Using Shade Cloth to Prevent Heat Damage in Summer Broccoli

Progress report for FNE21-996

Project Type: Farmer
Funds awarded in 2021: $10,320.00
Projected End Date: 11/30/2024
Grant Recipient: Urbanowicz Farm
Region: Northeast
State: Connecticut
Project Leader:
Andrew Urbanowicz
Urbanowicz Farm
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Project Information




Project Objectives:

This project seeks to determine whether or not a simulated shade environment will actually yield more uniform summer broccoli production. This study seeks to compare plots of a heat tolerant variety against plots of a heat susceptible variety, further comparing sub plots without cover, with tobacco shade netting, and with commercial horticultural shade netting. If the shading is successful, it would help other farmers to produce summer broccoli in the river valley, thereby improving their farm income. Further if the use of the shade tobacco netting is successful, it could create an adaptive reuse of a waste product that is piled away in many barns in the area. Lastly, this project will answer whether, if successful, the cost of shading the broccoli produces an economically significant boost in production over broccoli grown in the conventional manner.


 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.


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  • Susan Scheufele - Technical Advisor


Materials and methods:

The plan of the project is to establish 6 test plots in an existing vegetable field on the farm. Three of the plots would be planted with the variety Tendergreen, the other three would be planted with the variety Covina. Tendergreen is heat susceptible and Covina is heat tolerant. Both varieties have similar days to harvest, and should be ready for harvest at approximately the same time.

Each plot would consist of a block of 20 plants. Two blocks in each variety would receive grower standard treatment, planted at conventional spacing, and be in the full sun. Two blocks in each variety will be shaded with tobacco shade cloth (20% shade). The final two blocks in each variety will be shaded with horticultural shade fabric (30% shade).

All three blocks will be planted in a double row at 20 inch spacing in all directions. The space between blocks will be 42 inches to allow for access.

All three blocks will be watered by drip tape placed in the center of the double row if natural rainfall is insufficient.

The shaded blocks will have the fabric attached to a tomato stake and nylon twin gridwork that will keep the shade cloth several inches above the final growing height of the plant. The shade gridwork will extend 6 inches beyond the final diameter of the plant to prevent side light infiltration.

The blocks will be fertilized with conventional 5-10-10 fertilizer preplant, and sprayed with fish emulsion as a liquid side-dress two weeks post-setting. This is my standard fertilizer program for broccoli.

The blocks will receive spray for worms and borers with Bt and Radiant as conditions dictate and necessitate as is the norm for my broccoli production.

We will harvest ten random heads per plot. Data on head size, visual appearance, and marketability will be collected, according to methods described in the references above. Mature broccoli heads will be measured for head diameter, as well as being ranked as marketable or not (>6 inches). The visual appearance ratings will note heat damage in the form of bead uniformity/discoloration, which will be scored as a rating from 1 to 5 with 1 being the best score and 5 the worst as follows: e.g. for bead uniformity 1 = all beads perfectly uniform, 2 = uniform, but not perfectly, 3 = some lack of uniformity, 4 = irregular uniformity, and 5 = highly irregular. Individual plant measurements or ratings will be averaged to compute a plot mean for each trait., Finally, a marketability rating will be given that will combine the factors and indicate whether a particular head could be sold on the farm stand (over 6 inches in diameter and no/acceptable heat damage (< 3 damage rating)).

Means will be calculated and data will be plotted onto charts. The technical advisor will run statistical analyses on the data to determine if differences are statistically significant and will help interpret the data and create useful figures. The data from the shaded plots will be compared against each other and the data from the open field plots.

The entire experiment would be repeated identically in year 2 of the trial to examine whether consistent results from this practice can be achieved.

2021 Update

A broccoli crop was planted in line with the research plans, but we experienced complete crop failure due to flooding. Additionally, health issues in the project team will significantly delay work in 2022. We will extend the project end date to 2024 and hopefully pick the research up in 2023.

2022 Update

We are on track to seed the transplants in the spring and proceed with the research in 2023.

Participation Summary
Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.