Gunnera

Gunnera requires constant, abundant moisture to thrive, as its native habitat is wet bogs, stream banks, and rainforest edges. Never allow the soil to dry out completely; during hot, dry periods, water deeply 2-3 times per week, and mist the leaves regularly to maintain ambient moisture. Avoid overwatering in frozen winter conditions, as waterlogged frozen soil can rot dormant crowns.

Gunnera grows best in partial shade, particularly dappled sunlight that filters through overhead tree cover, which protects its large leaves from scorching in intense midday sun. It can tolerate full sun only in very cool, foggy coastal regions where temperatures rarely exceed 75°F (24°C) and soil remains consistently damp. Too much deep shade will slow growth and reduce the size of its iconic leaves.

Plant gunnera in rich, humus-heavy, consistently moist soil with a slightly acidic to neutral pH range of 5.5 to 7.0. The soil must retain water well but have minimal drainage to avoid drying out; adding large amounts of compost, leaf mold, or well-rotted manure at planting time will improve water retention and nutrient density. It thrives in boggy, poorly drained sites where most other ornamental plants would rot.

Feed gunnera monthly during the active growing season (spring through early fall) with a balanced, slow-release organic fertilizer, or apply a diluted liquid all-purpose fertilizer every two weeks to support its rapid, large growth. In early spring, top-dress the root zone with a 3-4 inch layer of well-rotted manure or compost to replenish soil nutrients and help retain moisture. Stop fertilizing in late fall to allow the plant to enter dormancy before cold weather arrives.

Gunnera is hardy in USDA zones 7 through 9, tolerating winter temperatures as low as 10°F (-12°C) if its crown is properly protected with a thick layer of mulch or cut, dried leaves during dormancy. It prefers cool summer temperatures between 55°F and 75°F (13°C to 24°C), and will struggle in extended heat above 85°F (29°C), especially if not provided with ample shade and water. In regions with harsh winters, it can be grown in large containers and overwintered in a cool, frost-free garage or shed.

Prune away damaged, yellowed, or frost-killed leaves at any time by cutting the thick stem at the base near the soil line to keep the plant looking tidy and encourage new growth. In late fall after the first frost kills back the foliage, cut all stems down to 6-12 inches above the crown, and use the cut leaves to mulch the crown for winter protection, removing the mulch in early spring once the risk of hard frost has passed. Remove spent flower spikes after blooming if you do not want the plant to self-seed, as it can become invasive in ideal wet conditions.

Gunnera is most easily propagated by division in early spring, just as new growth begins to emerge from the dormant crown. Dig up the entire root mass, use a sharp, sterilized knife to cut the crown into sections, each with at least one visible growth bud and a portion of the root system, and replant sections immediately in prepared moist soil. It can also be grown from seed, but seeds require cold stratification for 2-3 months before sowing, and seedlings grow very slowly for the first 2-3 years before reaching mature size.

Gunnera prefers high humidity levels of 60% or higher, matching its native rainforest and bog habitat, and will develop brown, crispy leaf edges in dry air. In dry climates or during periods of low humidity, mist the leaves daily with room-temperature water, or place a tray of water filled with pebbles near the plant to raise ambient moisture levels. Indoor-grown gunnera will benefit from placement near a humidifier, especially during winter when indoor heating dries out the air.

Container-grown gunnera requires repotting every 1-2 years in early spring before new growth starts, as its fast-growing, extensive root system will quickly fill a pot and reduce drainage. Use an extra-large, heavy pot (at least 20-24 inches in diameter for mature plants) to support its top-heavy growth and prevent it from tipping over, and use a rich, compost-heavy potting mix that retains moisture well. When repotting, gently loosen tangled roots and trim any rotted or damaged root sections before placing in the new pot with fresh soil.