Bring 5V DC 20 meters away
Unless you are prepared to dig a trench and bury an expensive weatherproof cable (or a conduit) to bring 220v across from the house or wherever to the garden, you should convert the 220v to DC beforehand.

However, you cannot just plug in a 220v AC to 5v DC adapter, and run a low-voltage line (which would not require a conduit or weatherproof wire), because the cable will have some resistance, and there will be a voltage drop.  Can't say how much, without knowing the current draw and and the gauge of the wire.

So it would be better to transmit a higher voltage, say 12 volts, and then have a 5v regulator at the far end which would provide the necessary power.

This article has a table where you calculate of a wire of various gauges.  Remember to double this, because you have two wires.