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

[Jerry De Santis]

If the grass is wet (Dew) or not cut very short, in my opinion it is an accident waiting to happen.

[mshac]

Originally Posted by Jerry De Santis

If the grass is wet (Dew) or not cut very short, in my opinion it is an accident waiting to happen.

I've had pilots argue that "grass is grass" i.e. every grass strip is the same. IMHO, this attitude comes from a lack of experience on grass. As Mr De Santis said, wet grass makes a big difference, especially on landing when you slide instead of stop. Long grass will increase your take off roll far more than the 10% the book says to add. Long grass combined with soft soil can cause a nose-over and prop strike upon touchdown. Soft, unimproved soil vs hard soil that's regularly rolled is another big variable. Soft/short fields are where the men are separated from the boys.