Several agastache varieties show yellow leaves with green veins (interveinal chlorosis) starting last summer. Iron treatment didn't help. Osmocote (macronutrients N-P-K) was applied. The pattern suggests magnesium deficiency, often caused by high soil potassium levels.
Dissolve 1 tablespoon of Epsom salts (magnesium sulfate) in 1 gallon of water. Water plants with this solution several weeks in a row. Practical for small areas.
Look for products at your local garden center that provide a full range of micronutrients, including magnesium.
Iron deficiency shows similar yellowing but magnesium deficiency is distinguished by mid-vein yellowing. Excess potassium can lock out magnesium, making iron supplements ineffective.
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