EVAPOTRANSPIRATION–BASED MISTING CONTROL FOR POINSETTIA PROPAGATION
Theoretical development and validation of a model–based misting control system for propagating poinsettias 
(Euphorbia pulcherrima ‘Freedom Dark Red’ Willd. ex Klotzsch) are presented. The control system predicts 
evapotranspiration (ET) using the Penman–Monteith combination model, with the canopy surface resistance term replaced 
by an equivalent surface resistance that includes the combined resistance effects of plant stomata, water uptake, propagation 
medium, and leaf adaxial surfaces. Different combinations of incident radiation (Ri) and air vapor pressure deficit (VPDair) 
were used to validate this technique by comparing measured and estimated values of ET of partially rooted cuttings under 
partially wetted conditions. Under dark conditions, four ranges of VPDair were applied (0.8 to 1.1, 1.3 to 1.6, 1.8 to 2.1, and 
2.3 to 2.6 kPa). Under light conditions, three levels of Ri (100, 200, and 300 W m–2) were combined with three ranges of VPDair 
(1.3 to 1.6, 1.8 to 2.1, and 2.3 to 2.6 kPa). Observed and predicted values of ET under dark conditions were in remarkable 
agreement, with the ET model overestimating actual ET by 6% and 8% at root emergence and after one week, respectively. 
During light conditions, predicted ET was at most 14% larger than observed values. The use of predicted ET in misting control 
for propagation of vegetative cuttings has several advantages over other techniques, such as timer–based scheduling and 
techniques that account for only Ri or VPDair, by applying only enough water to maintain water status of the cutting.