Nothing is more sought after than the hard to attain. Case in point: Gardeners with alkaline soils wanting to grow blue hydrangeas.
During April’s #SoilBasics #groundchat, I covered how soil pH affects gardening. Chris Sabbarese, @CoronaTools, asked how he could keep his hydrangeas blue. His wife loves blue hydrangeas.
Requirements for blue hydrangeas
Type of hydrangea
First it is important that you get the right type of hydrangea. According to the United States National Arboretum there are 23 species of Hydrangeas grown in North America. Thankfully, only 5 species are widely cultivated:
- Bigleaf, French, garden or florist’s hydrangea (Hydrangea macrophylla), has two main varieties:
Mophead or Hortensia (H. macrophylla var. macrophylla) UDSA Zones 6-9
Lacecap (H. macrophylla var. normalis) USDA Zones 6-9
- Panicle hydrangea (H. paniculata) USDA Zones 4-7.
- Oakleaf hydrangea (H. quercifolia) USDA Zones 3-8
- Smooth hydrangea (H. arborescens) USDA Zones 4-9
- Climbing hydrangea (H. anomala subsp. petiolaris) USDA Zones 4-7
Out of these five Hydrangea species, only the mophead and lacecap hydrangeas (H. macrophylla varieties) change from pink to blue in acidic soils. And even within this group there are cultivars that don’t change colour. They don’t produce the pigments that turn the flowers blue. Examples of non-bluing include:
- White or green-flowered lacecap or mophead cultivars. They remain white or green. No amount of fairy dust will transmute these babies to blue blooms.
- Few big leaf pink-flowered hydrangea cultivars also never produce blue flowers including ‘Alpengluhen’ ‘Pia’ and ‘Kardinal.’ In low pH soils, the flowers of these “non-bluing” cultivars turn a dull reddish-purple.
Michael Dirr’s Hydrangeas for American Gardens (Timber Press) has a “the best of the best” hydrangea list. I culled the list to include only the “bluing” hydrangea cultivars.
Best blue hydrangeas
‘All Summer Beauty’
‘Generale Vicomtesse de Vibraye’
Once we have selected the right hydrangea species and cultivar, we also need to check to see if have the right soil conditions.
The underlying reason for blue hydrangeas is availability of aluminum in the soil. Usually aluminum is toxic for most plants, but over millenia bigleaf hydrangeas have adapted to high soil aluminum and actually use the aluminum to make the desirable blue pigments.
Aluminum is freely available in acidic soils, which is why experienced growers suggest acid soil for blue hydrangeas. At higher soil pH the aluminum gets bound to the soil particles and the hydrangeas can’t take up aluminum. The hydrangeas remain pink.
- Soil pH 5.0 to 5.5: Blue flowers.
- Soil pH 5.6 to 5.9: Intermediate between pink and blue flowers.
- Soil pH 6.0 to 6.5: Pink Flowers.
If there are soil pH differences along the root system, blue and pink flowers can appear on the same plant!
What about southern California?
Corona, California, located in Riverside County, has moderately alkaline soil, with soil pH around 8.0. Hydrangeas growing in this soil are bound to be pink!
Usually gardeners add aluminum sulphate to the soil to bring down the pH and supply aluminum. But…
Hydrangeas! Hydrangeas! website says, “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.”
Solutions for alkaline soils
The best thing to do in this case is to grow the hydrangeas in raised beds or large pots filled with acidic potting soil mix, for example a Camellia & Azalea potting mix. Start adding aluminum sulphate when the plants are at least 2-3 years old, and then use throughout the growing season. Make sure that the hydrangea is well watered before you add aluminum sulphate mix, otherwise the hydrangea will burn.
Raised beds: 1/2 oz (1 Tbsp) aluminum sulfate per gallon of water.
Pots: ¼ oz aluminum sulphate per gallon of water.
Hydrangeas! Hydrangeas! website also points out that fertilizer choice is also important:
The choice of fertilizer will also affect the color change. A fertilizer low in phosphorus and high in potassium is helpful in producing a good blue color (25/5/30 is good. Potassium is the last number). Superphosphates and bone meal should be avoided when trying to produce blue.
And if you are really serious about keeping hydrangeas blue, Hydrangeas! Hydrangeas! website recommends that you test your water’s pH. The water should not be higher than 5.6.
One last note: don’t plant near a concrete foundation or sidewalk… lime leaching from the concrete will make the soil alkaline.
If you garden on southern Californian alkaline soils and want to get your fill of blue, why not grow blue agapanthus or even the delightful delicate blue plumbago?
To create blue hydrangeas you need freely available aluminum in the soil, conditions usually found in acidic soils. Unfortunately, Chris lives in Corona, southern California, USDA Zone 9b, which not only has low rainfall, but also alkaline soil. Of course, southern Californian gardeners love blue hydrangeas. Blue hydrangeas are tricky to maintain in the garden!
More on hydrangeas!
Explore the world of growing hydrangeas on Hydrangeas! Hydrangeas!
FAQ about hydrangeas on United States National Arboretum.
Missouri Botanical Garden Plant Finder: Hydrangea
Virginia Cooperative Extension. Bigleaf Hydrangea, Hydrangea macrophylla
p.s. Dr. Linda Chalker-Scott shoots down the practice of using coffee grounds to bring down soil pH. It’s a horticultural myth.
“A commonly held assumption states that coffee grounds are acidic, but this does not hold true experimentally. While two studies on coffee ground composting reported mildly acidic pHs of 4.6 and 5.26, others have measured neutral (7.7) to somewhat alkaline (8.4) pH levels.”