Broccoli, cabbage, and cauliflower are members of the cabbage family. They are also referred to as cole crops. Cole crops are cool season vegetables which grow best at temperatures between 60 and 70 degrees F. When planted in spring, these crops produce high quality heads before the arrival of hot summer weather.
Although cole crops are cool season vegetables, they should not be planted when temperatures are consistently below 50 degrees F. Broccoli and cauliflower plants may form heads prematurely when exposed to temperatures below 50 degrees F for several days. This premature head development is called buttoning. Buttoning occurs when plants are exposed to stressful conditions, such as prolonged periods of cold temperatures, drought, or infertile soils. Also, large, rootbound plants are more likely to button than smaller, younger plants. Plants that button do not form usable heads.
Plant broccoli, cabbage, and cauliflower in the garden in mid-April in central Iowa. (Gardeners in southern Iowa can plant in early April, while those in northern counties should wait until late April.) Sow seeds indoors 4 to 5 weeks before planting outdoors or purchase young, stocky transplants at a greenhouse or garden center. Harden or acclimate the transplants outdoors for several days prior to planting. Initially place plants in a shady, protected location and then gradually expose them to longer periods of sunlight.
Cole crops perform best in fertile, moist, well-drained soils. The planting site should also receive at least 6 hours of direct sun each day. To avoid disease problems, rotate the placement of cole crops in the garden. If possible, plant cole crops in a given area only once every 3 or 4 years.
When planting cole crops in the garden, space plants 18 to 24 inches apart within the row. Rows should be approximately 24 to 30 inches apart. After planting, apply 1 to 2 pints of a starter fertilizer solution to each plant. A starter fertilizer solution can be prepared by dissolving 2 tablespoons of a complete analysis fertilizer, such as 10-10-10, in one gallon of water.
Broccoli, cabbage, and cauliflower can also be planted in summer for a fall crop. Start seeds indoors in mid-June. Transplant the seedlings into the garden 4 to 5 weeks later.
Suggested broccoli, cabbage, and cauliflower cultivars for Iowa include:
'Arcadia,' 'Goliath,' 'Green Magic,' 'Gypsy,' 'Imperial,' and 'Packman'
'Blue Vantage,' 'Early Thunder,' 'Quick Start,' 'Stonehead,' and 'Thunderhead'
'Red Jewel,' 'Ruby Ball Improved,' and 'Ruby Perfection'
'Savoy Ace Improved,' 'Savoy Blue,' and 'Savoy King'
'Aquarius,' 'Fremont,' 'Graffiti' (purple heads), 'Snow Crown,' and 'White Sails'
For more about Growing Cabbage in the Home Garden, see the Iowa State University Extension & Outreach Yard and Garden news release from February 28, 2018.