How to automatically add R^2 to a graph with facet wrap using ggplot2?

Question

I am trying to add R^2 values to each facet in a ggplot2 graph. This is the code I have so far and this is what the graph looks like currently.

ggplot(BB_new, aes(x=Date, y=pH, color=Treatment)) + 
  geom_point(pch=19, size=1,alpha = 1)+ 
  geom_smooth(aes(color=NULL), method = "lm", formula = y ~ x + I(x^2), se = FALSE) +
  theme_classic()+ theme(legend.title = element_blank())+
  theme(legend.position = c(0.9,0.93))+
  scale_color_manual(labels = c("Reference"),
  values = c("coral1"))+
  facet_wrap(~Hor) + ylim(2.5, 6)