This idea is very interesting, since it combines ideas from other domains. For example, your idea of Gratitude Gardens incorporates architecture and the effects of architecture on people. The underlying belief is that architecture and fauna can positively effect humans, such as by providing more light and greenery, we feel more connected to our physical world. This provides contrast to our increasingly digitized workplaces. Your idea also incorporates the notion of continuity, that gratitude is endless and can be constantly memorialized to incentivize each other.
I like how this idea imbeds gratefulness into our daily work habits, such as logging into the company computer system. Your idea is very powerful because it forces us to stop and think about some idea we are grateful for. I would like to add that it would help if management can compile what everyone is grateful for every day. Then show the results (such as a Word Frequency on the items the coworker is most grateful for) to the specific coworker so that coworker can reflect upon their gratitude. I would also like to see this idea implemented so that coworkers can share gratitude to each other. Perhaps, they can show their results to each other and just reflect upon it. Either way, your idea is a great start!
Interesting idea. I like how your idea places a restriction on word count. This forces everyone to write brief, yet powerful and direct phrases to coworkers. Sometimes, it's better to get to the point with a message, especially when it comes to gratitude. I also like that coworkers need to view their sticky notes on Monday, rather than on Friday. Since Monday is the beginning of the week, this can increase worker morale at the beginning of the week!