Is Your Crop Ready for the First Fall Frost? How In-Field Data Buys You Time

For many growers, fall comes with the hardest part of the season still ahead: late tomatoes and peppers still in the field, grapes and apples still ripening, citrus sizing up, nursery stock hardening off. One cold, clear night can undo months of work.

Timing the first frost has also gotten harder. A Climate Central analysis of 200 U.S. cities found the first fall freeze now arrives an average of 11 days later than in the early 1970s, and at least two weeks later in 57 of those cities. A later frost gives you more growing season, but it also means the old calendar rules are less reliable. The first frost often arrives after a long run of mild weather, when growers aren't expecting it.

This year adds another variable. NOAA's Climate Prediction Center gives a greater than 90% chance of a very strong El Niño this fall and winter. El Niño winters often run milder across the northern U.S. and cooler and wetter across the South. But these are seasonal averages. A mild season can still bring a sharp freeze on any clear, calm night.

The answer is to watch what is happening in your own fields, not the regional forecast.

Two ways frost damages crops

Radiation freeze. This is the most common fall frost. On calm, clear nights, plants and soil lose heat to the sky faster than they take it in. Cold air settles near the ground while warmer air stays above, which is called a temperature inversion. Temperatures at crop level can be several degrees colder than at the height where a typical weather station measures.

Advective freeze. A cold air mass moves in, usually with wind, and replaces the warmer air. These events cover large areas and are harder to protect against, but early warning still gives you time to harvest what you can.

In both cases, the damage comes from ice crystals forming inside plant tissue and rupturing cells. The result is the limp, blackened, water-soaked leaves and fruit growers know well. Repeated freezing and thawing makes it worse.

Four things to watch besides the thermometer

1. Dew point

Humidity matters as much as air temperature. When water vapor condenses into dew or frost, it releases heat, which slows the temperature drop. On dry nights with a low dew point, temperatures can fall quickly and keep falling. When the dew point is below freezing, crops can freeze without any visible frost forming. This is sometimes called "black frost," and it often catches growers off guard. A station that measures humidity and dew point shows how steep the overnight drop is likely to be.

2. Your crop's damage threshold

A frost usually means 32°F. A hard freeze is 28°F or below. What matters is the temperature where your crop and its current stage take damage. Tender crops like tomatoes, peppers, beans and squash suffer at or near 32°F. Brassicas and root crops can handle colder temperatures. Grapes and apples still on the plant fall somewhere in between, depending on sugar content and how exposed they are. Set frost alerts for each crop, not one number for the whole farm.

3. Low spots

Cold air flows downhill like water and pools in low areas. The bottom of a block can be several degrees colder than a station on higher ground. If you grow on sloped or uneven ground, a single station can miss what's happening in your most vulnerable areas. Adding a sensor node in known cold spots gives you a truer picture. Learn more here.

4. Soil moisture

Moist soil stores more heat during the day and releases it more slowly at night than dry soil. A well-watered, firm soil surface can keep the air around the crop warmer on a frost night. Soil moisture sensors show whether your ground is ready to hold heat before cold weather comes, and whether a pre-frost irrigation would help.

When an alert comes in: common protection strategies

• Row covers and frost blankets for vegetables and low-growing crops. Put them on before sunset to trap the day's heat.
• Overhead or under-canopy irrigation. Water gives off heat as it freezes, which protects plant tissue. Start early, based on dew point and air temperature, before crop temperatures reach the danger point. Keep it running until all the ice has melted. Stopping too soon can make damage worse than no protection at all.
• Wind machines in orchards and vineyards work by pulling warmer air down from above an inversion. They help on calm radiation-frost nights and do little during a windy advective freeze. Temperature readings at different heights, or local knowledge of your inversions, tell you whether turning them on is worth the cost.
• Harvest early when a hard freeze is forecast and the crop can't be protected.

Each of these costs money in labor, water or fuel. The better your field data, the less often you turn them on when you don't need to, and the less often you get caught unprotected.

How Davis helps you stay ahead of frost

A Davis weather station measures temperature, humidity, dew point and wind at your site around the clock, and WeatherLink sends alerts to your phone when conditions cross the thresholds you set.

For commercial operations, EnviroMonitor with the Mobilize app goes further:

• Frost alerts for each field and crop, so the vineyard block and the pepper field each get their own warning point
• Sensor nodes for cold spots, soil moisture and soil temperature across your operation
• Chill hour, growing degree day, irrigation and pest reports in the same app, so the station keeps working after frost season

A typical starter system includes a Vantage Pro2 GroWeather sensor suite, an EnviroMonitor IP / cellular gateway, and a service plan. Learn more here.

Don't wait for the first frost warning

Set up your alerts now, while the nights are still mild. When the first cold, clear night arrives, you'll have the lead time to cover, irrigate or harvest.

Talk to a Davis Agriculture Specialist | Download the Agricultural Weather Solutions Buying Guide