By Bev Johnson

Master Gardener

Minnesota gardeners who have visited tropical areas drool over the bright tropical blooms. We have “flower envy.”  The University of Minnesota is working to increase winter hardiness in some of those exotic plants.

Cold hardiness is “a complex, dawn-out affair” states Neil Anderson, professor of horticulture.  He says If the plant dies down to the ground every year, those are the ones that survive. To be a good candidate for cold hardiness, it must have an underground storage system. It has to be able to tolerate zone 3 soil temperatures of at least 14 degrees. Frost is usually about 6 feet deep even though the soil is warmer than air in the winter. Our winter hardy plants can usually tolerate that. The exception is if we don’t have insulating snow cover.

During the winter of 2022-2023 the deep snow around the University test gardens, let their field gladiolus survive. These plants are native to South Africa! Now the U is working on making them cold hardy. The researchers screen hundreds of thousands of seedlings or cuttings in outdoor trials to measure plant growth and survival. The few that survive are intercrossed to get babies. They are then tested to see if they are winter hardy.

Winter hardiness is a genetic trait that segregates so you can cross a winter hardy plant with another that seems winter hardy but “that doesn’t mean you’re going to get any hardy ones out of it” says the professor. He wants a test winter with lots of snow cover and below zero air temps. If the plants survive in 3 winters in several locations, then the plants are hardy.

The University has a neat machine, a programable freezer. They take a plant, rooted cutting or seedlings and grow them out for one season, then harden them off naturally in falls cooler temperatures and shorter days.  Then they stick them in a walk-in cooler that is just above freezing. The temperature is slowly dropped to 12 below. Samples are continually taken. The temperatures are then slowly raised to normal, and the plants are moved to a greenhouse and scored for hardiness. 50 percent of the plants have to survive this experiment to be considered hardy. Anderson says “we call that LT.50, lethal temperatures at which we have 50 percent survival. They kill the plants, so we don’t have that sinking feeling in the spring when one of those new cultivars we bought last spring as hardy, isn’t. 

We can have a (sort of) tropical look in our gardens. Hibiscuses are hardy to minus 30.  There are two zone 4 yucca varieties, Y. glauca and Y. baccata that are not only hardy, but very tropical looking. 

Plants that aren’t hardy: Japanese Banana tree, Musa basjoo is zone 5 so it needs to be a house plant in the winter. In full sun, it will grow 6 to 14 feet tall in one summer. Not many people have that much room in their living room for her. Then there are the tropical looking lilies. Pineapple lily is a south African native. Aztec lily blooms in the summer and may re-bloom later. Canna lilies are big girls, up to 6 feet tall.  Peruvian daffodils have thin stalks that need staking as they bare huge cluster of flowers on them. Cala lilies like wet feet and part shade.  Gladioluses are a South African native. Their tall stalks add an exclamation point to the garden, but also need staking. All these flowering plants need to be dug up and stored for the winter for next summer’s gardens. Or, just let the darn things freeze and replace them next spring.  Add a few giant hostas or ferns and it’s Hawaii in Minnesota—sorta.