How to change the color of hydrangeas
The color of the flowers, especially in the subspecies and varieties of Hydrangea macrophylla (including the two major groups of "Hortensia" and "Lace-cap"), usually changes depending on the acidity or alkalinity (pH) of the soil. On substrates plus acids, with pH 5-5.5, the flowers of most hydrangeas are purplish; in those with pH 5.5-6.5, mauve or magenta; and in the more alkaline, with pH 6.5-7.0 or higher, roses.
In addition, the color can be changed if the plants are treated with certain fertilizers or mineral salts, except in the varieties with white flowers, which never change color.
The reason for the various colors
The mechanism of the color change is due to the presence or absence of heavy metals in the tissues of the sepals: if the heavy metals, especially the aluminum, are present in sufficient quantity, form with the pigment anthocyanin (causing the color red) complex salts that tint the flowers blue; if, on the other hand, aluminum is present only in limited quantities, the color will be purplish mauve; lastly, if there is no aluminum, the color will be pink.
Aluminum becomes soluble so that it can be present in the water of the substrate and be absorbed by the plant in acidic soils (with low pH).
How to modify the color
To change (always slowly) the color of the flowers of pink to blue it is necessary to supply the soil, in the irrigation water, between 125-200 g of soluble sulfur per m².
You can also add some peat to the soil.
Changes will be faster if the soil is watered abundantly with aluminum sulfate (1.5-3 g per liter of water) in March, April and May.
Later, to maintain the acidity of the soil, fertilizers with a low phosphorus (P) content, but high in potassium (K) and nitrogen (N) should be used; for example, an N: P: K ratio of 25:30:30.
To pass, instead, from blue to pink a dead lime solution (1.5-3 g per liter of water) or from dolomite (200 g per m²), always in March, April and May, trying not to cause an excessive alkalinity that would culminate in chlorosis of the leaves and flowers.
The fertilizer used must have a N: P: K ratio of 25:10:10.
In both cases, avoid spraying the plants with the solutions used to avoid damaging the leaves.
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