Tricky question. If there are people in the photograph you will need a release. If it is just lawn with no identifiable landscape features or architectural features I wouldn't worry about. Grass is grass. But if there are you may want to contact the garden and see what their policy is becuase it may depend on the institution. Whether or not you credit where the picture was taken is a judgement call too. If the picture is for a positive application I would credit it. If using as a negative application, such as this is a horrible looking lawn, I wouldn't credit it.

Also you need to look into whether or not there is a sign or other feature in the picture that may have a registered trademark associated with it. If there is you will need a release.

When in doubt you may just go for a release.

Most of the bontanic and community gardens including city parks in this area require a written release plus they charge a fee (even for wedding photos, whether or not the wedding takes place in the park). If there wasn't a sign posting their policies at the institution where you took the photos I would think you would be ok. But if any identifiable features are visible in the photos I wouldn't publish them without a release.