20 miles away and at the same latitude as ours, the brassicas suffered considerable damage when the temperature reached the mid-20s Thanksgiving morning. That same morning, ours sailed through 22 degrees with no discernible damage. Why the difference?
We play detective in this week’s podcast. Taking in turn each factor known to affect the cold-hardiness of brassicas, we try to determine what made the difference between the kale, broccoli, and cabbage growing in the Learning Garden at the Elmore County Extension Office at Wetumpka and the same crops here near Tallassee.
Listen – 20:49
Unfortunately, we don’t have a photo to share with you showing the frost damage to the kale and cabbage at the Learning Garden. Please take our word for it that they suffered considerable foliar damage but that they survived. With their damaged foliage pruned, they’re looking pretty good now and seem to have survived the Thanksgiving insult as well as can be expected.
What we can show you are a napa cabbage heading up on Veg Hill with no visible damage from the freeze, as well as some collards on the right that are growing in the next bed over. For what it’s worth, the collards at the Learning Garden seem to have suffered no damage from the freeze either.
If you’re one of those “bottom line” types who wants a quick answer, our guess is that the differences in levels of damage are due primarily to two factors:
- (a) elevation (we’re at 328 ft above sea level, and the Learning Garden is at 183 – thank you Google Earth) because cold air falls and warm air rises; and
- (b) the greater height of the beds at the Learning Garden, which meant that (1) the plants were farther away from the warm ground, and (2) the beds may have been drier by the time of the freeze. We know that damp soil holds much more heat and feeds it back to the plants more than dry soil.
If you want to delve deeper, Amanda is the guru on these issues, so I asked her to provide me the notes from her research that she used as we looked at each factor:
Texas A & M Extension, University of Illinois Extension “Gardeners’ Corner,” Botanical Interests site
In general, a frost (31-33 degrees F.) will kill beans, cucurbits, corn, nightshades, okra, and sweet potatoes. We experienced this November 13-14, when it got down to 27 & 28 degrees.
Colder temperatures (26-31 degrees F.) may burn foliage but will not kill broccoli, cabbage, cauliflower, chard, lettuce, mustard, onion, radish, and turnip.
The real cold weather champs are beets, Brussels sprouts, carrots, collards, kale, parsley, and spinach.
Any temperature below 25-28 degrees F is dangerous.
On November 28 (Thanksgiving morning) we had a low of 22.
Temperature is not the only factor:
1. Preconditioning. If drop in temperature is sudden, plant is at higher risk for damage.
2. Distance from ground. The further a plant or its parts are from the ground, the more likely it is to be damaged by frost. The ground is usually still warm in early fall and will radiate some warmth to plants that are close to the ground.
3. Moisture. Humidity can help protect plants from frost. Humid air holds more heat and reduces the drying effects of frost.
Wet soil can hold four times more heat than dry soil. The air temperature above moist soil can be as much as 5 degrees F higher than the air temperature above dry soil.
4. Air movement also has an influence on frost damage. When wind blows during cold nights, it sweeps away any warm air trapped near structures or the ground, eliminating their insulating capabilities.
5. Protection. Floating row covers. U of I says it offers 2-5 degrees protection. They did not specify weight of the row cover. We have Agribon 15 over one row of rutabagas & kale (insect barrier). It allows in 90 % sunlight. We are guessing it probably offers 1-2 degrees protection (only a guess; the product claims to offer “a little frost protection.”)
We also have deployed Agribon 30 over lettuce (overnight only); it allows in 70% sunlight. It claims to offer 4-6 degrees protection. The lettuce came through the 22-degree night unscathed.
6. Altitude. Cold air falls, warm air rises.
Our altitude is 328, Learning Garden is 183.
7. Length of exposure—we don’t have that information about the latest damaging frost.