will toss out content and only go for new posts when the site has reached a certain level of maturity.
Well, there's a perspective issue here. You're thinking purely as the owner of one site, and you've clouded your own vision.
Google hasn't forgotten or tossed out your site. It simply thinks that it isn't important enough to rank highly out of tens of millions. The site is indexed, remembered, and probably ranks well for some more specific phrases that relate to specific phrases and paragraphs on your site.
Now think of the broader world, and the millions of pages about water gardens, about gardens with water features, and about gardens that need watering. As I said before, from Google's perspective, this site appears abandoned and forgotten even by its creator. It may be as good as any other site, but that's not enough. I don't stop using what I have to use something as good. I only stop using what I have to use something
better than what I have.
Make something special. Something market-leading. You'll find that the SERPs, and your traffic, will react to that lead. We're back to Seth Godin's "Purple Cow" - something remarkable.
Your water garden site should have seasonal updates. A calendar of when to do what things perhaps. It should deal with issues such as ice, burst pipes, dry seasons, water shortages, unwanted insects, fungi, and all other important (to someone it will be vital) aspects of tending to the water garden in general, for all situations you can cover.
Then it becomes a top resource. Something special. Something people will love because they see that you love it. Something where they get to share your passion and interest. Something people will link to without being asked to, simply because they want to share it.
Suggested Inspirational Reading:
Unleashing the Idea Virus, Purple Cow - Seth Godin
Cluetrain Manifesto - Doc Searls, et al
Tipping Point - (Malcolm Gladwell?)
Note well that nothing here is about 'promotion' in the classic sense. Certainly no link swapping or mass link building via "push marketing". This is
real marketing. Having a product people will want, not one they have to be forced, tricked, or nagged into taking.