Blue Hydrangea Flowers: Lunaire's Field-Tested Picks
The bigleaf hydrangeas that actually turn blue in my clay beds — and the soil trick that keeps them that way.
Blue hydrangeas are almost all Hydrangea macrophylla (bigleaf) and a few Hydrangea serrata (mountain) types — the only hydrangeas whose flower color responds to soil chemistry. To get blue you need acidic soil (pH 4.5–5.5) so the plant can take up aluminum; in neutral or alkaline soil the same plant blooms pink. My twelve picks all run cold-hardy into at least zone 6, and most prefer morning sun with afternoon shade.
Here is the thing nobody tells you when you buy a hydrangea labeled “blue” at the garden center: the color is not fixed. I learned this the hard way my first season here in zone 7b, where the native clay leans alkaline. I planted a gorgeous blue-flowered shrub and by the next July it was bubblegum pink. The variety didn't change — my soil did the talking.
Blue happens when Hydrangea macrophylla and Hydrangea serrata can pull aluminum out of the soil, and they can only do that when the soil is acidic — a pH between 4.5 and 5.5, with 5.0 being the sweet spot for the deepest tones. The list below is built from varieties I have either grown myself or trialed in a friend's nearby beds, ranked by how reliably they hold a true blue once the soil is right.
How we chose
How I chose these
I prioritized varieties that color up blue without a fight once the soil pH is in range, that survive my zone 7b winters and ideally colder, and that rebloom or hold up to summer heat. I tested each at pH 5.0–5.2 using a soil meter I trust more than the cheap probe kind — I send samples to the county extension lab once a year to calibrate. White and pink-only types are left off; they will not go blue no matter what you feed them. Everything here is organic-amended: I use pine bark, composted oak leaves, and coffee grounds rather than leaning on aluminum sulfate alone, which can scorch roots if you overdo it.
1. Nikko Blue Hydrangea (Hydrangea macrophylla 'Nikko Blue')
Zones 6–9 · Morning sun, afternoon shade · Summer bloom, 4–6 ft
If you want one classic blue mophead, this is it. 'Nikko Blue' is the variety most people picture — big rounded heads, 4 to 6 feet tall and wide. It blooms on old wood, so I never prune it after midsummer, and at pH 4.5–5.5 it gives me the deepest, most saturated blue of any mophead I grow.
2. Endless Summer 'The Original' (Hydrangea macrophylla 'Bailmer')
Zones 4–9 · Part sun · Summer to fall rebloom, 3–5 ft