The world of Baros (b'a:rəs) has low gravity (around 0.5g), thick, moist atmosphere (2-3 bar), and lots of volcanoes. Humans mostly inhabit sunny elevated areas, separated by relatively dark and CO2-rich lowlands. The humanity is at roughly medieval tech level.
Now one of the features I want there is a plant that grows on bare cliffs, using symbiotic bacteria and its own roots to cut its way through micro cracks. This plant is domesticated and selected for rapid growth and deeper rooting. Bacteria are producing oxalic acid which is already used by lichens to crush basalt. UPD: The wild variation relies on birds for nutrients and seed distribution, and grows berries to attract them. Domestic one is dependent on humans' aid.
The rock in question is similar to Earth's alkali basalts (Mg/Ca/Na/K silicates with Fe, Ti and Al oxides) and should be able to withstand temperature changes, water, knocking, and scratching for ages.
Say I want the roots to penetrate 5-10cm of rock in 2-3 months. Afterwards other means (driving in wedges and making them swallow? Or maybe just a sledgehammer?) would be used to dismantle weakened rock into convenient small blocks of stone. Nice terraces are left behind.
Is it possible at all? Should it be a shrub? A grass? (Trees seem to be ruled out, for one cannot plant trees in tight rows - or...?) Maybe such plants already exist on Earth? Last but not least, is such technique feasible at all for a medieval society?