The front page shows the 4 most recent items posted to "Gardening Ideas". The idea is for the homepage to move quickly and keep showing new and fresh stuff appearing. Something that was posted 3 days ago isn't new anymore.
Since it was posted, there have been 4 new items posted, so it got pushed down.
But it's still available through all the other areas of the site, including, of course, under the Gardening Ideas tab.
