Like Shazam said, all the fabric will do is stop those weeds that spread by roots. The only ones I know that do that are grasses, and they don't root deeper than a couple of inches.
My rose bushes are 1 year old and have roots at least that deep. My tiger lillies are into their 5th season and they have roots going down at least 12 or 15 inches.
Landscape fabric is designed to be placed on the surface of the soil; like between gravel and soil. Not burried under it.
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