Manihot grahamii aka Graham's Manihot

Manihot grahamii is a fast-growing shrub that typically reaches 2.5-3 metres (8-10 feet) tall with a spread of 1-1.5 metres in a single growing season. In frost-free tropical climates, it can become a small tree reaching 4-6 metres. The plant has an open, airy structure with foliage concentrated near the top of slender, woody stems. In temperate zones 7b-8, it dies back to the ground in winter but rapidly regrows each spring, often reaching full height within a few months.

Manihot grahamii is native to South America, specifically northern Argentina, southeastern Brazil, Paraguay, and Uruguay. It grows naturally in subtropical and tropical regions of these countries. The plant has been introduced as an ornamental in other parts of the world, including the southeastern United States (Georgia, Florida, Louisiana, and Texas), where it has occasionally naturalized in disturbed areas and suburban forests.

Manihot grahamii thrives in well-drained, loamy soil but is adaptable to various soil types. It tolerates acidic, neutral, and alkaline pH levels equally well. Good drainage is essential; avoid heavy, waterlogged soils. For container growing, use a quality potting mix amended with perlite or sand to improve drainage.

Manihot grahamii is the most cold-tolerant Manihot species and can survive in USDA zones 7b-10b. Above-ground stems survive light frosts, and even if severe cold kills the aerial growth, new stems regenerate from underground parts. One specimen in Salt Lake City (zone 7b) survived over 5 years, and a Kansas trial saw survival at -17°F (-27°C) with heavy mulching. At the heat end, it tolerates temperatures above 100°F (38°C) for extended periods. Protect container plants from temperatures below 15-20°F (-9 to -7°C).

Manihot grahamii tolerates a range of humidity levels and is not particularly demanding in this regard. Plant Lust notes it tolerates humidity well. When grown under glass or in greenhouses, RHS recommends low humidity conditions. The plant's drought tolerance suggests it handles dry air better than many tropical-looking plants, making it suitable for a variety of climates from humid subtropical to more arid conditions.

Apply a balanced time-release fertilizer in spring when new growth emerges. As a fast-growing plant, Manihot grahamii benefits from regular feeding during the active growing season. For container plants coming out of winter dormancy, apply fertilizer in April or May when you move them outdoors. During the growing season, you can supplement with liquid fertilizer monthly. Reduce or stop feeding in autumn as the plant prepares for dormancy.

In spring, remove dead foliage from winter, apply fertilizer, and resume regular watering as new growth emerges. During summer, the plant grows rapidly and may need staking if top-heavy; water regularly and enjoy the summer blooms. In autumn, reduce watering as growth slows. For winter in cold climates (zones 7-8), the plant dies back to the ground; apply 12 inches of mulch over the root zone for protection. Container plants should be moved to a cold but frost-free location (garage, basement) with minimal watering.

Manihot grahamii produces distinctive bell-shaped flowers in summer, typically from April through August. The flowers are pale green to chartreuse with waxy red striping, appearing in clusters on tall stalks. They are followed by smooth capsules containing oblong seeds. The flowers attract bees and other pollinators, making this plant valuable in pollinator gardens. In warmer climates, the plant may self-seed if flowers are allowed to mature.

Manihot grahamii grows outdoors year-round in USDA zones 8-10. In zones 7b-8a, it survives as a herbaceous perennial that dies back in winter and regrows from the roots in spring; heavy mulching (12 inches of leaf mulch) dramatically improves survival. In zones 6 and below, grow it as an annual or in a large container that can be overwintered indoors. In frost-free climates (zone 9b+), it can become a small evergreen tree. Plant in a sheltered location with full sun for best results.

Manihot grahamii requires minimal pruning. In early spring, remove any dead or winter-damaged stems and foliage before new growth emerges. The plant naturally forms an open structure with foliage concentrated at the top, so routine shaping is rarely needed. In frost-free areas where it becomes tree-like, you can remove lower branches to create a cleaner trunk. Cut back spent flower stalks if you want to prevent self-seeding.

Manihot grahamii propagates easily from both seeds and stem cuttings. For seeds, lightly scarify the hard seed coat with sandpaper, soak in warm water for 12-24 hours, then sow 1-1.5cm deep in well-draining mix at 24-29°C (75-85°F). Germination takes 2-4 weeks. For cuttings, take softwood cuttings from mature stems at least 20cm long with several nodes, and root them directly in soil. Dormant winter twigs can also be used successfully; simply push them into the ground in spring.

Manihot grahamii is generally disease-resistant when grown outdoors. When cultivated under glass or in greenhouses, it may develop infestations of aphids, glasshouse red spider mite, or whitefly. Inspect plants regularly, especially in warm, dry conditions that favor spider mites. Control pests with insecticidal soap, neem oil, or by introducing beneficial insects. Good air circulation helps prevent fungal issues. Outdoors, the plant is typically trouble-free.

No, Manihot grahamii is not edible. Despite being closely related to cassava (Manihot esculenta), which is a major food crop, Graham's cassava is grown purely for ornamental purposes. Parts of the plant are poisonous if ingested. Like other members of the Manihot genus, it likely contains cyanogenic glycosides, which release hydrogen cyanide when plant tissues are damaged. Keep this plant away from children and pets, and do not attempt to eat any part of it.

Manihot grahamii is valued primarily as an ornamental plant for its exotic, tropical appearance. Its deeply lobed palmate leaves create a dramatic focal point in gardens. Common uses include tropical-themed landscaping, hedges, windbreaks, and large container specimens. The fast growth makes it effective for creating quick screening or filling gaps. The summer flowers attract bees and other pollinators, making it valuable for pollinator gardens. It is sometimes used in scientific research due to its relationship to the economically important cassava.

Manihot grahamii is pollinated by insects, particularly bees. The summer flowers with their distinctive pale green and red-striped bell shape attract various pollinators, making this plant a valuable addition to pollinator gardens. If flowers are allowed to mature and set seed, the plant can self-sow. The capsule fruits contain oblong seeds 10-12mm in size. To prevent unwanted seedlings, remove spent flower stalks before seeds mature.

Repot container-grown Manihot grahamii in spring before new growth begins. As a fast-growing plant, it may need repotting annually or every two years depending on growth rate. Choose a pot only slightly larger than the current one to prevent overwatering issues. Use well-draining potting mix amended with perlite or sand. When repotting after winter dormancy, this is also a good time to refresh the soil and apply time-release fertilizer. Ensure containers have adequate drainage holes.