Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Fall vegetable growth is greatly affected by daylength hours and heat units. Warm season vegetables also will be terminated by killing frosts. (Information on fall frost and freeze changes in recent years will be detailed next week.))
The following are some charts showing these effects.
By Labor Day, daylength will drop below 13 hours. This is the same number of hours as April 9. Chart from timeanddate.com
By the middle of October daylength has dropped below 11 hours. Loss of daylength will reduce hours of photosynthesis and thus potential growth.
Another major influence is heat units. As fall progresses, heat units also decrease significantly. A current question is as years become warmer, will there be more heat units for fall growth?
September 2021 Growing Degree Days dropped below 10 on 9/29
October 2021 Growing Degree days dropped below 5 on 10/19 in 2021