The only way to describe their "argument" is insane troll logic, maybe even worse than that, it just doesn't make any sense and doesn't seem to be related to their argument.
Some of that may be poor translation on my part. It's difficult to translate the flow of a long speech when you're translating one sentence at a time. I'll come back later and smooth out the choppiness.
Without the sarcasm and mouthing-off their argument boils down to asking mister vegetable if it's taking it easy. Since it doesn't start whining and demanding, they assume it's growing on its own.
Plant -> not a yukkuri -> not alive -> doesn't need food or water.
Human that gives food and water to plant -> human that gives food and water to dirt -> moron.
Well, it makes sense from a yukkuri point of view... kind of... if you don't recognise anything that doesn't move and make noise as a living thing... then a plant isn't living, is it? So it doesn't need water? You stupid? It was just there! It grew on its own!
...
I... I've been here too long...
Reimu "Yuyuyu? Why is Miss picking up mister shovel?"
Marisa "Little ones! Everyone wake up no ze! It's as usual ze!"
Reimyu "Yunya yunya... Reimyu'sh... shtill shleepy..."
Maricha "Mishter food? Ich mishter food? Maricha'sh waking up!"Miss "...wrong... umm... that's... en, enough... already!"Reimu "Going to dig Reimu's family a house with mister shovel?"
Marisa "Don't need such a poorly-shaped house no ze! Marisa's way is more skillful ze!"Miss "... even if you apologize... I... won't forgive!"Reimu "Mister vegetable is taking it easy with mister vegetable's circumstances!"
Marisa "So it's growing on its own no ze! Understand properly ze? Yu~n?"
Reimu "Mister vegetable absolutely won't grow with human's wishes!"
Marisa "Miss's lies to yukkuri need propper adjustment no ze!"