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The Hydrangea Blues

High consumer demand for blue hydrangeas creates a profitable opportunity for growers of Hydrangea macrophylla: blue hydrangeas sell faster than their pink counterparts, and are more popular as cut stems as well. Producing a crop of blue flowering hydrangeas is a fairly straightforward process, though some guidelines must be followed:

  • Be sure you’re working with the right kind of hydrangea. Only H. macrophylla and H. serrata can be manipulated to change their flower color, and even then, not all cultivars are equally capable of color change. Red varieties like Cityline® Paris and Tuff Stuff Red will only develop a slight purplish hue; white flowered varieties will not change color at all. Among those that can turn blue, some take to color change more readily than others, including:
    • Cityline® Mars
    • Cityline® Venice
    • Cityline® Rio
    • Let’s Dance® Starlight*
    • Let’s Dance® Rave*
    • Let’s Dance® Rhythmic Blue*
    • Let’s Dance® Diva!*
    • Let’s Dance® Blue Jangles®*
    • Tuff Stuff *
    • Tiny Tuff Stuff *

* Reblooming varieties

  • Aluminum availability determines flower color. Although color change is usually attributed to soil pH, it is actually aluminum that causes the change. If aluminum is present in the soil, the flowers will be blue. In the absence of aluminum, the blooms will be pink. Soil pH nonetheless plays a key role in color change: it controls the availability of aluminum for plant uptake, and aluminum moves more freely in low (acidic) pH levels. Therefore, both conditions - the presence of aluminum and acidic growing media - must be fulfilled for blue blooms to occur. Because most commercial growing media are acidic but lack aluminum, crops must be specially treated to ensure blue blooms:
  1. Plant your hydrangeas in a phosphorous-free medium, and use a low phosphorous fertilizer with high potassium (i.e. 25-5-30). Phosphorous ties up aluminum in the soil regardless of pH, so it’s crucial to minimize its presence during this process.  
  2. Treat your crop one of two ways:
    1. Drench with aluminum sulfate immediately after planting. A solution of ½ oz (1 Tbsp) per gallon is a good start. Be sure the plants are well-watered before applying the aluminum sulfate to prevent burning the roots. Drench again in 10-14 days then test the soil pH. If your soil pH is greater than 5.6, a third drench will be required. Though more labor-intensive, this method gives faster results.
    2. Apply a controlled-release form of aluminum sulfate, such as Blue Knight or Blue-Max. If selling your crop the same season it is potted, apply at potting time at the entire recommended rate. If selling your crop the next year, apply at half rate in autumn, with the second half given at bud break the following spring. This requires less labor and results in a deeper, longer-lasting change.
    3. Plan ahead: plants should be treated several months prior to sale to ensure the best flower color.

Please see the Spring Meadow Nursery or Proven Winners websites for photos of our hydrangeas in both color options. Where relevant, we depict both blue/purple and pink versions on our plant tags.

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