OK, I have to say straight away that there is not much research on therapeutic influence of keeping plants and looking after them in depression disorder, but there is a little bit more information about pets.
Pet therapy apparently is especially effective in PTSD (post-traumatic stress disorder), and as something encouraging social interaction and keeping depression at bay in elderly people.
As you know depression treatment involves socializing, communicating with people, even if you don't feel like it. Pets can act as your friends; they give you unconditional love and none of the baggage. They trigger memories of happiness and calm you down. They also definitely reduce stress and anxiety.
There was a study group in one of the nursing homes in U.S. few years ago which was given interaction with various pets trained for pet therapy - dogs, a rabbit, a goat, a ferret and a cat.
The results showed that the group decreased its social avoidance, the participants became less depressed.
There are also some therapists in U.S. who prefer to bring their pets to work, saying that it instantly comforts the patient and relaxes them. When emotions get too intense pets provide a momentary distraction, you can touch them when you need contact much easier than people.
Some of the benefits of having a pet (or "an animal counsellor" as they say it in pet therapy) is that they make you feel needed, useful, give you a purpose, which in turn increases you level of self-esteem. They also reduce your feeling of loneliness and isolation, which is always one of the telltale symptoms of depression.
Now, about the plants. What I could find is that plant therapy can be one of the components of depression help if you treat your plant as your pet, give it a name, talk to it and tell it about your fears and insecurities. You also involve colour therapy into the mix. Choose flowering plants - marigold, peace lily, etc. Pink flowers for anger management, purple - for creativity, blue - for soothing effect, yellow - for warmth and positivity.
Don't forget to choose an indoor plant and consider how much light it would need and how much light you get in your house.
But, honestly, I think pet therapy offers you an almost instant feedback and gratification where with flowers it looks like pretty one-sided relationships.
Also consider the severity of your depression before you get your pet/plant. If at times it becomes debilitating, you need a fail-safe, someone who can look after them when you can't. So ask your therapist for advice, and talk to one of your friends and relatives and see if they agree to help when you need it.
So, what do you think? Did you try this therapy? Please, share your experience and thoughts.