Broccoli and Summer Heat

Many vegetable growers in central and southern New England avoid growing summer broccoli because head quality suffers from the heat. Some grow it all summer, but have to deal with lower quality and more head rot during the hottest part of the summer. Even ‘early fall’ broccoli, harvested in early September, may be subject to summer heat during critical growth periods.

Growing

Growing quality broccoli through the hottest part of the summer is a tricky proposition, and while there isn’t a silver bullet that will ensure a perfect crop, there are ways that you can mitigate your risk and ensure the best possible broccoli crop all summer long.

Research done by Thomas Bjorkman at Cornell University, using the cultivar Galaxy, found that the critical period for heat sensitivity in broccoli only lasts for roughly ten days.  This ‘window’ of sensitivity corresponds to the time when the growing tip shifts from vegetative growth to flower bud initiation.  This is a period of about 10 days prior to when a tiny crown is visible in the center of the plant. Temperatures above 35 degrees Celsius (95 degrees Fahrenheit) for more than four days during that period causes uneven bud development at the bud initiation stage, resulting in heads that were uneven and poorly shaped.  Other references suggest that temperatures above 85 degrees can cause heat injury.

Other factors in addition to heat can cause reduced head quality and increased susceptibility to disease. Uneven or inadequate soil moisture exacerbates heat stress. Trickle irrigation may be helpful for supplying water on a regular, steady basis without increasing the risk of water sitting on the head. When individual buds or areas of the head are killed by heat stress, this allows entry of pathogens. Uneven heads also allow water to remain longer on the surface of the head, which increases the likelihood of disease development.

Inadequate nutrients and improper nutrient balance affect both head and stem quality. Boron deficiency increases likelihood of hollow stem, which is often not noticeable until harvest. However, hollow stem can also be exacerbated by excessive nitrogen fertilizer, imbalance of nitrogen and boron, or rapid growth after head initiation.  Cauliflower, turnip and rutabaga are also very sensitive to boron deficiency.   Conventional fertilizers can be purchased with added boron.  For broccoli, use 2-3 lb. actual boron if the soil test level is low (0 to 3.5 ppm), or half that much if the soil test is medium (0.35 to 0.7 ppm).  There are a number of soluble sources of boron, including Solubor and Fertibor, which are OMRI listed. Solubor is 20% B so you’d need 10 lb per acre to achieve 2 lb actual.  If you are broadcasting an organic blended fertilizer, ask if your supplier will add boron to the mix. Another way to apply it is to mix it in water, spray it on the soil with a boom sprayer, and incorporate it into the soil.

In general, even moisture and fertility are important in producing high quality broccoli heads.  Avoid large doses of nitrogen directly after head initiation. 

Handling and Storage

Broccoli is one of the more challenging crops for postharvest handling. Harvest in the early morning, so the crop comes in from the field as cool as possible. For best quality and shelf life broccoli needs to be cooled to 32 degrees F rapidly, under conditions of high humidity. Rapid cooling is the key is preventing yellowing of broccoli. Higher temperature will cause the floret to turn chlorotic very quickly so cold (32 F) must be maintained throughout the postharvest period.  In addition, broccoli is sensitive to ethylene, so it is important that the florets are not stored in a cooler with other vegetables or fruits that emit ethylene.

The main challenge for many vegetable growers is the lack of a facility to quickly cool down the florets. Most growers have ONE cooler where they store vegetables and they also use the same cooler to remove field heat from vegetables. This type of setup is inadequate for rapid cooling and the vegetables and fruits often remain warm even after storing in the cooler for 48 hrs. Ideally, broccoli should be pre-cooled in a different facility before being brought into cooler where all of the produce is stored. Not only will this allow the broccoli to quickly reach the target temperature, but it will also not raise the temperature of the storage room. 

There are many different ways to pre-cool fruits and vegetables, vacuum cooling being one of the methods. The least expensive methods are force-air cooling and hydro-cooling. The latter cools down the vegetables faster than the former. The mister method works as it is similar to hydrocooling. It will be even more effective if the water is chilled. Vacuum cooling is very effective but is one of the most expensive investments of all of the pre-cooling methods available.