Gunnera manicata, often known as Giant Rhubarb, is a prehistoric-looking perennial plant capable of producing leaves up to ten feet tall and fourteen feet wide. Growing it in containers allows enthusiasts to incorporate this tropical drama into smaller gardens. While the plant’s massive scale makes this a challenging endeavor, restricting its root run in a tub is the only way to manage its size while still achieving an impressive display. This method requires a commitment to high-demand care to compensate for constraints on its aggressive growth habit.
Selecting the Container and Substrate
The primary limitation for container-grown Gunnera is root space, making container selection the most important initial decision. To maintain a reasonably large specimen, a container of at least 50 gallons (approximately 200 liters) is strongly recommended; use the largest size you can physically manage. Heavy, non-porous materials like thick plastic or glazed ceramic are preferable because they minimize moisture evaporation through the walls.
Proper drainage must be engineered to balance the plant’s need for constant moisture with the danger of root rot. Instead of drilling drainage holes at the base, place them approximately four inches (10 cm) up the sides of the container. This creates a permanent water reservoir, or “sump,” at the bottom, mimicking the consistently moist conditions of its native boggy habitat while allowing excess water to escape.
The substrate must be a dense, nutrient-rich, and highly moisture-retentive mix, far heavier than typical commercial potting soil. Start with a heavy loam base and amend it generously with organic matter, such as well-rotted manure, leaf mold, and compost. Incorporating elements like coarse bark chips, perlite, or pumice stone (up to 10% of the total volume) ensures the mix remains aerated. This prevents the dense organic matter from compacting into a soggy mass and provides the rich food source the plant requires to thrive with restricted roots.
Essential Care During the Growing Season
Once planted, tub-grown Gunnera requires intensive maintenance from spring through early autumn to sustain its immense foliage. The plant is notoriously thirsty, demanding a constant supply of water to prevent its enormous leaves from wilting, especially during summer heat. During peak warm weather, deep watering may be required daily, or even twice a day, to fully saturate the dense substrate and replenish the reservoir.
The sheer volume of water flushes nutrients out quickly, meaning the plant must be treated as a “gross feeder” requiring heavy, regular fertilization. Begin the season by incorporating a generous dose of a balanced, slow-release granular fertilizer (such as a 10-10-10 formulation) into the topsoil layer. Supplement this with a high-nitrogen liquid feed every one to two weeks, sometimes applied at double or triple the recommended strength to compensate for leaching.
While Gunnera naturally prefers partial or dappled shade, it can tolerate full sun exposure if its moisture and nutrient needs are met without fail. Any lapse in water supply will quickly cause the leaves to droop dramatically, signaling severe water stress. Throughout the growing season, routinely remove damaged or yellowing leaves by cutting them cleanly at the base to redirect the plant’s energy toward new growth. Maintaining this consistent regimen of water and feed encourages the large leaf size that justifies container cultivation.
Protecting Tub-Grown Gunnera in Winter
Overwintering is the most precarious stage, as the root crown is highly susceptible to frost damage when confined to a container. Wait until the first hard frost blackens the foliage, signaling dormancy, before beginning winter preparations. Carefully cut back all withered leaves, leaving the stalks about a foot long, using a sharp tool and protective gloves to avoid the spines on the stems.
The cut foliage should not be discarded but used as the primary insulator for the vulnerable central crown. Stack two or three of the largest leaves directly over the growth point. Then, cover the entire crown with a thick, protective mound of the remaining leaves, securing them with the cut leaf stalks or a layer of straw. This organic material acts as a natural, breathable blanket, preventing direct contact with freezing air.
Because the confined rootball can freeze solid much faster than a plant in the ground, the entire tub should be moved to a sheltered, frost-free location for the duration of the coldest months. Ideal spots are an unheated garage, a cool basement, or a well-insulated shed where temperatures remain above freezing. Even in dormancy, the substrate must not be allowed to dry out completely. Check the soil every few weeks and apply a small amount of water if it feels arid to prevent the roots from desiccating.