Shortest field you've been to.

[general]

You could easily use 1800 ft but the trees have to be considered very carefully.
I regularly fly out of a 1400 foot field at about 1500agl feet/ no trees.
I normally have no more than one-third to half tanks. And only one or two people.
And I've never tried it over 90 degree f.
A lot of it depends on prevailing winds and your ability. I've been in and out of many jungle strips that were rough and less than 2000ft.
Do not take any chances. Also have you flown off of dirt/grass?
Again there's a big difference between a 30 foot tree and a 70 foot tree. Make sure you know all the limitations/ temperature, wind, field conditions, your aircraft, importantly your ability and weight. Don't even try it or think about it until you practice a lot of short/ takeoffs and landings at various locations and you get signed off by an instructor.
Another important factor is insurance