There's a compliance path for just this type of situation. As long as the property's landscaped area is equal to at least 5% of the total site area, the credit language and other documents state you are eligible for the credit. If the project is registered, check out the form in LEED Online for how to document compliance based on zero use of irrigation (and if it's not registered yet, it's a good idea to do so and always look at the LEED Online forms early on to get a full sense of the compliance and documentation options). Also note that you'll need to make sure there's no regular hand-watering going on, as sometimes even if there's not a permanent irrigation system the grounds are still regularly watered.