\[E(\mathcal{F})=\frac{1}{2}||\pi||^2= \frac{1}{2} \int _M g_Q(\pi\wedge * \pi)
\]
Theorem: Let \(\mathcal{F}\) be a foliation on a manifold \(M\) and \(g_M\) a Riemannian metric. Then all the leaves of the foliation are minimal submanifolds of \(M\) if and only if the canonical \(Q\) -valued 1-form $\pi : TM \to Q $ is harmonic.
Kamber, F.W., Tondeur, P. (1982). Harmonic foliations