Broccoli is a cool-season vegetable that offers two distinct planting windows for gardeners to achieve a successful harvest. Timing the planting correctly is crucial because the plant is highly sensitive to temperature extremes. The strategy revolves around ensuring the broccoli heads mature during a period of mild weather, avoiding the damaging effects of both unexpected spring frosts and intense summer heat.
Understanding Broccoli’s Temperature Needs
Broccoli, a member of the Brassica oleracea family, grows best within a relatively narrow temperature range. The ideal ambient temperature for robust growth and head development is between 65 and 75 degrees Fahrenheit. When temperatures rise consistently above 80 degrees Fahrenheit, the plant undergoes stress that triggers a process called bolting. Bolting causes the plant to prematurely flower, resulting in a bitter taste and loose, unusable heads.
Young plants can also suffer from exposure to prolonged cold, which can cause them to “button,” or form small, underdeveloped heads that are not desirable for harvest. This buttoning is often triggered by temperatures consistently below 50 degrees Fahrenheit. Therefore, the successful gardener must aim to schedule the plant’s final maturation stage to coincide with the mild, moderate temperatures of late spring or late autumn.
Timing for a Spring Harvest
The goal for a spring crop is to transplant seedlings into the garden early enough to mature before the summer heat arrives. The entire spring planting schedule is calculated backward from your area’s average last expected spring frost date. To give plants a head start, seeds should be sown indoors approximately six to eight weeks before this last frost date.
Starting seeds indoors allows seedlings to establish a strong root system and four to five true leaves. Transplant these established seedlings into the garden around two to three weeks before the last expected frost. Although young broccoli can tolerate a light frost, protecting the plants with a row cover during any unexpected deep freezes can prevent damage. This timing ensures the broccoli heads will form and be ready for harvest during the cool, mild weather of late spring.
Timing for a Fall Harvest
A fall harvest often yields superior quality heads due to gradually cooling temperatures. The planting window for a fall crop is determined by counting backward from the first expected fall frost date. Calculate the days to maturity for your variety (typically 85 to 100 days from seed), and then add about two weeks to account for the slower growth rate as the days shorten.
This calculation means seeds must be started indoors in mid-to-late summer, often July or early August, while temperatures are still warm. Seedlings are then transplanted outdoors about six to eight weeks before the first expected fall frost. This strategic timing ensures plants are large and established during hot weather, but head formation occurs during the cool transition of autumn. Using a heat-tolerant variety can extend the window, allowing plants to continue producing side shoots even after a light frost.
How Starting Method Affects Planting Dates
The choice between starting with seeds or using nursery transplants significantly modifies planting dates. Starting broccoli from seed indoors requires four to six weeks of additional lead time compared to purchasing transplants. Using a transplant bypasses this initial growth period, allowing the gardener to shift the outdoor planting date forward by over a month.
For the spring crop, using a transplant means waiting until the last expected frost date has passed before planting. For the fall crop, a transplant can be set out four to six weeks later in the summer, reducing the period the young plant endures intense heat. Floating row covers immediately after transplanting help protect young plants from spring cold snaps and intense sun exposure during the fall planting window.