158 J.E. FAUST, J.M. DOLE, AND R.G. LOPEZ
placing them in propagation to reduce leaf yellowing by increasing the
sink strength of the leaves. Root initiation is slightly delayed by the BA
and/or GA treatment.
Mist is intermittently provided to the cuttings to prevent desiccation
until root initiation takes place. The first day in propagation requires
approximately 100 mist events, and this number decreases daily until
the tenth and final day the cuttings are misted, when they may receive
as little as five mist events during the entire day. The total volume of
water applied to geranium cuttings over this 10-d propagation cycle
averages 22 L⋅m−^2 during the peak propagation month of February in
North America; however, the most water-efficient greenhouses use as
little as 5 L⋅m−^2 for geranium propagation (J.E. Faust, unpublished).
Commercial mist systems are controlled by one of two methods. The
most common method is to use a simple time clock that the grower
adjusts as needed as the weather changes and the cuttings mature.
A more sophisticated alternative is to control the mist by measuring
the greenhouse temperature and humidity and calculating the vapor
pressure deficit experienced by the cuttings (Davies 2005). The calcu-
lated vapor pressure deficit value is accumulated every minute until it
reaches a threshold number programmed into the system by the grower.
Then, a mist event occurs, and the vapor pressure deficit number is
reset to zero and begins accumulating once again. The benefit of this
system is that it responds to changes in the greenhouse climate without
human input. In both mist-control systems, the grower will strategically
reduce the frequency of mist events every day the geranium cuttings
are in propagation until the mist is turned off 10 d after sticking, by
which time root initiation has occurred on all of the cuttings. Following
root initiation, the cuttings will remain in propagation for an additional
2 weeks until they have become fully rooted liners and are ready for
transplant.
Some geranium propagators use irrigation nozzles rather than mist
nozzles for propagation. The irrigation nozzles supply 4–8 times larger
water volume per irrigation event compared to mist nozzles; conse-
quently, the number of irrigation events is considerably lower when
irrigation nozzles are used compared to mist nozzles. In this approach,
the propagation medium is kept moist while the leaves are drier due to
the reduced frequency of irrigation events. This method can work well
because geranium are particularly susceptible to gray mold infection
of the leaves when they are kept constantly moist and this technique
produces leaves that dry off between every irrigation event.
In addition to unrooted cuttings, geranium stock-plant growers offer
callused cuttings for propagators. In this situation, the cutting supplier