Heat Tolerant Broccoli Project

photo of heat trial in the field

This project aims to identify heat-tolerant broccoli cultivars for the Willamette Valley processed vegetable industry.

Introduction: With the hot summers of recent years causing significant heat damage to broccoli heads during harvest, it was deemed necessary to screen new broccoli cultivars for tolerance to heat. Temperature is often the main limiting factor in determining whether a broccoli variety can be grown successfully within a particular region, and mounting evidence of global climate change suggests the need for a more heat tolerant broccoli will only increase in the future. With most commercially available cultivars being bred for production in mild and stable climatic regions such as the Salinas Valley in California, these types of heat tolerance evaluations become necessary to ensure the long term viability of the broccoli industry in the Willamette Valley. From the perspective of commercial growers and farmers, heat tolerance alone does not make a broccoli variety suitable for large scale field plantings. More often than not, yield becomes of paramount concern for growers, while aspects of head quality and processing characteristics are the main concerns of the vegetable processors. The goal of this work is to identify broccoli hybrids with the most complete range of desirable characteristics, including heat tolerance, for Willamette Valley production.

Objective: The objective of the heat trial was to assess commercial hybrids and experimental breeding lines for suitability and overall tolerance to the climatic growing conditions commonly found in the Willamette valley.


Germplasm: Broccoli germplasm with the potential for heat tolerance was identified by contacting seed company representatives and examination of seed catalogs. Representatives of seed companies with broccoli breeding programs also identified yet-to-be-released lines of broccoli which were described as being tolerant to high temperature environments. Cooperating university and USDA breeders associated with the USDA- NIFA-SCRI funded Eastern Broccoli Project also sent seeds for evaluation in this trial.

Experimental design: The heat trial was designed with six different planting dates spaced one week apart to maximize the potential range of temperature variability within a single growing season. Studies conducted by Bjorkman and Pearson (1998) have found there is likely a specific physiological period of time in a broccoli plant’s growth wherein excessive heat is the most detrimental to the formation of a broccoli head with desirable characteristics and quality. This 5-7 day period of time occurs when the broccoli plant is transitioning from vegetative growth into a reproductive phase, and the initial enlargement of bud primordia begins. Having six planting dates maximizes the potential of being able to correlate declines in plant performance with increases in ambient temperatures. 

Broccoli lines were seeded the OSU campus greenhouses and approximately 4 weeks later transplanted into the field site at the University’s Vegetable farm in Corvallis. For each planting date, 15 seeds of each broccoli variety were started in seedling trays and the best 10 seedlings were then transplanted into the randomly assigned field plots using a transplanter (using one foot in-row placings). There was one replicate (plot) per treatment (broccolli cultivar/line) per transplant date. In 2020, the six seeding dates were 10, 16, 23 & 30 May and 6 & 13 June. Two temperature data loggers were placed into the trial to record the ambient air temperatures at canopy height for the duration of the trial.

Measurements:  The growth of broccoli and trial site was closely monitored for any pest problems and fertilized with standard agronomic inputs. Each head was harvested individually when it reached a prime stage of maturity and then evaluated for the criteria designated as being relevant to assessing the level of heat damage which occurred for that variety and planting. With each head being harvested individually, the number of days required to harvest a given planting can also estimate the cultivar’s propensity for maturing uniformly. Evaluations of broccoli head quality consisted of rating each head for: color uniformity, bead uniformity, head uniformity, head firmness, and head diameter.  Yield variables including head weight (cut to 6.0 in length), weight of total florets, and weight of usable florets (< 2.5 in) were also quantified. 


2021 Field Trial Report

Amer. Soc. Hort. Science poster presentation (August 2021)