Broccoli is a cool-season vegetable that thrives in moderate temperatures, ideally between 60 and 68 degrees Fahrenheit. Oklahoma’s climate, with its swift transition from cool spring to intense summer heat, makes precise planting timing essential for a successful harvest. Correct timing ensures the plant matures and forms a dense head, or “curd,” before heat causes it to “bolt,” or prematurely flower. The state’s variable weather requires specific strategies for both spring and fall crops to maximize yield.
Planting Calendar for the Spring Harvest
The goal for the spring harvest is to ensure the broccoli matures before average daily temperatures consistently exceed 80 degrees Fahrenheit, typically occurring in late May or early June. Plants must be started indoors well in advance of the last expected spring frost. Seeds should be sown indoors approximately six to eight weeks before the last frost date for your specific region.
The average last frost date in Oklahoma falls between early April (south) and mid-to-late April (north), making late February to early March the ideal time for indoor seeding. Transplants are ready once they have developed four to six true leaves and stems about the thickness of a pencil. They can be safely set into the garden from late February to mid-March, as mature broccoli tolerates light frost and temperatures as low as 25 degrees Fahrenheit.
Planting Calendar for the Fall Harvest
The fall crop is often more productive because plants mature during the naturally cooling days of autumn, which improves head flavor and density. Planning requires calculating backward from the first expected hard frost, which typically occurs between late October and early November in most of the state. Broccoli varieties usually take 60 to 100 days from transplanting to reach maturity.
Seeds should be started indoors during the residual heat of summer, specifically from mid-July to early August. Seedlings are transplanted in late August or early September, allowing the plants to establish themselves before cooler temperatures stimulate head development. Protecting young transplants from intense summer sun immediately after planting is important for the fall crop’s success.
Optimizing Soil and Spacing Needs
Broccoli requires rich, fertile, well-drained soil to support rapid growth and head formation. The soil should contain a significant amount of organic matter to ensure both good drainage and consistent moisture retention. An optimal soil pH level is slightly acidic to neutral, ranging from 6.0 to 7.0.
Before planting, amend the soil with aged compost and apply a high-nitrogen fertilizer. Broccoli is a heavy feeder, and pre-planting nitrogen application powers leaf development and eventual head formation. Space individual transplants 18 to 24 inches apart. This spacing allows for proper air circulation and gives developing heads enough room to grow large and dense.
Protecting Broccoli from Oklahoma’s Climate Extremes
Oklahoma’s climate extremes, particularly heat waves and pest pressure, pose the greatest challenge to successful broccoli cultivation. High temperatures, especially those consistently above 80 degrees Fahrenheit, stress the plant and trigger bolting, which renders the heads inedible. Consistent, deep watering is necessary to mitigate this stress, aiming for 1 to 1.5 inches of water per week, delivered directly to the root zone.
To keep the soil cool and moist, apply a heavy layer of organic mulch, such as straw, around the base of the plants. During heat spikes, temporarily installing a 30 to 40 percent shade cloth can reduce ambient temperature and prevent heat stress on the developing heads.
Common regional pests include the cabbage worm and cabbage looper. These can be managed effectively with applications of Bacillus thuringiensis (Bt), an organic bacterium that targets caterpillars after they ingest it. Aphids can be controlled with applications of insecticidal soap or neem oil.