Looking for a long-distance (~1km) physical layer
Yes!! For RS-485 network common grounding is needed. It is achieved by running a separate GND wires between all nodes. The RS-485 pair needs to be a twisted pair. And for these long distances you may want to consider the use of some type of shielding for the cable.

The common GND is needed to help maintain the common mode voltage between any two nodes to within the range needed to keep your receivers working. A normal range is something like -8V to +12V CM. You can get special transceiver chips with extended common mode voltage voltage range (i.e. TI's SN65HVD1785 family of parts) supports a range from -20V to +25V. But even with this you need to have a common GND wire. 

You can operate your RS-485 network at these distances with good cables and proper types of terminations at rates in the neighborhood of 115.2K baud. But keep in mind that your 1 km distance should be the total cable distance and not the distance between multiple different nodes.  

For successful design and deployment of RS-485 networks I strongly recommend that you read and comprehend these application note documents:

Application Note 847 FAILSAFE Biasing of Differential Buses

http://www.ti.com/lit/an/snla031/snla031.pdf

AN-1057 Ten Ways to Bulletproof RS-485 Interfaces

http://www.ti.com/lit/an/snla049b/snla049b.pdf

B&amp;B Electonics RS-422 and RS-485 Applications eBook

http://www.bb-elec.com/Learning-Center/All-White-Papers/Serial/RS-422-and-RS-485-Applications-eBook/RS-422-RS-485-eBook-graphics-embedded.pdf