How often should I fertilize color-changing hydrangeas?

The fertilization of hydrangeas that change color relies heavily on the timing and nutrient formula that you choose. Your schedule for blue colors will differ from that for pink colors due to the different nutrients they require. Blue hydrangeas require a low-phosphorus food, whereas pink varieties require a high-phosphorus formula. This is important because you want to avoid the binding or blocking of aluminum, which is especially important when hydrangeas are changing colors.

Timing coincides with the stages of growth for optimal impact. Stimulating growth with fertilizer can start when new leaves appear in the spring. I suggest you stop applications six weeks before your first frost. This will help avoid potentially tender growth. I write on my calendar once every month and give the hydrangea plant food to match its growing stages. Knowing when to fertilize consistently, from an aesthetic perspective, your hydrangeas will have richer color transitions.

Effective nutrient management also involves water management. Always thoroughly irrigate the soil before applying nutrients to prevent burning the roots and ensure that the fertilizer spreads evenly across the plant's root system. I will always water until the soil feels very moist at a depth of 4 inches, then I will add a diluted fertilizer to the soil after the irrigation event. This practice avoids pockets where the chemical is concentrated.

Steer clear of common fertilization blunders that ruin color. Do NOT use slow-release formulas that feed continuously while plants are dormant. DO NOT use any aluminum-rich products for pink hydrangeas. I label fertilizer containers clearly to avoid confusion between different beds.

Testing soil provides direction for adjusting nutrient applications. Test for nutrient levels in the spring, before the first feeding. Soil testing can indicate the buildup of phosphorus, as too much may inhibit aluminum uptake. I have noted success in keeping phosphorus levels low, preferably under 25 ppm, to have the best results for blue hydrangeas.

Read the full article: Hydrangea Color Change: Ultimate Guide