  We study black hole binary mergers in certain cubic Horndeski theories of
gravity, treating them fully non-linearly. In the regime of validity of
effective field theory, the mismatch of the gravitational wave strain between
Horndeski and general relativity (coupled to a scalar field) can be as large as
$10-13\%$ in the Advanced LIGO mass range. The initial data and coupling
constants are chosen such the theory remains in the weakly coupled regime
throughout the evolution. In all cases that we have explored, we observe that
the waveform in the Horndeski theory is shifted by an amount much larger than
the smallness parameter that controls the initial data. This effect is generic
and may be present in other theories of gravity involving higher derivatives.
