With different varieties of hydrangeas, specifically mopheads and Lacecaps, you have creative control over the color of the blooms. You can, to some degree, control whether the blooms are blue or pink hues. By adjusting the pH balance of the soil you can keep your hydrangeas blue.
How to get blue hydrangeas
First of all, you need to make sure that you buy the right varieties. Not all hydrangeas produce blue flowers. Some hydrangeas like Oak Leaf hydrangeas will produce only white or cream flowers, with pink tinges on the end near the end of their life span. The hydrangeas that bloom in a range of colors from pink through blue are typically mopheads or lacecaps and these are the hydrangeas you want to have in your garden in order to get that perfect blue bloom. If you have these varieties you can alter the chemistry of the soil to get the color that you prefer. With acidic soil, the blooms will take on a blue or color and alkaline soil will produce pink colors.
How to turn hydrangeas blue
Understand that because soil chemistry plays such an important role in determining the color of the bloom, the variable names you might see on mop heads or lace caps mean very little. You might see a variety that says Pretty in Pink or blue deckle, but any of these have the same chance of producing blue or pink blooms depending on the soil in which they are planted.
In order to manipulate the color you have to manipulate the mineral content of your soil and the pH of your soil. This is a gradual process, not one that you do just once. In order to truly experiment with the color of the blooms, we recommend you try planting in large pots because it will be much easier to control the soil and the chemistry therein by using pots compared to a large garden. Be mindful of the fact that plant health should be your first priority. It can be difficult to make changes to the soil pH balance especially if your plant is already established and doing so can impact the overall health of your hydrangea. So if you notice that the plant is starting to suffer and your shrub is no longer as healthy and vibrant as it was, de-prioritize the blue color.
The first thing you need to know is that to encourage blue flowers you need to have a soil pH between 5.2 and 5.5. You can apply a soil acidifier to lower the pH you can also lower it gradually and naturally by applying acidic organic mulch like conifer clipping or pine needles. If the soil you have is naturally quite alkaline, it is probably going to be more difficult for you to get that blue color even if you’re throwing in a lot of aluminium into your soil. Alkaline soil especially heavily alkaline soil tends to sort of lock up the aluminium in the soil which prevents it from getting to the plant. You’ll still have beautifully healthy hydrangeas they’re just going to be a bit pink, you could, of course, consider growing them in large pots with ericaceous compost.
Keeping hydrangeas blue
As mentioned this process is not a single event, it is a process, something you must do regularly. So, start by testing the pH level of your soil with a pH test kit which is readily available at garden centres and online.
This is something you should test on an annual basis. Once you know your baseline you can start to make changes to the soil and continually test the pH balance in order to measure the effects of your changes. The reason you have to do this annually is because native soil conditions will always be stronger so your soil pH levels will revert back to their original condition at some point or another.
In order to make the soil more acidic, you can add, as mentioned, a soil acidifier such as aluminium sulfate or ammonium sulfate, you can also try adding plenty of ericaceous compost to the soil. When you purchase this from a gardening store you need to follow the application rates as indicated on the packaging as you could easily apply too much or not enough.
Pine needles, sawdust, oak leaves, and peat moss, even coffee grounds to some degree can be added to organically change the levels. Elemental sulfur is another option. For good garden soil, you can add on average 1.5 2 to lb of elemental sulfur per 100 square feet.
Again, regardless of the method you use to change the soil pH, you will need to measure repeatedly to verify the results of your efforts so that you know whether to raise or lower the soil pH on an ongoing basis. This is why you may have more success growing them in pots, especially if you have just purchased new hydrangea ready to be potted on.