Ground rules on growling
Growling and barking are very natural ways for dogs to verbally communicate with people, other dogs, and other animals. A dog's growl may not be a pleasant sound, but it's an important part of their behaviour pattern—and one you should understand.
I'm watching you
When a good guard dog feels their territory is being threatened, they bark or growl. And since they do this at people or dogs that are passing by, they feel they did their job by protecting their home turf and the family in it that they love.
What can you do to control it? A simple way to help control a dog's growling in this situation is to block their view of the street or property line. In addition, keep your dog mentally and physically stimulated through exercise, play and toys.
I'm bored, therefore I bark
Working and sporting dogs often bark due to sheer boredom. These dogs were bred to run around and to work the great outdoors. So to make up for the lack of aerobic exercise, they choose to exercise their vocal chords by barking or growling.
What can you do to control it? You can help to alleviate their boredom by setting aside a couple of extra hours each day for some energetic and fun playtime. Also consider agility training and other dog sports such as flyball.
It's just nerves
If you obtained your dog from a shelter it is possible that they may have a history of neglect and abuse. Through no fault of their own, these dogs were never given the opportunity to develop proper social skills early on. Thus, they growl and bark a lot.
What can you do to control it? A great deal of patience is required in these cases. Dogs with a history of neglect or abuse take a bit longer to train, but with a little understanding and extra care in building trust, in time they can come around. It's important not to reinforce the behaviours you don't want your dog to repeat. Instead - reward the behaviours you do want to see.
Growling and senior dogs
Growling is not uncommon in ageing dogs. In some cases it's an early sign of senility or cognitive dysfunction.
What can you do to control it? Reward-training and disruption techniques can work well in some cases. It's important to be patient and consistent. Contrary to popular belief, you can teach an old dog new tricks. Also get a vet check up for your senior dog to ensure there isn't an underlying medical condition.
In more extreme cases, a dog's growling could be due to a genuine medical condition, such as joint pain exacerbated when 'kindly' strangers touch them. For those types of conditions, medication, modified exercise and weight control can help. If your senior dog has suddenly begun to growl often, take them to the veterinarian for an examination and to discuss your options.