If possible, greenery tiles must be placed next to another tile that you own. If you have no available area next to your tiles, or if you have no tiles at all, you may place the greenery tile on any available area. Place a player marker on it to mark your ownership.
When placing a greenery tile, you increase the oxygen level, if possible, and also your Terraform Rating (TR). If you can't raise the oxygen level you don't get the increase in TR either. Greenery tiles are worth 1VP at the end of the game, and also provide 1VP to any adjacent city.