Potatoes and Fennel
Antagonist Pair — Keep Apart
Fennel's suppressive effect on neighboring plants ranks among the more solidly evidenced antagonist findings on this site. The specific concern that a below-ground tuber is more directly exposed to soil-based allelopathic compounds than an above-ground fruit is a reasonable extension of that documented mechanism rather than something separately trial-tested for potatoes specifically.
Mechanism: Allelopathy (Growth-Inhibiting)
Potatoes share the nightshade family's documented sensitivity to fennel's allelopathic chemistry - anethole-family compounds released from its roots and foliage that suppress growth in a range of neighboring plants - but potatoes carry a distinct vulnerability among fennel's antagonist pairings on this site: the edible part of a potato plant develops entirely underground, forming and sizing up directly in whatever soil fennel's root exudates have been influencing all season, rather than hanging as fruit above ground the way a tomato or pepper does.
Potatoes raise a version of the fennel allelopathy concern that's a little different from tomatoes, peppers, or eggplant, because of where the part you're actually trying to grow develops.
The Edible Part Grows in the Soil, Not Above It
A tomato or pepper fruit hangs above the soil surface, only indirectly affected by whatever's happening at root level. A potato tuber is the opposite - it forms and bulks up entirely underground, developing directly within whatever soil chemistry fennel's roots have been shaping all season, and continuing to expand into freshly hilled soil as the plant grows. That direct soil-level exposure is worth taking seriously as a reason to keep real distance between the two, beyond the general allelopathy caution that applies to fennel's other antagonist pairings.
Spacing
Give fennel its own bed or container rather than trying to manage a close spacing near potatoes - remember, too, that potatoes are hilled with soil as they grow, so the zone you need to keep clear effectively expands over the season as more soil gets mounded around the developing plant.
Watch For
Beyond the growing season itself, consider fennel's impact on future potato beds too - because the potato tuber's exposure is a soil-level concern rather than an above-ground one, a bed that grew fennel is worth treating cautiously for at least a season before planting potatoes there, similar to the rotation caution recommended for eggplant elsewhere on this site.
Frequently Asked Questions
Does hilling potatoes with more soil make the fennel concern worse?
It could extend the affected zone somewhat, since hilling adds fresh soil around the developing tubers throughout the season - one more reason to keep real distance from any fennel planting from the start rather than assuming initial spacing is enough once hilling begins.