You will need some basic experience with soldering, breadboarding and using the Arduino platform for this tutorial.
This particular instructable is for a sensor that will upload it's data via a WIFI connection. If you house it correctly, you can keep it outside. If this seems a bit adventurous for you, check out the ethernet version here.
To build a plant sensor, you will need the following:
] 1 x Arduino Uno
] 1 x WIfly shield + headers
] 1 x USB A>B cable
] 1 x 5v Relay
] 1 x ZTX605 -type transisitor
] 1 x DHT22 Temperature + Humidity sensor
] 1 x LDR (Light dependent resistor)
] 4 x 10Kohm resistors
] 2 x Nails (Any nails will do for now)
] Plenty of hook up wire (single core wire)
] A Soldering iron & plenty of solder
] A Mac or windows computer
Once you think you have everything, you can continue to the next step to get started!