Wanted to close the loop on this.
The current Alexa API does not support "Alex, turn on zone 1 for 10 minutes".
In order to do that you would need to use the Alexa Smart Home Skills API.
Plus, your customers don't need to remember your skill name or a specific invocation phrase.
Unfortunately, the Alexa Smart Home Skills API at this time only supports lights and thermostats
The Smart Home Skill API currently supports lights, switches, smart plugs and thermostats, including turning devices on and off, dimming lights and thermostat controls.
I'm going to reach out to the Alexa team and see if they have plans on creating an irrigation profile which would help enable this.
We are looking at other ways to incorporate Alexa technology, so stay tuned!