Hydrangea not obeying chemistry

Last year a friend gave me a lovely bunch of clear blue hydrangea flowers. So when he was pruning it, I got some cuttings and rooted them. I planted out a couple (I admit they were root bound as I hadn't repotted enough). The photo shows the immature plant flowering bright pink despite the rhododendron in the background showing how acid the soil is. Is this just due to the flower being forced by root conditions? I hope so, as otherwise it's for the compost heap!

 

We are on clay soil and the plants go to white/pink. So we wanted a blue one. Tried all sorts but to no avail then we added into the bottom of the trough MANY nails that were rusted.

and this has been the result for some years now. Beautiful blue.

 

I don't think tight roots should be the problem. Hydrangeas don't seem to mind that and even in the ground their roots are small and shallow in relation to the size of the shrub above them, in my experience.

 

So, when did you plant out, what compost did you use in the pot, ericaceous I presume.

 

The soil around here is only just slightly acidic.
My neighbour has a hydrangea in the front garden which is sometimes pink and sometimes blue.
I have one in the garden too which is usually pink, but occasionally blue.

If you want it to be blue, then you can buy hydrangea blueing powder that will change the colour.

 

They can be both colours round here on the same plant. They're flowering earlier here this year and I've noticed a lot of them are double coloured - maybe more than usual, but perhaps it's just because the weather has suited them so well this year, being much milder when buds were appearing and growth starting sooner than normal.

Our soil is neutral to acidic - and that's fine for rhodos too. The vast majority really don't need acidic soil to grow well. It's poor, alkaline soil that affects them

 

Good point @pete. I only recently planted it out - if it's roots haven't yet ventured into my acid soil, the compost in the pot was just MPC, so that may be affecting it. The source plant is growing in a garden less than a mile away, with nothing done to the soil. So I had high hopes. Yours is a lovely colour @On the Levels but this one (originally) is a clear pale sky blue which I want.