Gunnera (Gunnera manicata), commonly called giant rhubarb, and garden rhubarb (Rheum rhabarbarum) have only one thing in common, their looks. Giant rhubarb grows in U.S. Department of Agriculture plant hardiness zones 7 through 10, while garden rhubarb grows in USDA zones 3 thorough 8. Both giant and garden rhubarb need digging and replanting periodically, but you can't follow the same schedule for both, as these two unrelated plants have different requirements.
Plants From Separate Continents
Giant rhubarb comes from tropical and sub-tropical South America, while garden rhubarb originated in the Far East and is widely grown as an edible in cold-climate gardens. These two unrelated plants look strikingly similar even through they come from different continents and plant families. Both have reddish stalks that support large, palm-shaped leaves. Giant rhubarb grows up to 10 feet tall with leaves 6 to 8 feet across while garden rhubarb grows just 2 to 3 feet tall and wide. Giant rhubarb isn't edible, but you can harvest and eat garden rhubarb stalks.
Dig Giant Rhubarb Before Winter
In USDA zones 7 through 10, you can grow giant rhubarb year-round outdoors. The first hard frost kills off the foliage and stalks but the roots survive, insulated by the soil through the winter. In spring, new shoots emerge from the soil. In mild, frost-free areas, this tropical-looking giant grows lush foliage year-round. Outside the growing zone, in USDA zone 6 and colder, you can dig up giant rhubarb roots and overwinter them indoors for spring replanting.
Replanting Giant Rhubarb
In fall, cut back giant rhubarb stalks and leaves at the soil line, then dig out the roots using a garden fork. Just be careful not to damage the fleshy tuberous roots. Pack the roots in buckets or crates filled with dry peat moss and store them at 40 degrees Fahrenheit through the winter. In spring, replant the roots in the garden upright at the same depth as they were originally in the soil. Giant rhubarb thrives in part shade in wet bogs or marshy spots.
Replanting Garden Rhubarb
Every six to 10 years, dig up and divide garden rhubarb to prevent overcrowding. Dig rhubarb roots in early spring as soon as the ground thaws enough to work and before new top growth begins. Use a garden fork to dig out a the root clump, then cut it into into 4- to 6-inch sections. Replant the root sections at the same depth in the soil, spaced 4 feet apart.
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