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Avoiding Primula Pitfalls: A Grower’s Guide to pH & EC Management

Thu, Feb 6th, 2025, created by W. Garrett Owen

Primula or Primrose (Primula acaulis and P. vulgaris) is a popular crop grown for the Valentine's Day market and early spring sales. Greenhouse growers should carefully manage substrate pH and fertility to ensure optimal plant quality and marketability at retail. Micronutrient deficiencies or toxicities are common during primula production. Interveinal or marginal chlorosis (yellowing) or necrosis (death) may develop due to high substrate pH or over fertilization or high soluble salts [referred to as electrical conductivity (EC)].

Substrate pH Management

Primula should be grown with a substrate pH range of 5.5 to 6.2. Substrate pH above 6.2 will inhibit iron (Fe) uptake, causing newly developed and recently matured leaves to become Fe-deficient and exhibit marginal and interveinal chlorosis. Corrective procedures for high substrate pH should begin within the range of 6.2 to 6.4. Maintaining substrate pH below 6.2 will reduce the likelihood of Fe-deficiency from occurring; however, applications of chelated micronutrients, such as Fe may be used to correct these deficiency symptoms.

Substrate pH below 5.5 will inhibit magnesium (Mg) uptake causing lower or older leaves to become Mg-deficient and exhibit interveinal chlorosis and intensify to overall leaf chlorosis. Corrective procedures for low substrate pH should begin within the range of 5.3 to 5.5. Monthly applications of supplemental Mg in the form of magnesium sulfate (MgSO4; Epsom salts) at a rate of 8 oz./100 gallons of water in areas with naturally occurring Mg in the water supply or 16 oz./100 gallons of water in areas lacking Mg in the irrigation water may be required if plants exhibit interveinal chlorosis of lower or older leaves.

Substrate EC Management

Primula are highly sensitive to excessive EC levels. Substrate EC should remain below 0.6, 1.3, or 2.0 mS/cm, based on the 1:2 Extraction, SME, or PourThru methods, respectively. Accumulation of fertilizer salts in the substrate may result in marginal leaf burn or necrosis and mimic water stress symptomology (wilting). To avoid high EC, it is recommended to keep the fertilization rate low (100 to 150 ppm N). If levels begin to increase, switch to clear water irrigations to avoid a build-up of fertilizer salts in the substrate. If EC levels become excessive, then leach the substrate with clear irrigation water twice before providing fertility. It is best to monitor the crop to avoid excessive EC levels than to waste fertilizer by having to leach it out of the pots. Furthermore, during winter months when plants are grown under cooler temperatures, fertilize with nitrate-based fertilizers such as calcium nitrate. Avoid fertilizers providing ammonium such as 20-10-20 or 20-20-20. Ammoniacal-based fertilizers can cause excessive vegetative growth and leaf expansion.




About the Author:

W. Garrett Owen

Assistant Professor of Sustainable Greenhouse and Nursery Systems, The Ohio State University

W. Garrett Owen is an Assistant Professor of Sustainable Greenhouse and Nursery Systems in the Department of Horticulture and Crop Science at The Ohio State University. He has an appointment in research, teaching and Extension. His area of expertise is plant nutrition; plant growth regulation; and production problem diagnostics.

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