Despite being called 'boneleaves', these monsters are neither undead nor plants. Instead, they're aberrations.
These creatures are large-sized, with 7 RHD. Their ability scores are decent: 25 strength and 22 constitution make for a strong melee character, and the other stats are pretty neat as well (with the exception of the 8 intelligence).
10 ft. land speed hurts a lot, though the (equally slow) burrow speed makes up for stuff a bit. +8 natural armor is okay, until you realize no armor shop ever is going to have something that fits a tree.
Boneleaves' natural weapons are okay-ish. They get two attacks per round, which may be tendrils, slams, or a tendril and a slam. Slams deal okay-ish bludgeoning damage with some slashing thrown on, while tendrils Improved Grab and Constrict instead.
Furthermore, boneleaves have a number of plant traits (because figuring out their typing wasn't quite confusing enough). They're immune to magical sleep, paralysis, stunning, and most importantly mind-affecting effects. They also have 50% fortification. They're also so alike plants that a DC 15 spot check is required to tell them apart, which I guess is nice for the occasional ambush.
Boneleaves can also create illusions, which is a vastly limited, short-duration Major Image. It's worth mentioning, but not very impactful on the LA.
Finally, boneleaves have the Pervasive Sentience Ability, which states that near each boneleaf are 1d6+6 other ones, which share senses. However, the ability does not explain what happens as individual boneleaves are killed, what happens when one travels away using its move speed, or even what happens when a boneleaf is disconnected from the nerve network by making use of a Fly spell or similar effect. For this reason, I'm going to need an asterisk here.
I think -0* LA is fine for boneleaves. Their low speed and unimpressive stats, combined with the load of less-than-stellar aberration HD and inability to use most equipment, make me doubt whether they'd be able to contribute as much as any other ECL 7 PC. I guess some niche builds could use Illusory lure and the burrow speed to ambush people and get around the low speed, but even then foes could simply disengage and leave you unable to retaliate.