Q: I want to grow the giant Scottish gunnera plant, but I’m in zone 6, and my books list it as hardy only to zone 7. Since Scotland is farther north than my garden, can I fool Mother Nature into letting gunnera survive here? — Dennis Goen, Godfrey, Ill.
A: Thanks to ocean currents, some areas along the northeastern Scottish coast have winter extremes similar to those in zone 8; a few pockets are so mild that it never freezes. But just how far is zone 6 from zone 8? (To determine cold hardiness, most growers refer to the USDA Plant Hardiness Zone Map, which divides the country into zones based on extreme winter low temperatures in an average year. The map is printed in plant encyclopedias and in general references.) Roger Gossler, whose zones 7/8 Gossler Farms Nursery in Springfield, Oregon, ships gunnera throughout the U.S., has a customer who grows the leafy titan in New York’s lower Hudson River valley (zone 6). This intrepid gardener protects each of his plants with a layer of leaf mulch 12 inches deep and 4 feet in diameter, then builds a wooden hut around it. If you plant a gunnera in well-drained soil and can prevent the ground from freezing around the crown, it will probably make it through the winter (the hardiest species is Gunnera tinctoria, also known as G. chilensis). But merely staying alive is a different matter than flourishing. Because gunnera leafs out early, around daffodil time, the emerging foliage is often scarred by late frost, even at Gossler’s nursery. He spreads a protective mulch: 18 inches of mixed leaf litter topped off with a blanket of faded gunnera foliage.