Here in southeastern Pennsylvania, the old hardiness zone of 6b has changed thanks to a warmer, wetter climate of late. According to the USDA, the Lehigh Valley is now considered zone 7a.
Welcome news? A little mixed, actually.
The U.S. Department of Agriculture (USDA) updated the national plant hardiness map in November 2023. Above is the rezoned Pennsylvania map. If you want to see the national map, here’s a link to it: https://planthardiness.ars.usda.gov/.
Southeastern Pennsylvania has warmed an average of 5 degrees since the last time the map was updated, according to the USDA. However, I wouldn’t take this as a sign that we can plant zone 7 perennials with abandon and expect them to thrive. Some will, some won’t.
As you can see, zonal boundaries often follow the topography because higher elevations are colder than lower ones. Having previously lived on a mountaintop, I know from experience that even a few degrees’ difference absolutely affects hardiness. These higher spots are still zoned 6b, so it all depends on your location.
Of course, sudden plunges in temperature kill more plants than sustained single-digit temps regardless of zone. You don’t need me to tell you that our weather has been especially volatile lately. Remember those weird days in March when it soared into the seventies, almost 25 degrees above normal? Everything started to wake up. Then the mercury dropped into the twenties overnight. March was colder than normal and rainy. Shifts like this can be deadly even for normally very hardy plants.
But for gardeners with a protected spot and a hankering for tender perennials, perhaps that zone 7 favorite you’ve had your eye on finally has a fighting chance. Still, I’m reluctant to try most zone 7 plants because of poor luck in the past. Another challenge is too much water as conditions turn rainier. I don’t have very many protected areas, so I’m not the best candidate for tender perennials anyway. But hope springs eternal, as the saying goes.
Plants to Try — Maybe?
I may not have a spot suitable for borderline-hardy plants, but maybe you do. Here are a few zone 7 perennials often touted as hardy to zone 6 “with protection” which I consider a safer bet. Everyone has their own micro-climate, so judge for yourself. Just know if you choose to plant any of these, you are taking a risk no matter what the salesperson (or tag) tells you.
Pink Muhly grass — A willowy, fine-textured grass prized for its bronzy-pink seedheads, culminating in a “pink haze” of color by fall. I fell in love with this grass and got to adore it for a single season before it promptly died over winter. If you can coax it to survive multiple years, bravo!
Hardy Camellia “April” Series – Developed explicitly for colder climates by Dr. Clifford Parks of the University of North Carolina, this series of hybrids includes pink, red and white doubles. I planted April Rose in an “ideal” location and it still struggled in fits and starts. Late freezes were especially damaging, and the deer didn’t help matters. (They love the flower buds.) I finally pulled it out.
If you want to try an April series camellia, position it in a protected niche next to a warm wall, provide well-drained humus, afternoon shade, and give it a burlap wrap in winter. Once established I think they will do fine here. It’s those first couple of years when they’re young and tender that are so challenging.
Chinese Paperbush (Edgeworthia chrysantha) – Here is an uncommon plant that I really covet. I still think it’s too risky to try an edgeworthia in this area unless you have the absolute ideal location, but I’m listing it anyway because it’s just. so. lovely! Admittedly this plant is still a rarity in the U.S. You’ll have to seek it out if you really want to try one.
Edgeworthias bloom very early, sometimes with the witch hazels in late winter. The silky, fragrant, golden-ombre blooms are simply exquisite. As you can see from the photo above, the form is semi-dwarf, compact and spreading, wider than it is tall. If you’ve got a spot by a warm wall or in front of a bank of windows where you can admire it on chilly days, you might give it a try. Edgeworthia is rated hardy to zones 7 or “6 with protection”. So… edgy, but surely worthy!
Crepe Myrtles — What’s not to covet about Southern myrtles in their saturated colorful robes in August! Numerous cultivars are touted as hardy in zone 6a, but really, for how long? Now that we’re in 7a can we grow them? Perhaps in a very secluded location you can get one to thrive for years. It does seem to be getting easier.
In the past, I’ve watched multiple myrtles thrive in other people’s yards for a few seasons only to suddenly die. Let’s hope as the hybrids get hardier and our winters warmer, local survival rates will improve. For now, consider a crepe myrtle for a protected niche shielded from prevailing winds. Perhaps like the cardinals, crepe myrtles will gradually become a common sight here as the climate continues to evolve.
These are just a few tender possibilities for those extra warm and protected nooks and crannies. Who knows, maybe we’ll soon be able to grow oleanders and more magnolias and other Southern delights!
No matter what zone you’re in, our weather is changing–wetter, stormier, hotter, more destructive. The best we can do is cope and adapt as best we can, helping our garden plants do the same. I try to look on the bright side and appreciate whatever new specimens will thrive that I couldn’t grow before. So bring it on, zone 7b– let’s see what new and exciting combinations we can create over the next few years!
Crepe myrtles…sigh…if I had a dollar for every one that died on me. There are some very beautiful large specimens in the area but sadly not in my yard. I lived in Wilmington DE for several years. It’s a definite zone 7 there and it makes a difference.
I hear you, Kate. Let’s hope someday!