When using a soil moisture sensor on trees, plants. flowers, or even turf it is important the soil moisture sensor is located at or near the root ball of your plant.
Daisy is primarily designed to be used in potted plants both indoor and outdoors but should be placed in such a fashion that the bottom of the sensor is adjacent to the root ball and that the top of the sensor is capable of communication to your cell phone. Therefore if you wish to grow trees, daisy as a product is probably not optimum.
For larger plants or trees we recommend ECCO PRO sensors. This device consists of the Daisy data logger (the computer at the top of daisy) connected to our professional line of soil moisture sensors called ECCO PRO. ECCO PRO sensors are tethered by a wire to allow you to place the sensors at the root ball of your crops while allowing the electronics to reside above soil and communicate to your smart phone or tablet.
One last thing. Farmers purchase soil moisture sensors called profile sensors ($1000 - 2000 ea) to manage their crops. Profile sensors are typically 36 inches deep and measures soil moisture at 4 depths. ECCO PRO will be offered with 2 or 4 sensors. Therefore you can purchase 1 device to be used in up to 4 plants if you choose to place one sensor per plant.
Alternately, if you wanted to make a poor man's profile sensor for monitoring crops, then place the 4 sensors at various depths. We suggest: The first sensor is located to measure soil moisture when the plant is a seedling, the second sensor placement at the root ball when the plant is at mid maturity, the third sensor placed at a depth of your plants root ball at maturity, and the 4th sensor can be placed below the 3rd sensor to flag if you are watering too much...
We are happy to have your support and thanks for your questions.