So they spell says that the plant can do stuff for you as long long as they are not uprooted.

Let's day I have a plant grow inside of a dead body. Will the plant be able to control the dead body if I use speak with plants?

It's not uprooted and it's not like it's attacking (or maybe it can)

It seems like you’re trying to use speak with plants to cast animate dead. I’d rule no. But another DM might let you, cause it is a pretty fun idea to have a zombie made of daffodils.

What if i remove all their organs and replace it with soil? And have a garden oh dead. They are not considered undead, they are just plants moving a body in a really creepy way

I mean, as long as you just the plant statistics, mechanically, and don't try to use the zombie stats, I might let that go. But either way, this is a conversation to have with your DM.

Keep in mind that the spell specifies a "simple" command. Puppeteering a corpse is not a simple process.

Not only does it require continuous simultaneous control over multiple limbs, joints, and pieces of anatomy, it would also require a strong sense of balance, which is not a given. Even if the DM allowed the plant to animate a corpse in a humanoid way, stacking on any additional behaviors would compound the complexity.

For the sake of mechanics, you could use the Pot of Awakening and Awakened Shrub for reference.

With a Strength score of 3, an awakened Shrub wouldn't be able to lift a stocky Halfling.

That said, and as Xalthu was speaking to, if you want to build your character around this concept, then that is a perfectly legitimate thing for you and your DM to collaborate on. A custom 3rd level spell to create a plant-based companion is good fodder for homebrew.

I'd say no.

Consider Pot of Awakening or the Awaken spell, or a reflavoured Animate Dead.