152 J.E. FAUST, J.M. DOLE, AND R.G. LOPEZ
In some species, the tissue directly above the wound site begins to swell
into a distinct ring and can be followed by a change in color, from green
to white or light tan. Roots do not form from callus tissue, but callus is
an indicator that environmental conditions are favorable for root forma-
tion (Klopmeyer et al. 2003).
During Stage 2, moisture management continues to be a main focus of
the propagator as the goal is to maintain a high relative humidity envi-
ronment so that the cutting does not wilt. The substrate should be thor-
oughly moist, but is not saturated. Propagation media moisture content
ranging from 85% to 95% saturation is ideal for adventitious root for-
mation (J.R. Crook and J.E. Faust, unpublished). However, only enough
water should be leached from the substrate that wet patches are visible
below the drain holes rather than a puddle surrounding the tray (Fisher
et al. 2009). In a survey of propagators, Santos et al. (2008) reported that
as much as 46 L⋅m−^2 of water was leached from the propagation media
in a 4-week crop cycle.
Tissue nutrient content decreases rapidly during the first week in
propagation due to the high volume of water typically used during mist-
ing (Svenson and Davies 1995). These nutrients must be replaced with
nutrients through the mist or irrigation solution (Fisher et al. 2009).
Typical fertilizer application rates during propagation range from 50
to 100 mg⋅L−^1 nitrogen and should include micronutrients, for exam-
ple, iron concentrations should be approximately 1 ppm (Gibson 2006).
Additionally, recent recommendations indicate that nutrients (50–
75 mg⋅L−^1 N) can also be applied through the mist for most, but not all
crops (Santos et al. 2009). Foliar feeding is not recommended for crops
that are sensitive to specific ion or salts on their foliage. For example,
phosphorus on the foliage of poinsettias during propagation will cause
leaf thickening and severe distortion of the young leaves (J.E. Faust,
unpublished).
Substrate temperatures should be maintained from 21◦Cto25◦C. A
common strategy for heating the propagation medium is to circulate hot
water through tubing on benches, under benches, or buried in concrete
floors (Dole and Bilderback 2006). The air temperature is also important
as it affects growth and should be maintained from 18◦Cto23◦C. The
combination of cooler air temperatures and warmer substrate tempera-
tures promotes rapid root initiation and growth without excessive shoot
growth. Cool air temperatures also reduce the respiration rate of the cut-
tings, which helps to reduce leaf yellowing during propagation (Druege
and Kadner 2007). Additionally, water used for misting should be above
21 ◦C, as it can reduce substrate temperatures and delay callusing and
rooting (Klopmeyer et al. 2003).