The main stem on these plants sometimes dies back, but the low-down branches will persist... this feature is especially evident in seedlings, where the main stem basically terminates when it branches early on close to ground level. I agree with Thijs' advice. You can restart the arms from cuttings but you may see the same thing happen afterwards to them. This is not a small plant in the ground and it tends to require some space in containers after a few years. I have never tried to trim the roots.