As hydrangeas come into bloom, so do the inevitable questions about their flower color. True blue flowers are elusive in the garden. Most of us turn to blue flowering hydrangeas to bring this cool color into our Dallas landscapes. Trouble is, the desired color doesn’t always stick around. What starts out as a stunning blue flower head on newly planted specimens can quickly fade to a wimpy shade of pink. If we’d wanted pink, we would have planted something pink. Yet, here were are with pink hydrangeas. Now what.
What’s with the color bait and switch? Basically, blue hydrangea varieties can fade to pink, or vice versa, dependent upon the presence of or lack thereof aluminum in the soil; and the soil pH. When aluminum is available, blue hydrangea varieties will stay blue. When there is a lack of aluminum they will trend to pink. But it’s not quite that simple. You can have plenty of aluminum in your soil, but it’s only available to plants at a low (acidic) soil pH of about 5.2-5.5. When soils are alkaline with a high pH, like our soils here in Dallas, then plants can’t absorb the aluminum.
So what can you do? To keep your blue hydrangeas as blue as possible, you’ll be forced to provide aluminum sulfate supplements to the soil. Aluminum sulfate will slightly acidify the soil around your plants so it will also help the absorption of available aluminum. Be careful not to over apply as you can burn plant roots. You can also add organic matter such as coffee grounds, which are slightly acidic, to the soil around your plants.
Now, don’t try turning a white variety of hydrangea flowers into blue: It won’t work. Nor will trying to dramatically change the color of a pink variety to blue in our soils. If you want blue, purchase blue flowering varieties such as ‘Endless Summer’ and then apply aluminum sulfate each spring and fall.
