  We study the periodicity of subshifts of finite type (SFT) on
Baumslag-Solitar groups. We show that for residually finite Baumslag-Solitar
groups there exist both strongly and weakly-but-not-strongly aperiodic SFTs. In
particular, this shows that unlike $\mathbb{Z}^2$, but like $\mathbb{Z}^3$,
strong and weak aperiodic SFTs are different classes of SFTs in residually
finite BS groups. More precisely, we prove that a weakly aperiodic SFT on
BS(m,n) due to Aubrun and Kari is, in fact, strongly aperiodic on BS(1,n); and
weakly but not strongly aperiodic on any other BS(m,n). In addition, we exhibit
an SFT which is weakly but not strongly aperiodic on BS(1,n); and we show that
there exists a strongly aperiodic SFT on BS(n,n).
