Yes, to use your Gro Sensor or Sensors you will need a Gro Hub. You can connect up to 32 Sensors to the same Hub as long as they are all within range.
The Gro Hub plugs into an indoor outlet and connects to your home network over Wi-Fi.
The starter kit available at mygro.com contains one Gro Sensor and one Gro Hub. Expansion kits of one Sensor are also available.