Update for info.
So I used last year and continue to use 6 ‘nodes’ of 4 ds18b20 one-wire waterproof sensors (potted in a stainless steel tube) on a single bus ~60m of outdoor grade cat5e. Each ‘node’ consisted of a set of 4 sensors taped at to a 50 cm long bamboo cane at intervals. Cane simple inserted into an augered hole in the soil and sensors pressed up against the side of the hole before backfilling. Sensor bus connected to a OWplug from taaralabs.eu and logged by a Sodaq Mbili with a GPRSbee sending the data by ftp to my server as csv files. Was pretty reliable last year, but seem to be having issues with condensation in the junction boxes this year. The sensors all record within about 0.1 to 0.2°C of each other when checked in a waterbath.