Using Shade Cloth to Prevent Heat Damage in Summer Broccoli

 I have observed that our summers have become more and more prone to extreme heat conditions in July and August, likely due at least in part, to the ever expanding amount of paved surfaces in the valley and generally warming temperatures of the climate. These months, however, are also a time where the demand for fresh broccoli is very high on the farm stand.

Broccoli is a favorite summer vegetable because it is quick and easy to prepare in hot weather, and we regularly sell out of broccoli all summer long. Broccoli sells on the farm stand for $2 per head and it is not unusual for us to retail 50 heads in a day. Additionally, prior to the pandemic, chefs would regularly purchase broccoli in the summer at a price of $1.50 per head and take 100 or more heads in an order several times a week. This crop adds $700.00 to $1,000.00 to our weekly sales and is important to have during all the weeks we are open.

While producing clean, uniform, and good quality heads of broccoli is relatively easy in the cooler spring and fall months, summer broccoli is a crap shoot at best during July and August. Sometimes, our summers can be rather cloudy and lacking in extreme heat days, and in these less common seasons, summer broccoli production is workable. As is more often the case, we end up with sunny hot days, many with extreme heating. Broccoli is a heat-sensitive crop, and has a critical period for heat sensitivity that lasts for roughly ten days. Plants are most sensitive when the growing tip shifts from vegetative growth to flower bud initiation—about 10 days prior to when a crown is first visible. Temperatures above 86 degrees Fahrenheit for more than four days during that period results in uneven and poorly shaped heads. The amount of time that plants are exposed to high temperatures makes a difference – higher temperatures will cause damage more quickly than lower temperatures. The heat damage then manifests as irregularly shaped heads, burnt appearance on the heads, and poor quality generally. In addition, broccoli can be triggered to flower too soon if they are stressed, by heat or by other factors. This results in a head that is abnormally small, also known as buttoning.

I estimate that 50% of the broccoli planted for summer harvest never generates a marketable head. This creates a waste of resources from the time spent taking care of the plants to the inputs used to produce the plants. This results in summer produced broccoli using almost double the resources to harvest a head of broccoli. Improving the amount of plants we are able to harvest will reduce waste and make this summer season production more sustainable.

On my farm, we have access to tobacco shade netting that was left over from previous shade grown tobacco operations in the area. This netting reduces sun light transmission by 20%. I hypothesized that using this shade netting could reduce the heat broccoli plants experience during this critical period, and save plants from buttoning or other heat-related disorders that reduce marketability. Since the netting was available as a waste product, we were able to run informal trials using it on broccoli in both 2019 and 2020. These trials were not conducted to the standard of scientific inquiry that this project is intending, but they did provide anecdotal evidence that shading broccoli with such netting produced broccoli heads that were cleaner, more regular and visually appealing. With this preliminary data, we would like to undertake the study again and with the help of UMass Extension, in order to determine if the differences we observed are statistically significant, repeatable, and hold true over a few seasons. We will evaluate our 20% shade-cloth side-by-side with a commercial shade cloth that provides 30% shade, and a full-sun control. We will have each treatment replicated on the farm and will repeat the experiment in each of two project years. Our technical advisor, Sue Scheufele of UMass Extension, will help us design the study so that we can conduct statistical analysis of the data. Conducting the study in this way, and with the help of Extension, will allow us to have confidence in our results and to share our findings with other farmers in New England who suffer the same heat-related losses in summer broccoli. We will share our findings with annual research summaries and field days in both year one and year two.