top of page

Hydrangea Blues- don't be a basic pink

Hydrangea season approaches! Blue is coming.

blue hydrangea starting to open, by sayo ts 2

The special and huge draw of these shrubs for many gardeners is the unusual color of certain hydrangea. They are a vibrant rich blue. You’ve seen blue flowers of course, but if you think hard, you’ll notice they are actually quite rare. Which makes the blue hydrangea both pretty and pretty special.

ombred hydrangea, by Cindy Gustafson

These plants are not grown just anywhere either. They are more picky than many shrubs and thus the more a delight when you can have them in your yard blooming their unusual flower heads like a cluster of tiny dogwood flowers. Even white and pink flowered species are beautiful. But blue is cool! To get that hue, hydrangeas need acidic soils and mild climates (hardiness zones 5-8 generally but species vary). While they CAN be grown in hot dry climates like southern CA, Nevada, and Texas, you don’t see them very often because they need a lot of help surviving. “Finicky” is how some describe them.

The iconic blue hydrangeas are like a fun micro science experiment in your yard. Their flowers (of the big leaf hydrangea species) literally change color depending on their soil’s pH level, sort of. It’s actually about the aluminum availability and by the phosphorus levels. Aluminum solubility is higher in a lower pH soil and with low-moderate phosphorus levels. A plant with more aluminum levels (and not too high of phosphorus or pH) will have bluer flowers, while less aluminum (or high pH or phosphorus) will have more of a pink flower color. Should we test planting some in big coffee cans then?

color range of big leaf hydrangea, by Alabama Extension

Similarly, coffee grounds as well as pine needles, and lawn/grass clippings can make your hydrangeas more blue by increasing soil acidity (lowering pH) and if you are into pink, you can instead add crushed eggshells to increase the soil pH. I would put coffee grounds on one side and eggshells on the other to make a full ombre plant! For SCIENCE!

ombred hydrangea flower, by Alabama Extension
new hydrangea flower, hydrangea flower, young hydrangea flower

This leads to a single shrub that can have several colors or an ombred effect across its flowers. Adding to the hue changes, early flowers are often more cream or green colored and then gain their color as they mature so each week will look different for these shrubs.

Blue hydrangea belong to the Hydrangea genus (love it when common names ARE the genus name). They are native to both Asia and North America with cultivars grown in gardens all over the world. The main species we’ll see blue in cultivation are Hydrangea macrophylla, plus some H. serrata. There is also H. arborescense, H. paniculate, H. quercifolia, and a climbing vine species- Hydrangea anomala, all of which are generally white. There could be quite a lot more species out there, possible 20-80, but only the above handful are in general cultivation in the US. They are deciduous and long-lived shrubs with big floral displays each year (if kept healthy) and the large leaved H. macrophylla is again- unique among flowers as being very blue. Not that "sort-of bluish" but a bit too purple that blueberries and blue-eyed grass actually are.

blue-eyed grass flower, sisyrinchium sarmentosum
Sisyrinchium sarmentosum (aka pale blue-eyed grass), also not a grass (iris family)

Many flowers labeled as "blue" are more on the violet or plum side of hues. This is where I get in to blue arguments with some people, but I am entirely convinced that I have accurate (so to say) color vision because I see the colors of oil paints as they are also described. Paynes gray- is a smoky deep gray with some blue. So I’m for sure always right about color. Of course! What was I talking about…?

See? Bluish smokey gray, right? -->

blue-eyed mary flowers,
"Blue"-eyed Mary flowers (large- Collinsia grandiflora, and small- C. parviflora), more purple still but hint of blue on the small-flowered Mary. Spot the true blue forget-me-not hiding in the back?

That is why hydrangea-blue is so prized (remember- acidic soil with some aluminum) and unique in the plant would, and our gardens. This is when they begin to really shine in full bloom and can last for months. The leaves aren't bad to look at either when the flowers lend to a still persistent dried seed heads and brown dried flower bracts. Their flowers aren't just interesting for their color, but their anatomy, like fore-mentioned dogwood flowers. When fully open, the flower head (corymb) has dozens of small flowers with actual leaf-like yet colorful bracts instead of petals. The true flowers are at the center of these four bracts (blue, white or yellow) a lot like a dogwood flower, but with tighter enclosed bracts and fewer flowers within. Some have a single flower within their large outer bracts and a group of many bract-less flowers inside. These are called "lacecap" hydrangea. Varieties differ. There are also "mophead" and "panicle" forms.

These are an overall attractive and interesting plant if you can grow them without too much fussing. If you care to try a little floral science experiment, this is a rewarding genus to try out and even in winter, the vase-shaped medium shrub with nice tan bark in the winter. If you have them in your area, enjoy the show for the summer.

And ponder on the rareness of BLUE.


28 views0 comments

Recent Posts

See All
bottom of page