If your ground isn’t frozen yet (in the Northern hemisphere) or you haven’t wrapped your hydrangeas for winter (maybe you’re not doing that anyway), now is the best time to work on changing the color of your hydrangeas.
Many big leaf (Hydrangea macrophylla) and mountain (H. serrata) flowers can be changed to either blue or pink, depending on the soil pH. Knowing the ins and outs of how pH affects this shrub is, therefore, basic knowledge.
The value of a soil testing kit
Having your own soil testing kit (easily purchased from most garden centers or on line) is the first step, as is knowing when and how to use it. You may also want to check the pH of your water, too, especially if you’re on a well. Your water source may thwart your best efforts for changing the color of your hydrangea flowers, so get a handle on it before you start. Here’s one version of a soil test kit sold at garden centers
Exactly what is pH?
Now for your science lesson: what is pH? Technically, pH is the logarithmic measure of hydrogen ion concentration. In normal language, it’s the scale used to measure the degree of alkalinity and acidity based on a numbering system of 1 through 14, with 7 being neutral. A number below seven is acidic while 8 to 14 is alkaline.
That pH reading tells you about your plant’s ability to absorb aluminum in the soil, which is the real deciding factor for the color of your hydrangea flowers. If the soil is too alkaline, the aluminum gets “tied up” and isn’t available to the plant roots. It turns your hydrangea flowers pink. That’s why plants at the base of a house foundation or around a concrete walkway are more likely to be pink. The continual leaching from the concrete foundation or walkway affects the soil, making it alkaline. If you have an especially rainy early summer, those flowers you worked so hard to turn blue might fade closer to pink right before your eyes. The adjoining alkaline concrete repeatedly gets washed into the soil by Mother Nature.
When Should I Amend the Soil
The right time to amend your soil to change the color of your hydrangea flowers is in fall. The soil needs time to absorb amendments in order to change. Of course, you’ll need to continually test and amend on an ongoing basis, possibly each season. Each cultivar responds differently to this soil pH treatment so the “formula” that works for one plant may not be the same for another.
Start with the right flower color
Don’t necessarily assume that if you buy a pink- or blue-flowered big leaf or mountain hydrangea you can change the color. Many of today’s newer cultivars are not pH sensitive. This is generally indicated on the plant tag or description. If causing a hydrangea to flower pink or blue is a choice you want, make sure you purchase a pH-sensitive plant.
Some big leaf hydrangeas are neither pink nor blue, as is the case with the beautiful, pure white ‘Madame Emile Mouillère’. No amount of amending will change its flower color, so make sure you know that before you plunk your money down.
Changing the soil for pink flowers
If you want pink flowers, aim for a soil pH from 6.0 to 6.2. You can use dolomitic lime to reach that pH. If you go too far and the hydrangea leaves start to turn yellow, you might have induced an iron deficiency. Check the soil pH and see where it is. If it’s above 6.4, iron deficiency is likely. Consult your local garden center for the right product. Don’t worry; you can easily fix this and your big leaf hydrangea will thank you for it pretty quickly.
Fertilizers with high levels of phosphorous might help to encourage flowering. But the phosphorus also prevents the plant from “taking up” aluminum, keeping the flowers pink. For the record, treating the soil with phosphorus is not one of my favorite methods. It doesn’t move easily into the plant and runs off into the environment, polluting waterways and groundwater. I don’t recommend it.
Changing the soil for blue flowers
On the other hand, a big leaf hydrangea planted under the protection of pine trees will likely have blue flowers because of the natural soil acidity. But don’t be fooled thinking it’s the pine needles that cause the acidity. Not true! For blue flowers, aim for an acidic pH level of about 5.2 to 5.5. You can even go as low a soil pH as 4.5. That could yield a much deeper blue to violet color.
Rules of the road for acidifying the soil
- Test the pH before applying any product.
- If the pH is high, you have a choice of soil acidifiers. Elemental sulfur or iron sulfate can bring the pH down to the desired level. Aluminum sulfate is a third amendment that you can apply to lower the pH. A word of caution: be aware that if you have used a high phosphorous fertilizer to boost flower production, you run the risk of the phosphorous tying up the aluminum, preventing development of blue color in the flowers no matter how much acidifier you apply. Too much phosphorous can take the blue right out of your flowers over time as no aluminum can get to them. No aluminum = no blue. Again, your local garden center can help you find the right product to change the flower color. Always follow the directions on the product label.
- Water the day before acidifying to ensure the plant is well hydrated. You don’t want the fertilizer to “burn” it.
- Apply acidifying products on a cloudy or cool day, never during the heat of the day; again, you don’t want to cause any burning.
Remember to incorporate the amendment you use
Once you have applied the correct amount of your amendment to your plant, take the time to scratch it in. This will ensure that the amendment doesn’t get washed off with the next rainfall.
What to expect
As mentioned earlier, each plant responds differently to this soil pH treatment so the “formula” that works for one plant may not be the same for another. Plan to monitor your plant’s flowers next season to see what changes – if any – you need to make to keep your desired flower color. Remember this can be an annual chore. If you’re not up for that, maybe you should move the plant to a more hospitable site or learn to love what happens naturally without your intervention.
And that’s it!
You can read more about changing hydrangea flower color and all other aspects of caring for this plant in my internationally best selling hydrangea book, Success With Hydrangeas, A Gardener’s Guide.
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