1" ID low pressure black poly line. Depth just has to be deeper than the ground will be worked. Try to use the minimal amount of connectors underground as the plastic like shrinks and expands with the temp change. Cost is a location thing, find a landscaper that puts in irrigation systems and they can get in the ground pretty cheaply without tearing up the ground if they have a vibratory slitter. I've had them slit/pull 1" 900 feet with no issues, and 1 1/4" line over 1/4 mile.
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