This is actually been around awhile and it hasn’t worked well for me. I ended up using IFTTT with tigger receipes…
I am brain dead these days! I got the email from Amazon today! And I tried it out once and it did work! 
As such Jersey already hitting 90’s, the native Rachio app is what I prefer! Set it up and forget it!
Yea, I’ve been able to do the same using IFTTT, but this is nice as it removes the need for IFTTT all-together… and this is the last thing I have IFTTT running in the middle for in my setup.
It’s worked for me for a couple of weeks. Just wish there was an option to “ask Rachio to start schedule”, rather than just one zone.
I think the problem I have is saying the word “Rachio” doesn’t seem natural to me. This is why I’d rather say “Alexa trigger sprinklers on”.
I used it couple of times in the past month to delay schedule when it was raining. It worked well for me.
Can’t you just say “start watering”? I thought I saw that somewhere… Yeah, here - How do I use Alexa with my Rachio?
I’ve had this “hook-up” for about a month. I think “start watering” is a new option. But I could be wrong, that’s what my wife tells me all of the time. With about 7 inches of rain here in Omaha the last 3 weeks I haven’t had a chance to play with this. Now it looks like we’re gonna dry out a bit. So at least I got that to look forward to.
I have been experimenting with controlling Rachio named zones using Alexa Routines. My fiance and I are both handicapped and use Alexa for voice control for almost all of our ST devices. Currently, Rachio is working on enhancing SmartThings’ Rachio device support. I found that if you have the Alexa Rachio Skill installed, this will NOT work. You must uninstall the Rachio Skill. Since I brought this “bug” to the attention of Rachio developers today, maybe they will fix this in the future where you can have the Rachio Skill installed and use Alexa’s Rachio devices in Routines.
Here is a link for instructions on how I created a Alexa Routine to turn on a single Rachio zone for 15 seconds and then turn off the zone: https://www.echotalk.org/index.php/topic,5664.0.html
Yes, the actions could be created in SmartThings, but not with voice control.
Following the structure I used, you could modify it to add more zones and timers to create custom watering Routines.