Unless the manufacturer of the power unit for the system specifically allows a differant cable type or conductor type in their installation instructions, aluminum would not be permitted IMO.

In UL's White Book, under the heading of Landscape Lighting Systems, Low Voltage (IFDH), the conductors should be listed SPT-3 or SPT-2-W.

Under the heading of Flexible Cord (zjcz) in the first paragraph " All conductors are stranded copper."

Few things are harder to put up with than the annoyance of a good example.