what is it again you add to the soil?
Lime?
If your blue Hydrangeas turn out pink...
Acid soils will make them more blue, alkaline more pink, but some varieties are bred to be more of one colour. Lime will make a soil more alkaline, therefore more pink. I believe rusty horseshoes used to be said to make them blue, perhaps the iron oxide making it acidic. Other than that people here use Miracid I think, my soil is acidic anyway.
Look I found a rusty nail one, 6th from the bottom!
http://www.hauntedcomputer.com/scottst40.htm
To add to what wallaby1 is saying:
The important thing to have in the soil is aluminum. Without that, you won't get blue regardless of the pH.
Potting soils rarely contain enough (if any) aluminum to matter, so you'll always get pinks instead of blue, unless you add some aluminum (by way of aluminum sulfate, for one.) Greenhouses take care of that during production. I'm talking about when you grow, say, cuttings at home or pot on an plder plant and the soil has been depleted of whatever was put in at the grower's.
So, if there is aluminum present in the soil, the uptake is cut off by keeping the soil alkaline, resulting in pink blooms. Acid soil for aluminum uptake. And all colors in between due to uneven soil pH or aluminum levels.
It's really hard to make a large older plant go one way or the other because of the huge root system, it being difficult to get the aluminum dispersed properlyf or even coloring. If it's just a matter of correcting the pH, (aluminum present at sufficient levels in the soil) that can be done over time. Don't expect to do it overnight or with just one "treatment". And some say that you might just have to dig up the plant and replace the amended soil entirely: no easy task, but it can be done.
Robert.
Edited to add PS:
http://www.hydrangeashydrangeas.com/colorchange.html
This may help.
This message was edited Jun 1, 2006 1:01 AM
I have heard of aluminium making the difference, I must be lucky as I have H. Mariesii which remains a light blue, and an unknown name I bought as a larger plant but it's been in the ground for 5 years and remains blue, but I have notice a purple tinge coming in. It will still have it's main root ball in the original compost, which is mostly peat and useless to any long living plant. I mostly rip as much off as I can now, some plants will die after a time in that, but if they have rerooted into soil just above on the stem they will grow on.
Mariesii I bought as a young plant in a 7cm pot, and it's always been a pale blue.
Wallaby your soil much be at the Ph range that Hydrangeas like to be at to create blue then.
Yes, it is acidic but I don't think strongly so. I also mulch with lots of leafy compost and that is supposed to make it more alkaline and shouldn't be done too often.. It must be 5 years since I mulched the bed with Mariesii, it's due another as the weeds have started to get a hold again, it keeps them and moss down. Mariesii is a blue variety anyway, I do have a pink mophead one in the same bed and it remains a deep pink with no hint of blue.
You can see it behind the hosta bed, with a H quadricolor next to it. That is a lacecap with white tinged with lavender flowers and white, green, greyish green and yellow leaves.
Mariesii is behingd the pink on the other side.
They look a bit purplish in the pic but don't look like that in reality.
