Why I Ignore USDA Hardiness Zones
Plus, what’s "native" in a warming climate?
Source: Environmental Protection Agency
Dear Avant Gardener, We’re originally from Pennsylvania, where when it’s too cold to work outside, things go dormant so you don’t seem to lose any ground. Here when it’s too hot to work outside, the weeds and invasive plants take over! Our heat index reached 115 degrees this past summer, so we stay indoors watching everything we don’t want take over, dreading the effort in fall to get back to where we were the previous spring. I read recently that climate change is moving us into zone 10. We can use all the help we can get! — Retired in South Florida
Oy, what a can of worms! Invasives. Climate change. Hardiness zones. I’m going to tackle them in reverse order.
I generally ignore hardiness zones. Contrary to what it says on the USDA site, hardiness zones do not tell you “which perennial plants are most likely to thrive at a location” — unless you are assuming irrigation, fertilizer, and other horticultural interventions that enable people to grow plants not adapted to their regions.
The limitations of hardiness zones
As the USDA explains, hardiness zones merely indicate “the average annual extreme minimum winter temperature, displayed as 10-degree F zones and 5-degree F half zones.” The new zones reflect 1990 to 2020 averages, the prior ones represented 1976 average, which were lower in many areas. These zones will help you figure out if something that didn’t evolve in your region — a fruit tree or ornamental shrub — will live through winter to see the spring. But hardiness zones don’t tell you much that is vital.
Hardiness zones don’t indicate annual precipitation, precipitation in the driest month, duration of cold weather, or high temperatures and their duration. Los Angeles and coastal Sarasota are both in zone 10, but Los Angeles gets only 14 inches of rain annually whereas coastal Sarasota receives about 52. Obviously, the vegetation that thrives without intervention in L.A. is completely different from what thrives in Sarasota.
If you pick a species native to your location, you can generally ignore its hardiness zone, because the species evolved in your weather conditions. Just respect its preferred sunlight and moisture conditions. If you buy local ecotype plants raised near your location, it is even more likely to thrive.
Climate change and ecoregions
Unfortunately, climate change is upsetting the evolutionary balance of our ecoregions. And the location of your yard is at a pivotal point that exaggerates this impact: Your zone change from 9 to 10 indicates your location used to experience frost and now it rarely will.
Most of Florida, as well as nearly the entire area north and west to Wisconsin, is in the Eastern Temperate Forest Level I Ecoregion. Plants in this ecoregion adapted to freezing temperatures. In fact, their seeds often require a couple of months of freezing temperatures to germinate.
As your area warms, plants native to broad swaths of the eastern U.S. may no longer thrive there. Common wildflowers like cardinal flower, downy phlox, Ohio spiderwort, spotted beebalm, for example, are native to north Florida but not south Florida.
Biodiversity and invasives hotspot
On the other hand, native and exotic plants that didn’t adapt to freezing temperatures will now thrive in your yard. The southern tip of Florida, where it virtually never freezes, is in the Tropical Wet Forest Level I Ecoregion, along with the Yucatan. The heat and humidity that keep you inside in the summer make South Florida a global biodiversity hotspot — yay! — and a global invasive species hotspot — boo!
Invasives are plentiful in South Florida because the subtropical environment allows for many non-native species to thrive, and Miami acts as a port of entry. — National Park Service
The poster child for destructive tropical invasives is the Burmese python, of course. But tropical plants can wreak havoc, too. Brazilian peppertree and Australian pine, both brought to Florida by the nursery trade, have escaped cultivation and are aggressively displacing native plants in natural areas. This threatens not only native plants, but the insects and other wildlife that depend on them. (See Florida’s Invasive Plant Directory for comprehensive, species-level information.)
Eradicating invasive plants
But don’t despair! One reason home landscapes are important in the broader ecological picture is that with vigilance we can prevent the spread of invasives within them. With persistence, you will eventually prevail against the invasives in your yard.
The key is to remove the invasives without disturbing the soil when possible, then densely planting or seeding the area with aggressive natives. You can do this by cutting the invasives to the ground to remove them, instead of pulling, which just encourages more invasive seeds to germinate. For more on controlling invasive weeds, including rhizomatous ones, read The Latest on Weeds.
More plants!
Sophie Martine of Florida Native Plants nursery recommends planting shrubs and groundcovers to combat invasives in Florida. “Wildflowers don’t compete as well,” she notes. “Once you have the invasives under control, you can add that mid-tier.”
Martine’s top shrub recommendations include firebush (Hamelia patens), which attracts butterflies and hummingbirds, and wild coffee (Psychotria nervosa), which also attracts butterflies, as well as birds that eat its fruits.
For groundcovers that will spread quickly and suppress invasives in sunny areas, Martine recommends frogfruit (Phyla nodiflora) and sunshine mimosa (Mimosa strigillosa). Mimosa spreads by rhizomes and roots right into mulch; just avoid planting near a septic field where its deep roots damage piping. For shade, Martine recommends creeping sage (Salvia misella), though it may die back in summer.
