You basically have 2 choices.

1. a proper installed run of Underground Feeder (UF) cable from the house to the bed.

2. A run of conduit with the proper sized conductors to the flower bed.

A stake with a section of conduit and a weather tight outlet box attached to it will have to be installed at the bed.

If you have no knowledge of electrical wiring, hire an electrician to do it.

The major cost will be the time to dig the trench since the electrician will be charging for time & materials.
You might ask if you can help save some $$ by digging the trench yourself but for that distance you'd probably have to rent a trencher to get the necessary depth needed.

Get at least two bids...........