he space to get a good head on the top.  If you buy plants from a nursery, the plants can be stressed out and pot bound.  If this is the case, your broccoli might mature too fast and "button".  The means the plant will produce the fruit or flower much earlier than it should.  The broccoli flower will be too small and may be bitter.  To avoid this, plant from seed or buy plants that are small and don't have woody stems.  Broccoli likes well drained soil that is kept evenly moist throughout the growing season.  Use compost and manure to keep nutrients available to the plant.  Remember, this is a cool weather crop, so don't plant with your warm weather crops like tomatoes.