Pain in the Grass:

Protecting Cold-Sensitive Plants Over Winter

Question:

We have a lovely purple fountain grass (planted in the spring), and I keep hearing that it will die in the winter and we would have to plant a new one each year. Others say to dig it up, pot it, cut it back, put it in the house near a window, and replant it in spring. Another view is it will be fine in winter; just cut it back in spring and it will flourish! So we are confused. I also considered purchasing a small pop-up greenhouse and putting it around the plant to fight off frost and also allow it to get sun. Any help you could give would be most appreciated.

- Les Bender, Northeast Rio Rancho (6,000 ft)

Answer:

I understand this conundrum. Conflicting horticultural advice is often an indication that there’s no single correct answer. You’re right that purple fountain grass ( Pennisetum setaceum 'Rubrum') is perhaps “marginally hardy” in your area, and is listed by several sources as being cold hardy to USDA Hardiness Zones 8 and 9, which means this species can make it through winters with temperatures reaching as low as 10°F and still grow back in the spring. The next step is to determine the hardiness in your yard. According to the USDA Hardiness Zones, you’re likely to be in USDA Hardiness Zone 7, with average annual extreme minimum temperatures (from 1976–2005) of 0 to 10°F. But it’s not quite that simple because 1) average temperatures have gone up, even in the past decade, and continue to do so; and 2) you may have microclimates, like up against a low west-facing wall, that get plenty of sun and stay considerably warmer than other, more exposed areas. For more details on how to find and use this info in your landscape planning, check out my May 2020 column on picking the right shrub for your zone by searching my Desert Blooms blog for the terms “match maker.”

This purple fountain grass is thriving after a single growing season, but it may not be cold hardy enough to stay alive through the winter. Photo credit Les Bender

The rock mulch shown in your photos can also be expected to impact the microclimate by increasing daytime soil temperatures and keeping nighttime temps higher with residual heat. This may be a serious concern for our Southwest landscapes because of related urban heat island effects, continued global warming, and heat and drought stress on landscape plants. But, in terms of keeping microclimates warmer, the gravel around your ornamental grass may help it survive the winter.

I don’t trust myself to keep a plant like this alive indoors over the winter. To me, the plant may be more likely to die with the combined stress of being dug up and potted, sporadic neglect while being “protected” inside, and being transplanted again in the spring than if I left it in place. So I cannot recommend this method. Plus, we may have an unseasonably warm winter ahead of us. Keep an eye on the weather. If a cold snap is coming, you can easily add short-term protection by covering your grass with a sheet or blanket overnight.

An important tip is to keep that beautiful foliage intact through the entire winter. This is true for other ornamental grasses as well. On top of it being pretty through the winter, the above-ground plant material provides a warming micro-microclimate (let’s call it a “nanoclimate”) for the root zone and plant crown. In the early spring, it’ll be time to cut the leaf blades down close to the ground; you can simplify cleanup by tying them in a ponytail beforehand.

I joke that a title for this column could be “Pain in the Grass,” but it’s not the grass’s fault. These are the problems associated with planting species that are marginally cold hardy. We have several other species of beautiful ornamental grasses (both native and adapted) that are plenty cold hardy in New Mexico landscapes. My personal favorites are sand lovegrass (Eragrostis trichodes), blue grama (Bouteloua gracilis), and muhly grass (Muhlenbergia capillaris). For more info, including links to recommended landscaping books and a video on selecting ornamental grasses for your yard, check out the blog version of this week’s column at Desert Blooms.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

Links:

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook page.

Please copy your County Extension Agent and indicate your county of residence when you submit your question!