How to Grow American Groundnut (Hopniss)
Apios americana
Hopniss is a perennial legume that produces edible tubers along underground runners. Plant tubers in spring in a warm, sheltered spot with a strong trellis or fence for support. It is slow in the first year but builds vigour and tuber yield steadily from year two onwards. Harvest tubers in autumn once vines die back — leave a few in the ground to regrow the following year. The fragrant purple-pink flowers appear in midsummer. As a nitrogen-fixer, it improves soil fertility and needs little feeding. Contains the tuber chain in a defined area or use a root barrier to prevent unwanted spread.
Yearly Lifecycle
Care Essentials
Light feeder once established. Incorporate compost or well-rotted manure before planting. Hopniss is a legume and fixes its own nitrogen, so additional feeding is rarely needed. Avoid high-nitrogen fertilisers which reduce tuber formation.
Watch For
- Aphids on young shoots in spring
- Vine weevil larvae on tubers
- Japanese beetle (in North America; less common in UK)
Companions
Corn, Sunflower, Jerusalem artichoke, Comfrey
Care Requirements
Light
Full sun to partial shade
Produces best tuber yields in full sun. Tolerates partial shade but growth is less vigorous and flowering reduced. A warm, sheltered wall or fence is ideal.
Watering
Moderate; drought-tolerant once established
Water regularly in the first season to help establishment. Once the tuber chain is established, hopniss is reasonably drought-tolerant. Avoid waterlogged conditions which can rot tubers.
Fertilizing
Minimal; nitrogen-fixing legume
As a legume, hopniss fixes atmospheric nitrogen and needs little feeding. Incorporate compost before planting. Avoid high-nitrogen feeds which favour leaf growth over tubers.
Pruning
Cut vines to ground in late autumn
Cut all vine growth to ground level in late autumn after the first frosts. Tie in new shoots in spring as they emerge. Thin dense summer growth to improve airflow and light to the lower plant.
Support
Strong trellis or fence essential; minimum 2 m tall
Hopniss is a vigorous twining vine reaching 3 m or more. Install a strong trellis, fence, or wire system before planting. Bamboo canes are not sufficient for established plants.
Spacing
60 cm apart with trellis support
Plant tubers 60 cm apart. The plant spreads by underground runners over several years — a root barrier or annual harvesting keeps it in check.
Spacing & Planting
| Plant spacing | 60 cm |
| Row spacing | 90 cm |
| Mature height | 300 cm |
| Mature spread | 150 cm |
Hopniss is a vigorous climbing vine that spreads by underground tubers. Allow 60 cm between plants and provide a trellis or fence at least 2 m tall. In garden beds, contain spread with a root barrier or harvest tubers annually.
