The actual placement of an outlet within your landscaping project needs to be thought out carefully as they are difficult to relocate if you place it wrong the first time. Installing an outlet outdoors means purchasing the materials that were specifically designed for the application - weatherproof / outdoor.
Weatherproof utility boxes and outdoor receptacle covers, as shown in Figure 1, are a necessity if you do not want the circuit to trip every time it rains or when you start watering your garden.
Figure 1 - Outdoor outlet with weatherproof cover
Powering the circuit through a GFCI (ground fault circuit interrupter) is mandatory for safety purposes.
It is best if you can install the circuit directly to the main distribution panel / load center, powered through a GFCI breaker. If that is impracticable, because of access, then the circuit can be powered from a GFCI outlet / receptacle. Note: Only one GFCI control device is required on any circuit. All electrical outlets / receptacles and devices wired from the GFCI are protected by that one device.
It should be noted that multiple GFCI protection devices on the same circuit will cause false tripping.
Outlets should never be placed horizontally, facing up or down, as water will penetrate the device and cause a short circuit. The proper position is vertical. The bottom of the outlet should be a minimum of 6” above the top of the ground in your garden, and that includes any mulch that you use as a ground dressing.
When determining the best placement for the outdoor electrical outlet, keep in mind that you will be doing your day-to-day gardening around it. Keep it away from the path that your lawnmower takes!
It is also wise to consider the electrical outlet placement as it relates to any sprinkler heads. If possible, situate the front of the outlet so that it will face away from the direct impact of sprinkler spray. Placing electrical outlets in area that may be inundated with shrubs is not a wise decision, as someday you will want to be able to plug and unplug electrical devices.
The same weatherproof outlet devices can be used on decks to provide power to lighting, barbeque accessories and other general electrical needs.