An asparagus fern tells you when it needs a new pot. When the the root ball gets too big, the roots are so strong they break the growers pot or start forcing the soil up over the edge of the pot. If this isn't happening and you still think your fern needs a larger pot, gently pull the fern out of its pot. If the roots have filled the pot and there is very little soil left, it's time for a new container.