Hydrangeas are an old garden favorite and are very easy and rewarding to grow. They thrive in moist shaded locations with plenty of light. They are also excellent for cut flowers, fresh or dried. There are many varieties of Hydrangea exhibiting many shades of blue through to reddish pinks and white. Peak flowering season in Sydney and on the Central Coast is during November and December making them a popular Christmas gift.
While white Hydrangea flowers are always white regardless of pH, they will fade to very pale pink or blue depending on the soil pH as they finish. Pink and blue flowering varieties on the other hand are basically the same as each other, it is the pH level of the soil that influences whether they flower pink or blue.
How do I make my Hydrangea blue?
If your soil is acidic with a pH of around 4.5 to 5.0, the flowers will probably be blue. However, most garden soils are much less acidic and the flowers will be purple to pink, especially on beach sands derived from shell. Basically if you want to change the colour from blue to pink, apply Dolomite lime to the soil. For changing pink flowers to blue you need to acidify the soil and add Aluminium sulphate. You must change the soil pH before flower buds form if you want to change the colour and ensure the pH is at the desired level by August.
Aluminum is required for the plants to manufacture the blue pigment. Aluminium is readily available at a pH of 4.5, you will also need to supplement the supply with Aluminium sulphate. If you have too little Aluminium available or the pH is up around 6.0, your Hydrangea’s hues will be pale blues, and mauves rather than intense blues.
To enhance the colour of the blue flowers, treat the soil with one or two doses of Aluminum Sulfate. Water with 1 tablespoon per 5 ltres of water. Don’t overdose and don’t splash any on the leaves; as it will burn the foliage. Only do this during September and early November, just as you see buds beginning to form. Timing is critical as you want to affect the colour, but not cause a nutrient imbalance.
When your soil pH gets down to 5.5 or lower, Phosphorus in the Soil is not available to the plant. Don’t feed your Hydrangeas with fertilisers containing Phosphorus before they flower as it will block your Hydrangea from getting enough Aluminum to bloom in Blue. Phosphorus will interfere with absorption of the very Aluminum this plant needs to make the Blue petal pigment.
Lowering soil pH.
One potential problem that will make it completely impossible to lower the pH. and get blue flowers is free lime ie. Calcium. A Soil containing free Lime/Calcium can not be fixed to grow ‘acid-loving plants’. To test your soil for free lime/calcium place a spoon full in a clean container without touching it with your hands. Get some vinegar and drizzle over the soil and check for bubbles. Bubbles mean there’s a reaction taking place and you are not going to grow blue Hydrangeas in that soil no matter what you do.
If you see no bubbles, you can a purchase a pH test kit from us or bring in your soil sample and we can check it for you.
After establishing that it is feasible to lower the pH you need to drop it to a pH of 4.5. Add Sulphur to the soil, reapply every couple of months, this is a gradual process and can take a few seasons. Also work Iron sulphate into the soil at the rate 430 grams per square metre this will speed up the process. Peatmoss has a pH of 5 and incorporating this into the soil will also aid in soil structure and water retention.
Hydrangeas tend to get sunburnt when in full sun as indicated by a scorching of the flowers and or foliage. They hate hot dry winds and thrive in dappled shade or on the south side of the house. Hydrangeas don’t like to dry out too much and will wilt when they need watering. They have fibrous roots close to the soil surface so mulch is recommended. Overall, hydrangeas are really tough and will survive almost anywhere, even in a windy coastal spot.
Once the flowers finish, cut them off at 6 nodes/joints from the flower head. You can even cut lower if you wish. Do not cut the stems that are yet to flower, leave them and they will produce the first flowers of next year.
Hydrangeas are prone to Powdery mildew during humid summer weather. Try to water in the mornings so the foliage dries quickly. Any fungicide for Powdery mildew is a suitable treatment.
During November and December we have plenty of Hydrangea varieties available in flower.