High-light Injury in Poinsettia
Christopher J. Currey, Graduate Research Assistant and R.G. Lopez, Associate Professor of Horticulture
Although poinsettias (Euphorbia pulcherrima) are considered a high-light crop, exposure to excessively high-light levels can cause leaf injury and damage (ie. yellowing or scorch) following transplanting rooted cuttings. In some cultivars, such as ‘Chianty’, high-light can promote such yellowing (Figure 1). This yellowing can be easily be confused with a nutrient deficiency, such as magnesium (Mg) deficiency.
Poinsettia propagation occurs during some of the hottest times of the year and shading is generally utilized to reduce the greenhouse heat load. Additionally, shade is frequently utilized during propagation to minimize desiccation of un-rooted cuttings. However, cuttings of poinsettia are typically transplanted when light levels can still be very bright. Using shade to reduce light levels by 10 to 30 % after transplanting for 1 to 2 weeks or until roots start to grow into the substrate can reduce the incidence of leaf scorch, burn or yellowing. After this point, high light levels from 4500 to 6000 f.c. (900 to 200 µmol·m2·s1) should be maintained until bracts are mature, at which time light may be reduced to 2000 f.c. (400 µmol·m2·s1) to suppress color fading out of the bracts.
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Figure 1. Leaf yellowing induced by high-light exposure during production of poinsettia.