Root zone temperature optima for root initiation and root elongation stages for rooting in poinsettia (Euphorbia pulcherrima Willd. ex Klotzsch `Freedom Dark Red') cuttings was determined to be 28 and 26 °C, respectively. Threshold temperatures where rooting development was slow (>24 days) or did not occur were ≤20 and ≥32 °C. Time to visible rooting and postemergent root elongation was modeled based on cumulative daily mean root zone temperatures in growth chamber studies using a thermogradient table to provide simultaneous temperatures between 19 to 34 °C. Time to root emergence at different root zone temperatures was best described using a nonlinear growth rate derived mathematical model, while postemergent root elongation up to 100 cm could be described using either a linear thermal time model or a nonlinear equation based on elongation rate. These temperature-based mathematical models were used to predict rooting in six greenhouse experiments. Using a root zone base temperature of 21 °C, observed vs. predicted time to visible root emergence was highly correlated (r2 = 0.98) with a mean prediction error (MPE) of 1.6 d. Observed vs. predicted root length using the linear thermal time model had a r2 = 0.69 and an MPE of 14.6 cm, which was comparable to the nonlinear model with an r2 = 0.82 and an MPE of 14.8 cm.
If the inline PDF is not rendering correctly, you can download the PDF file here.