About how far is the distance of allelopathic plants from one another?

The length of the root is the distance of allelopathic plants from one another. The plants in question -- aster, black walnut, broccoli, forsythia, garlic mustard, goldenrod, juniper, Kentucky bluegrass, knapweed, perennial rye, purple loosestrife, sugar maple, sunflower -- may release toxins -- of which the most famous is the black walnut tree's juglone -- to control or inhibit the growth of other vegetation. Their effects -- which may be minimized by well-aerated, well-drained soils -- will be strongest as far across and down as the root system extends for herbaceous plants and the drip line for woody plants.