Squidward's Flower Garden, so far, only appears in the TV movie "Truth or Square" in a flashback of Squidward. Before SpongeBob lived beside Squidward, Squidward had a small flower garden, where he probably watered and worked on his garden everyday.
A boat from above water tipped, and a pineapple that is now SpongeBob's House landed on it, destroying all of his plants and the little fountain that was there. This could possibly be the reason that Squidward hates SpongeBob. There is a rumor that the plants around SpongeBob's house are from the garden, and the windows, door, and pipe are made from the fountain. The stones could also be part of SpongeBob's walkway. Squidward's garden is on the side of his house closest to SpongeBob.
The garden is bordered with the same flowers that are on the sides of SpongeBob's house with normal flowers in between just behind them, half the ground surrounding the statue is stone, the other is grass, there is a statue of a squid (that resembles Squidward) spitting water into the small fountain below it. But currently only the flowers remain, and possibly some remains of the fountain.
The outline size of SpongeBob's house could be the same size as the garden, since the flowers on the sides of SpongeBob's house could be the flowers from Squidward's garden.