There are sterile or seedless forms of many forest garden plants including many weedy species. I'd love to hear from people here about any sterile froms of the species profiled on the wiki.
TREES
Diospyros - some Asian persimmons are self-fertile and seedless if not pollinated.
Cudrania - Hidden Springs offers a seedless che fruit.
SHRUBS
Hippophae - seaberry. Male clones will fix nitrogen but not make fruit.
VINES
Vitis - there are many seedless grapes.
HERBS
Allium - "Profusion" chives is seedless.
Rumex - "Profusion" sorrel never even flowers.
Rumex - a male clone of sheep sorrel would be easy to find and seedless.
Urtica - nettle - an all-male clone is probably out there somewhere, and would be a great addition to forest gardens.
Petasites - Fuki. The giant form is apparently sterile. This is good because what a weed!
Lotus - Birdsfoot Trefoil is fairly weedy, but the "plenus" prostrate form is sterile.
Asparagus - many all-male clones are available and are the industry standard for commercial production.