What Temp Is Frost?
Living here in the western Piedmont of North Carolina, we have the fortune of gardening well into fall. The first local “Frost Alert” has been issued and it looks like our more tender crops will be taken out by Mr. Jack. We still have a bumper crop (for our little space) of sweet peppers and a number of Japanese eggplant bushes that are still yielding. Alas, saying goodbye to what remains of our summer and early fall produce is bittersweet.
Our observations regarding frost indicate it’s not just the temperature. Temps can be as high as 38 to 41 degrees and we’ll get patchy frost. At times when the temps approach freezing, there will be little to no frost. Conditions such as clear skies, little to no wind, a down hill (lower) elevation, and soil moisture all have an impact. From 38 degrees down to just above 32 degrees, chances of frost increase. Of course, from 32 degrees and below it’s frost/freeze territory. 1)https://www.weather.gov/arx/why_frost
These first few cold snaps usually prompt us to layout frost blankets that will stretch the growing season just a little more. Before we delve into frost protection, let’s consider a few resources that can guide our decisions as to when and how we begin to extend our growing season.
Your Plant Hardiness Zone
Eventually, we just have to roll with the seasons. North Carolina has plant hardiness zones ranging from 8B in the Coastal plain down to 5B in the mountains. We happen to be in Zone 7A. Of course, these are averages, but you get a general picture. It’s like gambling with the USDA as your odds maker. Plant sellers and customers who follow the Hardiness Zone Maps and assume “normal” temps will prevail are essentially gambling.
Back in the early 80’s, while working in the Asheville area as a landscaper, a severe cold snap killed off a slew of hollies. The Burford hollies (Ilex cornuta) were decimated. Rated as hardy within Zones 7 to 9, the Burfords had been planted for decades around many of the stately homes we serviced. Some had grown to be like small trees.
We spent most of that spring and early summer, removing the dead hollies and replacing them with hardier plants. It was a bust for many homeowners, but a boon for our company.
The object lesson here is to know the risks of assuming normal. We suggest selection of plants based on a margin of safety-at least a zone lower, if not two. We take a chance on a Zone 7A plants now and then, but are much more comfortable with 6A.
For a more specific hardiness map of your location there are USDA maps that can be zeroed in via zip code. These local maps take into account elevations. Usually, the higher the elevation the colder the zone. Hence, our upper garden is likely more like a 6B since it’s at a higher elevation and close to Pilot Mountain State Park.
Finding Frost Temperature for Your Area
We remind ourselves occasionally that the Ag Zones don’t specify frost dates. For that info, we go to the NOAA for state by state climate data. You can look up frost and freeze data on their home page by clicking on your state and then finding the closest town/city/airport that is collecting data. There you’ll find projected dates of last and first frosts and freezes. Only this time, they’re more specific. Below is a section on North Carolina.
For our own area (near the Greensboro Airport), we see that the first frost date is pretty accurate. The weather forecasts a potential first frost on October 22.
Once we have an idea of upcoming weather conditions, we’re better informed on when to start using plant covers to extend our growing season. A proven method is the use of various thicknesses of frost cloth.
These covers don’t just keep the cold from intruding. They also keep the ambient warmth from radiating outward. In this way, garden row covers are like cloud cover that interferes with the earth’s heat radiating into space. Laser thermometer measurements indicate a cover can conserve as much as 12 degrees more heat.
We borrowed this chart from a maker of agricultural row covers to illustrate the types of coverage and frost protection. The more weighty the fabric, the more protection but also the less light transmission. Weather conditions permitting, consider using your heavier fabrics mainly overnight and leaving the crops open to sun during the day.
Floating Row Covers
Floating row covers are rather gauzy materials that are often used for barriers against insects. They are lightweight enough to rest directly on the plant without damage. On our fall crops, we already have one layer. With the onset of “Frost Alerts”, we ad an additional layer of the same lightweight fabric. We prefer to tent ours over a taut string or wire. Then, we pin the sides with landscape staples. The ends can be closed off with clothespins or metal binder clips. Metal hoops, sold by on-line row cover suppliers, can be used to create a floating row tunnel. There are also various pre-fab tunnels that offer varying levels of frost protection.
An upgrade of plant protection for winter, we’ve used frost blankets successfully now for several years. They’ve been particularly useful over crops like broccoli, collards, carrots, Bok Choy, spinach and Swiss chard. We’ve also deployed frost blankets over cattle panel arches as a kind of stand-up cold frame. This allowed for harvesting arugula all winter long.
A few pointers about these heavier fabrics. They do have weight and it’s better to use wire hoops, bamboo, PVC pipe, or conduit pipe as a support. Your tents and tunnels will also need to stand up to the wind and snow. If you can engineer a way to easily remove and replace the frost cover, all the better for allowing the plants to soak up the sun on warmer days. This removable feature will also allow you to check plants for any problems and to harvest.
We are always eager to hear from our fellow garden enthusiasts. Let us know what you’ve been doing to extend your growing season. We’re especially keen to hear about your take on plant covers. Comments, photos, references are all welcome on our Facebook page or by email.
G. Coleman Alderson is an entrepreneur, land manager, investor, gardener, and author of the novel, Mountain Whispers: Days Without Sun. Coleman holds an MS from Penn State where his thesis centered on horticulture, park planning, design, and maintenance. He’s a member of the Phi Kappa Phi Honor Society and a licensed building contractor for 27 years. “But nothing surpasses my 40 years of lessons from the field and garden. And in the garden, as in life, it’s always interesting because those lessons never end!” Coleman Alderson
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