  Given a connected reductive algebraic group $G$ and a spherical $G$-variety
$X$, a $B$-root subgroup on $X$ is a one-parameter additive group of
automorphisms of $X$ normalized by a Borel subgroup $B \subset G$. We obtain a
complete description of all $B$-root subgroups on a certain open subset of $X$.
When $X$ is horospherical and either complete or contains a unique closed
$G$-orbit, we determine all $G$-stable prime divisors in $X$ that can be
connected with the open $G$-orbit via the action of a suitable $B$-root
subgroup.
