when you have to defend your mansion at 7 but enter a landscaping competition at 8
Anyway: topiary guardians are, as the descriptive text explains, not animate plants, but rather golem-like beings that just happen to use plants as a solid form. The statblock then proceeds to ignore all that and makes them plant-type anyway.
The guardians are surprisingly detached from the animal they mimic: hit dice, ability scores and natural armor bonus are all size-dependent, while movement modes other than land speed are lost.
The guardians also all have fire vulnerability, DR 10/slashing, tremorsense, and the ability to deal nonlethal damage without penalty. Furthermore, they can hold themselves motionless and appear to be a shrub to anyone who doesn't succeed on a DC 30 Spot check.
However, the issue is that Topiary Guardian is typically going to be increasing something's HD to the point where it loses net viability. I guess there's a few niche cases where the template gives a boost, like by applying it to a legendary wolf or something, but those cases are rare and more based on lost RHD than any real benefit from the template itself. In any case, I doubt the resulting creature would qualify for more than -0 LA.
+0 LA for the guardians in rare optimal cases, but if something it's only +0 LA on a handful of creatures it's hardly worthy of being called +0 at all. -0 LA, secure your garden in some other way.