Soil moisture sensors are an option.
Humidity and temperature sensors give you an idea on the amount of evaporation, so how much water your plants need.
The local weather forecast (some services may provide an API, otherwise you have to web scrape) can give you a forecast of whether it will rain (but not how much).
Rain gauges can tell you if it rains, and how much (if it's just a few drops after a hot, dry day, you still have to water).
Even brightness sensors may help tell you if rain is coming, as it tends to get darker before rain (thicker clouds).
The soil sensors I thought would be a great idea so I tested 3 of them in some plants around the house. They lasted a couple of weeks. Not a reliable situation on the test I have done. Maybe someone has had better luck with them and could share their findings.
I am looking at quarrying info on local weather report Throwing a few if maybe hope so and than bingo come out and decide when it will rain or has it rain. I wonder if any one has done something like that with the Arduino??
Thanks for the feedback