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

W. Garrett Owen
Assistant Professor of Sustainable Greenhouse and Nursery Systems, The Ohio State UniversityW. 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.
