Brad, you may have answered this before but do you provide any cold protection for your seedlings during the winter? I live in a similar climate to you and have planted out a large number of mango seedlings in ground. Lost a few to the chill last year, but they were mostly the weaklings. Trying to determine if there is anything else I can do this winter in case of a rare cold front so that I can protect my 1-2 year old seedling plants.
OK I'm not Brad, but I have solved this problem after a bad frost a couple years back damaged some of my young avocado trees. When I was traveling for work, I returned home to find major frost damage on the young avocados so I set out to find a solution which would work even if I am not home.
I already had a personal weather station (PWS) which gave me the temperature right in my grove, literally in the middle of the avocado section. I bought an outdoor "smart outlet", and set up an IFTTT account - IFTTT is a website (If This Then That) that lets users easily create an "applet" that automates one action based on one other input. In my case I set up an applet that turns on the smart outlet when the PWS temp drops below 35F. I connected the smart outlet to a string of old incandescent christmas lights and wrapped them around the young avocados. The strings are 175 watts each, but you can daisy chain them of course to get more heat. So now in my grove, when the temp in the avocado section drops below 35, the lights come on and produce heat for the young trees. IFTTT turns them off when the temp rises above 35 again. IFTTT is free for simple users like this and only a few $ per month for very complex users, creating this one applet would be free.
A PWS is about $175-200. A smart outlet is $20-30 on Amazon. Lights and extension cords are maybe $50-100 depending on what you have lying around. A PWS is not actually required but without your own you are depending on a weather station nearby which may be unreliable or may not match your micro climate in your yard. You can use Weather underground's closest station to trigger IFTTT but I wanted control so I use my own PWS.
I use the PWS and IFTTT to also automate my sprinkler timer (Rachio) so that if the temp rises above 105, the Avocado's get an extra watering session. Kept them all cool and happy through the multiple 115+ degree days we had in Sept.
Hope this helps anyone who is looking for a solution to keep Mango's thriving when it gets cold out. If you have questions feel free to ping me and I will help you out, this sort of automation is super easy but its also my day job, so if you have questions, just ask. I've read this whole thread and I really appreciate the info here... I'll be doing a little more research then stopping by again for some feedback/questions. Til then, happy growing.
/Dave