Considerations with Later Fall Vegetable Planting Dates

Emmalea Ernest, Extension Fruit & Vegetable Specialist; emmalea@udel.edu

As we get warmer fall weather conditions people often wonder if the planting dates for fall harvested vegetables can be pushed later in the season. Three things to think about when considering later plantings are the first frost and hard freeze dates, average fall temperatures and decreasing daylight hours. The frost/freeze dates and average fall temperatures will be impacted by climate change. Daylength will not be affected by climate change, however wildfire smoke has potential to decrease available light to crops and this will have a more acute affect in the fall when light becomes a limiting factor in crop growth.

The most recently calculated fall frost and hard freeze probabilities for four locations in Delaware are shown below in Table 1. The growing season for warm season crops ends with the first frost event. This typically occurs before early November and certain sites will be inclined to frost earlier in the season.

Table 1. Freeze and Hard Freeze Probabilities for Four Locations in Delaware for 1991-2020.

* Rare = 1 year in 10; Uncommon = 2 years in 10; Typical = 5 years in 10
Data is courtesy of Kevin Brinson, Associate State Climatologist and DEOS Director

Growing degree days are one way to account for temperature affects on crop growth. Figures 1 shows August through November Base 50 °F and Base 40 °F growing degree day accumulation at UD’s Georgetown research farm from 2018 through 2022. Growth rates for warm season adapted vegetables can be approximated with the Base 50 GDD model. These crops will slow or stop growth in early October when average temperatures are no longer above 50 °F. Cool season adapted crops will slow or stop growth in early November when average temperatures are no longer above 40 °F. The annual variation in base 50 GDD accumulation occurs mainly in September and variation in base 40 GDD accumulation occurs mostly in October. In the five years included in the Figure 1 charts there was more variability in base 50 GDD accumulation which would be observed as more variability in the fall growth of warm season adapted crops.

Figure 1. Base 40 °F and Base 50 °F growing degree day accumulation at the Georgetown, DE research farm from Aug 1 to Nov 30, 2018-2022

Daylength decreases in the fall, with less than 11 hours per day of light by mid-October. Decreasing daylength reduces hours of photosynthesis and thus potential growth. Even in warm years, crop growth rate will be lower in the fall because of reduced daylength. Periods of cloudy (or smokey) weather will have more impact on crop growth in the fall compared to the summer because of the already lower available light.

The trend toward warmer fall conditions means that later vegetable plantings will be successful in some years, but continued variability in temperature means that such plantings will still be a gamble. Also, warmer temperatures and later freeze dates will not overcome the slower fall crop grow that results from less light. There are more daylight hours on March 1 (with subsequent increasing daylength) than on November 1 (with subsequent decreasing daylength).