Pia (hydrangea macrophylla ‘Pia’ also occasionally sold in the trade as Pink Elf™) is a great hydrangea for two reasons–first it mostly h0lds its pink color even in very acidic soil (remember, my soil test showed a pH of 3.5–no wonder this wants to turn bluish!)
Next, it’s a true dwarf hydrangea–no pruning required. It stays under 3′ no matter what you do. I’ve had Pia for probably at least 15 years and it’s still no bigger than knee high on me–now that’s a true dwarf!
I’ve seen conflicting reports in the literature about how big Pia will grow–some say that it stays as small as 1′; others give the range as 1′-3′. Since as we all know by now that plants can’t read (and that most plants are very unhappy in my dreadful clay soil) Pia is at the top of its mature height after 15-ish years.
The other issue this poor plant has is that it’s very badly sited. It only gets a few hours of sun, but unfortunately, those hours are heat of the day, 11-2 sun. That means that transpiration constantly causes it to wilt, and in the tiny spot it is in, there is no way for the ground to hold the water it needs. Still, it has help up beautifully, wedged in between an aluminum sun porch wall (just showing in the photo), a slate walk, and the garage. And it gives me great color there all summer.
I can’t recommend this hydrangea more highly, both in terms of how well it performs in adverse conditions and how well it holds its color in acidic soils. It’s also great for those tight spots where you don’t want to have to prune back larger hydrangeas and risk losing the blooms. But you have to want a pink plant!