Weeds will compete with the avocado roots for oxygen, nutrients and soil moisture. Keep the area under the tree and just beyond the drip line free of weeds. Long grass can also be a health and safety risk as they can mask ground hazards (e.g. open drains, rabbit burrows).
Three methods of controlling weeds are application of mulch, chemical treatments and mechanical removal. For more information on weed control download the weed control chapter from the grower manual below.