Emmalea Ernest, Extension Fruit & Vegetable Specialist; emmalea@udel.edu
Since mid-March of this year, temperatures have been warmer than typical and in the last 30 days temperatures in much of Delaware have been 4 to 6°F above average (Fig. 1). This could have an impact on the timing of vegetable management practices and harvest. One way to understand and predict the harvest timing effects of temperature is to keep track of growing degree days (GDDs). Below is a discussion of warm temperature effects on cool and warm season vegetables, using GDDs to understand how this season might be different from past years.
Figure 1. 30-day departure from normal temperatures for the Northeast US. Obtained from the Delaware Climate Office.
Cool Season Vegetables
Early maturity in cool season vegetables like peas, lettuce, and cole crops can already be observed this year. This isn’t surprising when you look at a chart of this season’s base 40 growing degree days (GDDs). For example, the fastest maturing pea varieties are typically harvested at 1250 base 40 GDDs. At UD’s research farm in Georgetown, we are forecast to reach 1250 GDDs on May 20, for a crop planted on March 15 (Fig. 2). In the previous five years (2020-2024) we did not reach 1250 GDDs from a March 15 start for another 7 to 14 days (May 27 to June 3). This means that peas are maturing a week, or more, early. Potatoes are also growing quickly this spring and could be ready earlier than usual if warm temperatures continue in late May and June.
Figure 2. Accumulated base 40 GDDs with a March 15 start date at the Carvel Research and Education Center in Georgetown from 2020 to 2025. Chart created by the author based on weather data from DEOS and forecast from NWS.
Warm Season Vegetables
For warm season vegetables, base 50°F (or higher) GDDs are used to predict growth. Base 50 GDDs have also been accumulating quickly this spring. Figure 3 is a chart of accumulated Base 50 GDDs for Georgetown, DE starting April 21. This chart was created using Cornell’s Climate Smart Farming Degree Day Calculator. This tool allows you customize base temperature, planting date and location. The blue and purple lines on the chart indicate 15- and 30-year averages and the gray area indicates the high and low extremes for the period of record. This year’s conditions are tracking above the gray highlighted area, indicating that we have higher base 50 GDD accumulation and warmer conditions than what’s been observed previously.
Figure 3. Accumulated base 50 GDDs with an April 21 start date for Georgetown, DE. Chart created using the Climate Smart Farming Degree Day Calculator.
Early sweet corn and melons have established well under this season’s warm conditions and the quickly accumulating GDDs could result in early maturity. Keep an eye on GDD accumulation using a tool like the CSF Degree Day Calculator. Also, pay attention to key crop phenological stages that are indicators for management practices. They may be happening earlier than usual this year! For example: apply row middle herbicide sprays before melon vines get too long, place bees in time for early melon pollination, stake pepper and tomato transplants that are growing quickly, and keep an eye on sweet corn silking for spray timing. Besides all that, keep an eye on the weeds; it seems like they are growing rapidly already.
For more on using GDDs for sweet corn management see these past WCU articles:
Using Growing Degree Days to Schedule Fresh Market Sweet Corn