LIMITATIONS TO HYDRANGEA COLOUR CHANGE:

1. White hydrangeas can NOT be changed to pink or blue by the grower. (Mother Nature sometimes adds pink and red to blooms as they age).
2. It is most unlikely due to weather conditions in the British Ilses that a hydrangea will achieve a "true red". No matter how convincing those pictures in the catalogues are or how much lime is added to the soil, one can only achieve a very deep or dark pink, but not a true red (although we would love to hear from anyone who might have a true red hydrangea).
3. There is little that will affect the intensity of a colour (how strong or pale the colour is). The intensity develops for a number of reasons: the heredity of a particular hydrangea variety, weather conditions (hot or cold, humid or dry), health of the plant, and possibly other natural factors. Fertilizing hydrangeas once or twice a year may result in a little more saturated colour simply because the health of the plant may be improved.
4. A few varieties of hydrangeas tend more toward the pink or the blue range of colours, but will not retain even this colour if soil conditions are not right.

• For hydrangea blooms to be pink, the plants must not take up aluminum from the soil. If the soil naturally contains aluminum, one must try to keep it away from the hydrangea's system. Following are a few tricks that might work:
• Add dolomitic lime several times a year. This will help to raise the pH. Shoot for a pH of about 6.0 to 6.2 (If it goes above 6.4 hydrangeas might experience an iron deficiency). Since hydrangeas take up aluminum best at lower pH levels, raising the pH will help to keep the bluing effect of aluminum out of the hydrangea's system.
• Use a fertilizer with high levels of phosphorus. Phosphorus helps to prevent aluminum from creeping into the system of the hydrangea. Choose a fertilizer close to the ratio of 25/10/10 (Phosphorus is the middle number).
• In areas that naturally produce blue hydrangeas (soils with aluminum), consider growing pink hydrangeas in large pots. If hydrangeas are grown in pots, it would be best to use soil-less mixtures, since these mixes would not have aluminum in them. In a pot, it will be much easier to control the requirements for growing pink hydrangeas.

• To obtain a blue hydrangea, aluminum must be present in the soil. To ensure that aluminum is present, hydrangea colourant may be added to the soil around the hydrangeas.
• Always carefully read the manufacturers directions for applying the colourant. If in doubt check with your local garden centre before applying.
• Important: Water plants well in advance of application and put solution on cautiously, as too much can burn the roots.
• To make the aluminum available to the plant, the pH of the soil should be low (5.2-5.5). Adding a hydrangea colourant will tend to lower the pH of the soil. Another method for lowering the pH is to add organic matter to the soil such as coffee grounds, fruit and vegetable peels, grass clippings etc.
• If the soil naturally contains aluminum and is acid (low pH) the colour of the hydrangea will automatically tend toward shades of blue and/or purple.
• The choice of fertilzer will also affect the colour change. A fertilizer low in phosphorus and high in potassium is helpful in producing a good blue colour (25/5/30 is good. Potassium is the last number). Superphosphates and bone meal should be avoided when trying to produce blue. Your local garden centre will be able to help you make the right choice.
• Remember it is virtually impossible to turn a hydrangea blue for any length of time if it is planted in soil with no aluminum and that is highly alkaline (chalky).
• Perhaps the best idea for growing blue hydrangeas in an area with alkaline soil would be to grow them in very large pots using lots of compost to bring the pH down. The above suggestions for bluing would also work for a potted plant. Reduce the strength of the Aluminum sulfate to 1/4 oz per gallon of water. In a pot, it will be much easier to control the requirements for bluing.
• Planting hydrangeas near a concrete foundation, pavement or pathway will often affect the colour since the pH of the soil may be raised considerably by lime leaching out of these structures, making it difficult to obtain blue.
• One last suggestions for those who are serious about this process. It is important to have your water tested so that it will not "contaminate" the soil that you have so rigorously balanced. The pH of the water should not be higher than 5.6.