Getting ready for frost, harvesting sweet potatoes

Sweet potato harvest. Photo Nina Gentle

Sweet potato harvest.
Photo Nina Gentle

Our average first frost date is October 14. Actually from our own records it has averaged 10/22 over the last 11 years. So any time now, we could get a frost. We had a very light patchy frost 10/10 with a recorded overnight low of 36F, but now we are having a warm spell (very nice). It’s good to be prepared. We harvested our sweet potatoes (not a good yield, too many deer). We are getting ready to harvest our white potatoes.

DIY weather-forecasting

I recommend learning the local weather patterns by keeping records and watching what happens. Here’s what I’ve learned about ours:

  • Our mid-Atlantic climate is controlled by three weather systems,
    • mainly by moisture from the Gulf of Mexico,
    • the Bermuda High Pressure area in summer,
    • recurrent waves of cold Canadian air in winter.
  • Rain (fairly evenly distributed throughout the year in our county)
    • has slight peaks in January, February and March
    • and again in early June and August.
  • Some parts of our area can experience long periods of drought.
    • September-November is the drier season but it’s also the hurricane season, so the net result is very variable.
  • We use Wunderground.com but subtract 5F° from their forecast night lows for our nearest town, and mentally downgrade the chance of rain by 10%, as rain often passes us by as it scoots along the river valley north of us.

Local Information

According to  a Freeze / Frost Occurrence Data pdf for Virginia which I have, in Louisa County the threshold of 36F has a 50% likelihood on Oct 3; the 32 F threshold has a 50% likelihood by Oct 13 and the 28F threshold is as likely as unlikely by Oct 27. The 90% chances occur by Oct 14, Oct 28 and Nov 13 respectively. The data for spring are available here. probably the site has the fall info too, but I’m out of time to look, due to three hours without internet this afternoon.

Frosty daikon leaves. Photo Bridget Aleshire

Frosty daikon leaves.
Photo Bridget Aleshire

Help predicting frost

We keep this reminder list handy at this time of year, when trying to make our own frost forecasts:

Frost is more likely at Twin Oaks if:

  • The date is after 10/14 or before 4/30.
  • The daytime high temperature was less than 70°F (21°C).
  • The sky is clear.
  • The temperature at sunset is less than 50°F (10°C).
  • The dew point forecast is low, close to freezing. Frost is unlikely if the dew point is 43°F or more.
  • The Wunderground 3.30pm forecast low for Louisa Northside is 37°F (3°C) or less.
  • The soil is dry and cool.
  • The moon is full or new.
  • There is little or no breeze, although if temperatures are falling fast, the wind is from NW and the sky is clear, then polar air may be moving in, and we’ll get a hard freeze.

We also make for ourselves a Frost Alert Card, so we can quickly know which beds we need to attend to if we decide a frost is likely. (RBs means raised beds)

Task Crop Notes
Harvest all edible Asparagus beans
Harvest all edible Eggplant
Harvest all edible Okra
Harvest all edible Tomatoes Incl green
Harvest all edible Peppers exposed to the sky
Harvest all edible West Indian gherkins
Harvest all edible Pickling cucumbers
Harvest all edible Corn
Harvest all edible Beans #4, 5 Uncover once mild again
Thick row cover Beans #5,6 Ditto
Thick row cover Squash Spring hoops or none. Ditto
Thick row cover Slicing Cucumbers Spring hoops or none. Ditto
Thick row cover Celery
Thick row cover Last Lettuce bed Double hoops – leave covered
Set sprinklers Slicer tomatoes Overnight from before 32F till after sun shines on plants
Set sprinklers Romas and peppers Ditto
Set sprinklers Other vulnerable RBs Ditto

FrostProtectionFundamentalsPracticeandEconomicsFAO.pdf

is a 126 page book which includes explanations of freezes and frosts (both radiation frosts and advection frosts), and a chapter on recommended methods of frost protection for food crops. probably none of us will need the information on wind machines and helicopters (maybe little drones work for small scale growers?). The publication includes frost forecasting and frost damage physiology (for growers not in a panic about immediate frosts, who want to understand what the plants are going through).

Potato harvest November 2015 Photo Lori Katz

Potato harvest November 2015
Photo Lori Katz