I have left my little trees out, unprotected, ever since early April, unless temps were
projected to go below 40°. They are now in pots, buried about 5" into the ground,
so they don't come in at all now. I could show you a picture, but my camera is being
wonky and I don't know what to do to fix it, so the only "current" pic i have is from
my phone- and it really shows. I think you can leave yours out, but keep it protected
from too much sun during the day.
Oh, I got one of those meters too, but I had to get it at a nursery- amazingly only $15
there- Lowe's, Home Depot, WalMart (around here) didn't have them. Mine also has
a measure for "fertility". I guess it tells you how much plant food is in there? I've only
used the moisture and pH functions (I can tell by looking around how much light they
are getting!).