American Groundnut

Set whole tubers two to three inches deep in early spring, in moist sandy or gravelly loam with humus worked in, in full sun to part shade. Plant them intact rather than cut, and expect the first shoots to be slow and thread-thin. Groundnut is a perennial legume that grows from tuberous swellings on slender rhizomes, so the tuber is the propagule, and it needs warm moist soil and a full season underground before it strings out new tubers.

After the shoots are established and you can see where the plants are, mulch the bed to shut out weeds and grass. Groundnut sends up slender vines that compete poorly against grass at ground level, and mulch also holds the steady soil moisture the rhizomes need to run and set tubers.

Do not feed the bed nitrogen fertilizer. Compost or humus worked in at planting is enough. If you are starting from seed rather than tubers, or planting into ground that has never grown a legume, treat the seed with cowpea-group inoculant so it nodulates. Apios americana is a nitrogen-fixing legume that nodulates with cowpea-group rhizobia and supplies its own nitrogen, so bought nitrogen is wasted on it. Compost at planting covers what it needs.

As shoots emerge, set a trellis, arbor, cattle panel or fence beside them to twine up, and cage or fence the bed while the plants are small if rabbits work your ground. The stems twine and climb rather than stand, and vines left flat on the ground smother each other, while a first-year planting is small enough that a few nights of browsing can set it back a whole season.

Water whenever the top few inches dry out through the summer. Groundnut tolerates soil that stays occasionally wet, so err toward too damp rather than too dry. In the wild it grows in wet meadows, low thickets, stream banks and moist woodland soil, and in poor dry ground the plant struggles instead of bulking tubers.

Once a summer, walk the perimeter of the patch and dig out vines that have run past where you want them, tracing back to the rhizome and lifting the tubers with it. Give groundnut a bed you have edged, or grow it where colonizing is welcome. Groundnut spreads by rhizome to form colonies and can turn weedy - every stray tuber sprouts in spring and fills in the ground between rows, and pulling the top growth alone leaves the tuber to come back.

Wait until the vines yellow and die back, then dig gently and follow the rhizome outward - the tubers come up as a string of knobby beads along an underground runner, not as one clump. Expect several one-inch tubers per plant after a full year. They hold in the ground and can be dug any time from late fall through early spring. The tubers are best from late fall through early spring, after the vine has finished pulling the season's starch down into them, and cold ground stores them better than any cellar.

As you dig, put the small tubers back in the loosened soil and firm it down. Dig with a fork and work slowly so you break as few of the underground runners as you can. Every tuber left in the ground sprouts next spring, so the harvest is also the replanting, and a patch worked this way keeps producing year after year without ever being started over.