Yes you can shorten your column by trimming the column shaft from the bottom. The bottom of tapered columns is the largest end. Many columns have a moulding around the top end of the column shaft. Do not trim this end of the column shaft. Trimming a tapered column may affect the fit of the base. Be sure to pre-fit the column base to the shaft prior to trimming it to length to ensure a proper fit. Some columns sit on the base, and others pass through the base. Be sure to check this before trimming your column to the required overall height.
