|We can have severe weather.|
I've written often in recent weeks about the USDA Plant Hardiness Zone system. Today represents why it was created. The expected overnight low temperature puts us smackdab in the middle of the average annual minimum temperature depicted for Zone 5. Average minimum temperatures for Zone 5 can be expected to range between -10 and -20 degrees F. One of the nice things about the USDA system (and the Hardiness Zone Map for Europe) is that you don't need to keep a record of your own personal temperatures; you can determine your Zone by looking at their map.
Plants handle cold temperatures differently. Some can shrug off severe cold, some can't take anything near freezing. Many seed companies, nurseries, and plant growers identify their plants by the respective Zone. It's becoming standard and works very well. Any plant identified as appropriate for Zone 5 can handle the temperatures I'm expecting today.
The system doesn't have to be restrictive or limiting. Gardeners who want to grow plants not suited for their region just need to do make more effort. I can grow Zone 6 plants, suitable for minimum temperatures of 0 to -10 degrees F. On nights with colder temperatures the plants need to be protected from the severe cold. This can be done with smaller perennials, not so easily with large bushes or trees.
I planted a number of Lavender plants last year, all identified as appropriate for Zones 5-10. I avoided purchasing a French Lavender identified for Zones 6-10. If I did have that plant I would need be concerned about the forecast for tonight, but it wouldn't have to be deadly to the plant if I took action. I would only need to find a buffer of five degrees to bring the plant into a safe zone.
Covering it with a plastic tarp, blanket, or large bucket should be enough to trap warmer air from the day and provide slightly warmer air near the plant. A combination of the three provides extra protection. If I wanted to ensure warmer temperatures I could string Christmas lights (the kind that get hot, not LED) in a frame over the plant and cover it. Colorado State University conducted tests of various coverings for plant frost protection and found the light and plastic method gave 6 to 18 degrees F of protection. Covering with a space blanket extended the protection by up to 30 degrees F. With extreme cold you might not get similar results, but we're only seeking a few extra degrees.
Ideally I would have recognized the possibility of severe cold and planted this Lavender close to my house, a structure that actually generates heat during the winter. Between the residual heat of the house and a plastic cover, it might be enough to protect it from the cold. The south side of a stone fence or any similarly protected area could work.
Even with a covering, it's likely that part of the plant would suffer; exterior branches and buds might die, but the main plant would survive. If the plant was particularly prized by me, it might be worth the effort to save it.
I don't always want to work that hard, however, so it's easier to plant plants that are appropriate for my Zone. Toiling outside at 7 degrees F can be painful and I try to avoid it if I can. It makes more sense to put in perennials that can easily handle the cold so I can sit in my chair enjoying the fire warming the room while it's cold and snowy outside. Especially with the snow acting as an insulating blanket, I'm not worried about any of my plants tonight.
To have even less worry, I try to plant with a colder zone in mind. If I can find Zone 4 plants, suitable for -20 to -30 degrees F, I don't need to lose any sleep on the rare nights when the temperature drops to -25. The cherry tree I planted is suitable for Zone 3. Assuming I can avoid deer damage, I'll never need to worry about cold temperatures affecting it.
I don't enjoy this severe cold any more than the plants do. Both of us know that warmer weather is on the way. Surviving the extremes is all either of us needs to be concerned with, but with planning and a little research before planting, the concern is eliminated. Right now I'm enjoying my fire, looking out at the snow, knowing it will be VERY cold tonight, and not worried about my plants at all.