A fall broccoli trial was conducted in a high tunnel at Southwest Purdue Ag Center in 2016 to test the potential of growing broccoli in high tunnels after tomatoes. This article describes what we found from the trial.
Broccoli is a cool-season, frost-tolerant crop. The harvest portion of broccoli is the compact, slightly dome-shaped head that is comprised by numerous immature flower buds. Broccoli that forms a single large head and thick stalks requires 50-70 days to harvest. Vegetative growth occurs over a wide range of temperatures, but high-quality head development requires temperatures in the range of 54-68 ºF. If temperatures are below 41ºF, plant growth is significantly reduced. In Indiana, fall production of broccoli in an open-field can be challenging because of the relatively long growing season. But with increased heat accumulation in high tunnels, it is possible to have a second crop of broccoli following tomatoes.
Broccoli can be transplanted or direct seeded. But for the high tunnel production in the fall, it is better to start with transplants to shorten the season and achieve a uniform crop. Growing seedlings should start no later than August to ensure head formation before temperatures drop below 50 ºF. Transplants are ready 4-5 weeks after seeding. Optimal seed germination temperature for broccoli is around 85 ºF, higher than many other cool-season vegetables. If a greenhouse is not available for growing transplants in the summer, broccoli seeds can be successfully germinated under a shade cloth in July and August.
Caterpillar damage was the major challenge we encountered during the period of growing transplants. The culprits hide on the underside of leaves. They were hard to detect until significant damage was observed. Products with the active ingredient Bt are fairly effective in controlling them. More information about caterpillars in crucifers can be found in Rick Foster’s article https://vegcropshotline.org//article/caterpillars-in-crucifers/. Seedlings were transplanted on 2 Sep. (seeds were planted on 1 Aug. ) on beds with plastic mulch and drip tape in our trial.
Figure 1. Caterpillar damage on broccoli seedlings.
Plants started to form heads in middle October. First harvest of the earliest variety in our trial ‘Blue Wind’ was on Oct. 25. Central heads were harvested when they were about 6 inches in diameter with a stem about 8 to 10 inches long. If broccoli is harvested too late, the heads start to loosen and separate, and the individual flowers start to open. Harvesting the central head stimulates the growth of side shoots. Heads forming on the side shoots are smaller and less compact. Field grown broccoli is normally harvested once for the central head. Relatively warmer temperatures in high tunnels allowed formation of some nice secondary heads. We harvested these secondary heads between the middle and the end of November. Among eight varieties we evaluated in the trial, variety Green Magic had the highest yield. In average, each plant of Green Magic produced one central head and two secondary heads.
Figure 2. Broccoli ready to harvest.
In the trial, we had two rows of plants grown on a 4’ wide bed. In-row plant spacing was 18’’ apart. In a 30’ × 96’ high tunnel, it is possible to grow 600 to 700 plants with this spacing. A yield of up to 700 lbs of high-quality broccoli can be expected.