You can cut the top at any point that you like. As it grows the cane is the mature trunk that forms as it sheds its leaves. I've never heard of purchasing just the mature cane unless it has some top growth. You can take the top portion you cut off and root it and have another plant. New growth will sprout from the cane you leave in the soil. It would look something like this.
You could also propagate the top portion by air layering or cut the cane in sections and propagate that as well. Take a look here on how to propagate.
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.