In Spring 2025, we evaluated eight broccoli cultivars at the Southwest Purdue Agricultural Center. The cultivars included: Asteroid, Castle Dome, Emerald Crown, Eastern Crown, Millennium, Imperial, Green Magic, and Eastern Magic. Seeds were sown in the greenhouse on March 10 and transplanted into the field on April 1. The soils at the site were sandy loam, with an organic matter content of 0.7%.
Approximately one week after transplanting, the crop experienced a cold spell with nighttime temperatures dropping to 28°F and daily averages remaining below 50°F for about a week. Nearly six inches of rainfall occurred during this period. Despite the stressful conditions, most plants survived and resumed vegetative growth once temperatures normalized. Overall, the spring weather remained relatively mild. Average daily temperatures stayed below 70°F for most of the season, with a few days with maximum temperatures exceeded 85°F in mid-May and early June.
Physiological disorder due to heat stress
Among the cultivars, Castle Dome was the earliest to reach full crown size, followed by Green Magic and Emerald Crown. These cultivars were harvested on May 22 and May 27. Eastern Crown, Eastern Magic, and Asteroid were harvested on June 3. All six cultivars produced decent-quality heads. Millennium and Imperial were the latest maturing cultivars. They were harvested on June 9 and 12.
Between June 3 and June 8, daily minimum temperatures were around 65°F or above, high enough to cause injury to broccoli heads that hadn’t been harvested. Millennium showed symptoms of brown bead; Imperial had less brown bead, but some plants showed loose head formation. Overall, Imperial produced better quality heads compared to Millennium after the heat stress. The side shoots of Castle Dome, Green Magic, and Emerald Crown developed during the heat wave displayed uneven bud development.
Figure 1. Physiological disorders of broccoli head after heat stress: brown bead (left), loose head (middle), uneven bud development (right) (Photo by Wenjing Guan).
Broccoli is highly sensitive to heat, particularly during early head formation. Dr. Thomas Björkman explained that the minimum temperature is the most critical during this period, with a threshold temperature around 62 to 65 °F. The severity and type of heat stress symptoms may vary depending on the cultivar and the developmental stage of the head at the time of exposure. Given this sensitivity and the unpredictability of spring weather, spring broccoli production in southern Indiana carries a high risk.
The Eastern Broccoli Project has recommended cultivars for spring planting/early summer harvest. This can be an excellent resource for Indiana growers to select cultivars. Additionally, Dr. Thomas Björkman mentioned the new heat-tolerant hybrid EXP 3622, developed by the Eastern Broccoli Project, which is now commercially available.
Stem hollow and discoloration
We observed minor symptoms of stem hollowing and discoloration. Hollow stem was most pronounced in the variety Green Magic, with Emerald Crown also showing a slightly higher incidence compared to the other cultivars. The stem discoloration is likely caused by boron deficiency. While varieties can differ in their susceptibility, no clear varietal differences were detected in this trial. Interestingly, one specific experimental plot exhibited a high percentage of the stem discoloration symptom, which may be attributed to soil conditions in the area or uneven fertilizer application.
Figure 2. Stem hollow (left) and discoloration (right) observed in the trial (Photo by Wenjing Guan).
Dr. Thomas Björkman provided insight into the cause of these symptoms. He noted that while a hollow stem can be associated with boron deficiency, it is not the most common cause. A more typical sign of boron deficiency is browning of dead or dying cells lining the hollow cavity in the center of the stem. In contrast, hollow stem is more often the result of rapid growth driven by high nitrogen levels, warm temperatures, abundant water, and wide in-row spacing. Dr. Björkman suggested that if growers frequently experience hollow stem, reducing in-row plant spacing may help lower its incidence and severity.
This broccoli cultivar evaluation project is a collaboration with Dr. Emmanuel Torres Quezada at North Carolina State University. The same cultivars were evaluated across multiple states under a range of environmental conditions. We will share the full project report once it becomes available.