Ficus - just about any ficus will give you the banyan effect.
Wood, if you want wood, and don't care that it will rot - use wood. The deterioration could be viewed as part of the dynamic nature of the piece. Wood rot is likely to proceed faster than the ficus will develop, that's the only draw back.
Plenty of different 'plastic' wood substitutes.
Concrete can be cured quite simply by filling a bucket large enough to submerge the piece. Soak the concrete piece for 2 to 4 months. Change water in the bucket once every couple weeks. When the water tastes like water, no acrid or metallic taste, or when a goldfish dropped in lives for at least a week. It is fully cured. Will never leach significant Ca(OH)2 again. Can then be used for plants or fish tank.
Fish tanks and ponds were made from concrete since concrete was invented, once cured it is perfectly safe.
Or coat with a water proof product as suggested by Sorce.
Wear gloves while "creating" with concrete, while it is still plastic, before setting. "Wet" or still soft concrete can burn the skin pretty badly, and do it without causing pain at the time it is happening, pain will come later. So wear gloves while shaping your concrete. And eye protection. Just in case cement dust gets into the air.
hahaha
