There are several possible ways to implement a custom weather algorithm, but first of all, please understand that to translate this idea to algorithm, it needs to be more precisely defined. In your example, a condition like ‘If over 85F’ needs to be defined more precisely: did you mean the maximum temperature of a day, of the past 3 days, of the past week, or average temperature, or what is the temperature referring to? Also the action ‘water every 3 days’ — how long should this action persist before it re-checks the condition again? For example, if yesterday’s max temperature is more than 85, but today it’s less than 85, what should be the appropriate action?
In short, while your description seems intuitive and makes sense, to translate it to a computer algorithm requires more precise description.