Marisa Y. Thompson
Question: Should I bring in my potted trees and shrubs for the winter? I have a willow, a poplar, and a berry shrub, and I live at 7,300 feet.
Ora N., Seed to Supper participant
Reply: As we are all rushing for winter, I received several questions about plants to protect from the cold and how to do it. Are plants safer in the house, in a greenhouse-like structure, in pots or in the ground?
The answer, as usual, is that it depends! How cold is it generally in your area? How will it be cold this winter? What is the listed cold hardiness of each plant? How big is the plant? How big is the container and how long has it been in a jar?
Let’s start with your location and data from past winters. Find the most recent USDA plant hardiness zone map at https://planthardiness.ars.usda.gov/. Enter your address in the search prompt, and a small box will appear with your “plant hardiness zone” based on the coldest average annual temperatures between 1976 and 2005. Mine in Los Lunas is 7a, which means that the extremely low temperatures in my area averaged between 0 degrees Fahrenheit and 5 degrees Fahrenheit during this time.
Next, research the cold hardiness of each plant that worries you. I have a “Johnson’s blue” geranium in a pot, so I went google “Johnson’s blue geranium cold hardiness” and found that this particular variety is frost hardy up to zone 4 ( minus-30 degrees to minus-20 degrees). It makes me feel a lot more comfortable leaving it outside all winter – as long as I don’t forget to water (more on this in a moment).
If the plant was only cold hardy up to Zone 10 (30 degrees to 40 degrees), it is unlikely to survive outdoors anywhere in New Mexico. I added that word “likely” because we could have an unusually warm winter in which this plant could very well overwinter in the warmer parts of the state. Plus, as esteemed arborist George Duda says of trees, plants make all of us liars. The second I say that a plant cannot survive, I get an email with photos attached that prove me wrong. Lucky for me, I like to be wrong.
It is important to research the cold hardiness of your exact species. Common names can be misleading, and in many cases the range of hardiness zones will be too great for entire genera, such as willows. For example, the cold hardiness of coyote willows (Salix exigua) that grow along the bosque is zone 4. Weeping willows (Salix babylonica) are listed as cold hardy up to zone 6. And most varieties of desert willow (Chilopsis linearis, not true willows at all) should be hardy up to zone 7b (5 degrees to 10 degrees).
Cold hardiness aside, the biggest question for me and my potted plants is, “Can I be trusted to keep them watered in pots outside all winter?” Quickly followed by “Can I be trusted to water them indoors?” And the answer to both of these questions is “Unlikely”.
Certainly, properly watering container plants outdoors during the winter can be very difficult. We want to keep the soil in the root zone moist but not soggy. If you are watering with a garden hose, you may need to wait until it warms up enough for the ice in the pipe to thaw.
While writing this article, I took a break for a phone meeting with Andrew Lisignoli, another local arborist and president of the nonprofit Think Trees New Mexico. I asked Lisignoli what he thinks readers should consider. He pointed out that, compared to soil in the ground, soil in containers tends to heat up more during the day (especially in direct sunlight), dry out faster, and become much colder at night. However, Lisignoli also warned that bringing plants indoors was not necessarily a quick fix. If you know, you know. And if you don’t, you’ll learn that pest problems can become a huge problem indoors. All of a sudden, your plant is covered with aphids or mealybugs, or some other common pest.
If your plants are deciduous in the wild (that is, they drop their leaves and go dormant every winter), they may need to stay outside and be exposed to the cold. to survive. We’ll keep the topic of dormancy for another time, but basically if the plant is cold hardy, keep it outside and water it regularly.
Finally, perennials kept in containers develop circular roots over time which can eventually cause the plant to strangle. Consider planting these hardy trees and shrubs before the roots get too tangled. You will still need to water during the winter!
Quick Set for Seed to Supper and Think Trees New Mexico: NMSU’s Seed to Supper Program via ICAN (Ideas for Cooking and Nutrition) is a free, online, self-paced gardening course that has been modified by our own NMSU Food Systems Specialist, Sally Cassady, to be web-based and focused on New Mexico. Classes are offered in English and Spanish. Register at https://ican.nmsu.edu/seedtosupper.html.
Think Trees New Mexico is a non-profit organization that will be hosting its 36th Annual Urban Tree Care Conference in Albuquerque from February 10-11. The range of speakers is exceptional – more information on https://www.thinktreesnm.org/.
For more information on gardening, visit the NMSU Extension Urban Horticulture page at http://desertblooms.nmsu.edu/ and the NMSU Horticulture Publications page at http://aces.nmsu.edu/pubs/_h/. Find your local cooperative extension office at https://aces.nmsu.edu/county/.
Marisa Y. Thompson, Ph.D., is an Urban Horticultural Extension Specialist in the Plant Science Extension Department and is based at the New Mexico State University Agricultural Science Center in Los Lunas.
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