Plant in fall
Plant the native replacements in the fall immediately after removing the invasives to give them plenty of time to establish before the following summer. Buy groundcovers in four-inch pots and space closely — one per square foot, if possible. You can stretch your budget by direct-seeding easy-to-grow shrubs like coralbean (Erythrina herbacea), another hummingbird attractor. Learn more at the Florida Native Plant Propagation Facebook group.
Martine recommends covering the soil with a three-inch layer pine straw, live oak leaves, or melaleuca mulch before planting, clearing mulch six inches away from the bases of trees and shrubs. “Melaleuca mulch is made from trees harvested in the everglades, where they are invasive,” explains Martine. “It breaks down slowly, allows native plants to reseed, and helps restore the everglades.” Definitely a win-win.
In all our gardens, the best defense against invasive plants is a good offense: More (native) plants!
— The Avant Gardener
Fabulous Florida groundcovers (source: Florida Native Plant Society by, left, Marjorie Shropshire and, right, Ginny Stibolt
Why, How, Wow!
Why?
Native means a species occurring naturally in a particular region, ecosystem, or habitat. Our current definition freezes “native” at a time more than 500 years ago.
The word native should always be used with a geographic qualifier (that is, native to New England [for example]). Only plants found in this country before European settlement are considered to be native to the United States. —
But a warming climate challenges what "occurs naturally." In fact, earlier climate changes may have already altered the ranges of native plants.
According to paleoecologists, as the climate cooled and glaciers expanded during the Pleistocene era, forest plants of the Northeast survived by migrating to so-called refugia in the South. When the climate warmed and the most recent ice sheet receded, some species were able to recolonize the habitat they had lost. But others [were not]. —Yale Environment 360
Species migrate — armadillos into Florida, possums and coyotes into the Northeast, and barred owls into the Pacific Northwest. Plants move, too, their seeds spread by air, water, and animals. And invasive plants often migrate more quickly than native ones in response to climate change.
[B]iologists detected significantly fewer shifts in elevation by plants in California than by other organisms such as birds and mammals. They also found that the migration upward in elevation of non-native invasive plant species was nearly five times greater than that of the flora overall, and even more when compared to localized endemic plants. — Yale Environment 360
Government agencies are inconsistent in how they treat changes in the ranges of plants and animals native to this country. For example, U.S. Fish & Wildlife recently announced a program to shoot hundreds of thousands of barred owls in the Pacific Northwest to protect locally native spotted owls they are displacing. On the other hand, Florida law prohibits landowners from removing mangroves – which protect shorelines – even in counties where they are not native and are displacing locally native saltmarsh grasses, as at my Florida home. Hmmm.
Source: The Washington Post
How
So, what’s an ecologically-minded home gardener to do? While ecologists debate the merits of assisted migration —moving native plants to cooler climes ahead of climate change — they don’t recommend you undertake it on your own.
“Native plant horticulture is giving us some fascinating insights into what is likely to happen with climate change,” says [biologist Jesse] Bellemare. “But we’re not at a point where most botanists and ecologists would feel comfortable advocating” that gardeners help protect plants by moving them to cooler climes, he adds. — Yale Environment 360
I recommend choosing plants native both to your county and south of there, like the species Sophie Martine of Florida Native Plants recommends above. In general, plants with wide ranges are easy to grow because they thrive in more varied habitats. Here’s how to come up with a preliminary list to discuss with the experts at your local (or regional) native plant nursery:
Use the combination search function to find plants that match your needs and conditions and are native to your state at the Lady Bird Johnson Wildflower Center. (Or search a relevant region-specific database: California’s Calscape, Florida’s Florida Native Plant Society, or New England’s Native Plant Trust.)
Check county-level native status, by searching the genus then species and looking for light green areas in your state at the Biota of North American Program (BONAP).
In Rhode Island, I also plant some favorite species native further south that thrive north of their ranges — notably, oakleaf hydrangea (Hydrangea quercifolia). I consider these more like well-behaved exotics and don’t count them as part of my “⅔ for the birds.” Who knows, I may be helping save the species.
Coralbean seeds to sow in my Florida garden, after cutting the outer shell with a cuticle nipper (Photo: Heather Evans)
Wow!
The gorgeous central Florida native plant garden below contains many northern temperate meadow species, as appropriate to its location near Paynes Prairie. As you can see in the bottom photo, it also contains southern species like longleaf pine (Pinus palustris) — the Dr. Seuss-looking plant in the foreground, native to southern states but not tropical Florida — and cabbage palm (Sabal palmetto), native to all of Florida.
Garden and image: Harriet Festing and Stephen F. Eisenman
Garden and image: Harriet Festing and Stephen F. Eisenman
Related Resources
Unfamiliar with your state’s native plant society? Find it here.
Need to brush up on your ecological landscaping terms? See my short glossary.
Curious how poet Emily Dickinson is inspiring botanists today? Learn more at Yale Environment 360.
