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Plant fertilization requirements, nutrient monitoring and nutrient disorders have been a major research and extension focus at North Carolina State University. With nutrient disorders, our group has induced, photographed and recorded symptomology and critical tissue concentrations in more than 50 crops.
Most growers have standard fertilization practices that result in healthy poinsettia plants year after year. Problems usually occur with a change to production practices (new cultivar, new water source, different substrate or fertilizer blend) or when a fertilization equipment malfunction occurs. With poinsettia season just around the corner, we wanted to share results of our research program and help you recognize poinsettia nutritional problems and what steps to take to avoid them.
Low Electrical Conductivity (EC). When the root substrate EC is too low, plants are stunted and mineral deficiencies occur. Deficiency symptoms such as lower leaf yellowing (nitrogen, Figure 1, page 38) or lower leaf speckling (phosphorus) are common when EC values are below 0.25 mS/cm (1:2 extraction), 0.75 mS/cm (SME extraction), or 1.0 mS/cm (PourThru extraction). High EC. Even though poinsettias are heavy feeders, too much fertilizer also can be a problem. Plants can be darker green and stunted, similar to what occurs with PGR overdoses. If the plants are allowed to dry out, a marginal leaf burn also can occur (Figure 2, page 38).
Phosphorus Deficiency. Symptoms appear at the bottom of the plant. Older leaves may develop either of two patterns. With cold growing, having a waterlogged substrate or when root growth may have been impaired, older leaves will develop the classical reddish coloration (Figure 3, page 38). Another symptom is also common when the growing conditions are optimal, but phosphorus is withheld: the lower leaves will initially turn a dull green, followed by yellow spotting in between the midrib and secondary veins. With advanced symptoms the entire leaf may yellow and grayish-green spots will be observed (the spotting differentiates it from nitrogen deficiency) (Figure 4, page 38).
Calcium Deficiency. Symptoms appear at the top of the plant. Young leaves may develop marginal leaf burn and distortion (Figure 5, page 39). Leaf edges may become necrotic. Typically, symptoms in young plants occur during periods of overcast weather when the plant’s ability to uptake calcium is inhibited. Bracts also can develop a marginal necrosis and late season preventative applications of calcium chloride can be made.
Magnesium Deficiency. Lower leaves develop interveinal chlorosis (Figure 6, page 39). Under advanced conditions, the leaf margins turn necrotic. On younger plants symptoms appear on the lower leaves. On flowering poinsettias, symptoms tend to develop on the top half of the plant (Figure 7, page 39).
Sulfur Deficiency. The upper portion of the plant develops an overall yellowish coloration (Figure 8, page 40) and is most commonly observed during the last half of the growing season. Sulfur deficiency symptoms vary from both nitrogen deficiency (or low EC), which occurs on the lower leaves and from iron deficiency caused by high pHs, which is an interveinal chlorosis of the upper leaves.
Molybdenum Deficiency. Symptoms appear as chlorosis (yellowing) of the recently mature leaves (middle of the plant), rolling of the leaves and leaf edge burn (Figure 9, page 40). The leaf chlorosis of molybdenum deficiency resembles magnesium deficiency, except that the thin, marginal band of chlorosis is expressed from the leaf tip to the leaf base. Molybdenum deficiencies can cause distorted leaves due to the failure of the interveinal areas to expand normally.
High pH. High substrate pH can induce nutrient problems in poinsettias. The recommended pH range in a soilless substrate is 5.8 to 6.5. Iron deficiency is the most common problem at substrate pHs above 6.5. New leaves exhibit an interveinal chlorosis (yellowing) (Figure 10, above). Generally poinsettias do not exhibit foliar symptoms of low pH like other crops such as geraniums or marigolds. Slow plant growth may be the only symptom observed.
Monitoring what is occurring in the root zone by continual sampling the fertilizer solution and the substrate for pH, EC and nutrient levels. This will help assure your poinsettia crops’ success.