Please be very careful with this.

If you must have the plant, keep it potted in a container well away from ground soil; Japanese knotweed is highly invasive, threatens native species, quickly spreads underground well beyond the actual visible portion of the plant, and is hard to control.

From USDA APHIS, emphasis is mine:

>Invasive knotweeds were introduced into North America from Japan during the late 19th century. Japanese, giant, and bohemian knotweeds are large herbaceous perennials, which have spread throughout much of North America. The greatest infestations are located in the Pacific Northwest, the Northeastern United States, and eastern Canada. Knotweeds have become especially problematic along the banks and floodplains of rivers and streams, where they crowd out native plants and potentially affect stream nutrients and disrupt local ecosystems.

Basically, if you plant it at the edge of your property, it will be crossing that border into neighbors' yards or unmanaged/unmonitored woodlands and will continue to spread.

You do you, obviously, but if I were you, I'd look to get capsules or tinctures through a medicinal or holistic supplier.