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Hydrangeas
Expert guides on every fertilizer we carry, step-by-step crop growing instructions, and the science behind healthy soil.
Crop Overview
Hydrangeas occupy a unique place in the fertilization world because soil chemistry doesn't just affect plant health — it directly determines flower color in bigleaf (Hydrangea macrophylla) and mountain (Hydrangea serrata) varieties. The mechanism is aluminum availability, which is controlled by soil pH. In acidic soil (pH below 5.5), aluminum becomes soluble and is absorbed by roots, producing blue flowers. In less acidic soil (pH 6.0-6.4), aluminum is locked up and flowers turn pink. Between these ranges, blooms appear purple or display a mix of colors on the same plant. This means fertilizer choice has dramatic visual consequences beyond simple plant nutrition. For blue flowers, use low-phosphorus, high-potassium fertilizers (such as 25-5-30) because phosphorus binds aluminum in the soil and prevents root uptake. For pink flowers, high-phosphorus formulas (25-10-10) block aluminum absorption. White-flowering species like Annabelle (H. arborescens) and PeeGee (H. paniculata) do not change color regardless of soil chemistry. Beyond the color issue, hydrangeas are moderate feeders that respond well to several light applications during the growing season rather than one heavy dose. They are water-demanding plants with shallow root systems, making them sensitive to fertilizer salt burn during dry periods. The most critical nutritional consideration for New England growers is iron availability — if soil pH rises above 6.4, hydrangeas develop iron chlorosis (yellowing leaves with green veins) even when iron is present in the soil, because alkaline conditions make it insoluble.
How to Apply ?
Seedling
Newly planted hydrangeas benefit from compost-enriched planting holes and a light application of balanced slow-release fertilizer; avoid heavy feeding during the first season while the root system establishes in its new location.
Vegetative
Apply a slow-release granular fertilizer in early spring as new growth emerges, choosing the NPK formulation that supports your desired bloom color; spread under the canopy drip line and water in thoroughly.
Flowering
Cease fertilizing once flower buds begin to show color; late-season nitrogen pushes vegetative growth at the expense of bloom quality and can delay hardening off before winter in zones 4-5.
Fruiting
N/A — hydrangeas are ornamental and not harvested for fruit; after bloom, avoid fertilizing to allow the plant to harden off naturally for winter dormancy.
Common Mistakes
Using high-phosphorus bloom booster fertilizer when trying to grow blue flowers — phosphorus actually blocks the aluminum uptake needed for blue color|Raising pH above 6.4 with excessive lime which causes iron deficiency chlorosis regardless of bloom color goals|Fertilizing too late in summer which stimulates tender growth that cannot survive New England winters|Over-fertilizing in a single heavy application rather than several light feedings throughout the season|Ignoring soil pH testing and guessing at amendments needed for desired flower color rather than using data
Organic Options
Wicked Organics Flower Blend provides gentle balanced nutrition suitable for hydrangeas. For blue flowers, supplement with aluminum sulfate drenches and cottonseed meal (naturally acidifying). For pink flowers, add bone meal (high phosphorus) and garden lime to raise pH. Composted pine bark mulch maintains soil moisture while gradually acidifying — ideal for blue flower production.
Nutrient Deficiency Signs
Nitrogen
Leaves are uniformly pale yellow-green across the plant, new growth is sparse and weak, flower clusters are small and fewer in number, and overall plant vigor declines noticeably.
Phosphorus
Foliage develops a dark bluish-green color with possible purplish tints on stems, root growth is sluggish, and flowering is delayed or sparse — though note that low phosphorus may actually benefit those seeking blue blooms.
Potassium
Leaf margins and tips scorch brown, flower color intensity is reduced, blooms are smaller than expected, and the plant shows poor winter recovery with increased dieback in spring.