends to be more complex than for annual species. Many perennial
species have evolved in temperate climates where they experience a
cold winter that may increase the flowering response through vernaliza-
tion. The majority of perennial species are long-day plants that flower
as the night length gets progressively shorter during the late spring and
summer (Whitman and Runkle 2013). As the night length increases fol-
lowing the summer, some perennials will go dormant or produce com-
pact, rosette-like growth until shorter night lengths return in the spring.
When perennial species are grown in greenhouses in the tropics, no ver-
nalization signal is received for flowering and the natural night lengths
are never shorter than 11.0 h d−^1. These conditions promote vegeta-
tive, not reproductive, shoot development in perennial stock plants.
However, photoperiods from 10.5 to 12 h d−^1 are sufficiently short to
cause some perennials species to go dormant or produce extremely