RTK is for real-time centimetre-level accuracy. I don't think that you need such a bazooka to locate trees in a farm (I assume that those trees are isolated if you talk about a farm and not a forest). Furthermore, RTK relies on signal emitted from a ground based station which might not be available in your area (especially if it is mountainous).
On the other hand, there are intermediate solutions between the 100 $ trekking GPS and the 25000$ precision GPS. For instance, a "WAAS-enabled" receptor could help you consolidate your observations (and to achieve around 1m precision in the USA). If you are in an area where it is difficult to have GPS signal (mountain, important tree cover, high latitude...), having GPS+GLONASS receptors and/or a good external antenna could also be helpfull.
Finally, you should consider the protocol of acquisition:
a good planning of the time of the day for your observations (using almanac and some free softwares) help you make sure that you will have enough satellites available in your mountainous area
standing at the exact same position away from your tree (e.g. 15 m North) helps you get more signal
repeated measurements improve your confidence interval. If possible more than one minute because there is some temporal autocorrelation of the errors.