What makes broccoli heads form properly?
Written by
Paul Reynolds
Reviewed by
Prof. Samuel Fitzgerald, Ph.D.Care of broccoli. Broccoli head formation requires certain conditions for a tight, tasty crown to develop. Temperature is the most critical factor, in my experience. Constant daytime and nighttime temperatures of about 60°F (15°C), generally around 65°F (18°C), indicate to the plants that conditions are right for heading to take place. Below the prescribed figures, plants form loose heads; above them, they bolt. I always keep thermometers placed near the plants, connected to a wireless system.
Temperature Management
- Maintain 60-65°F (15-18°C) during heading phase
- Use shade cloth above 75°F (24°C)
- Employ row covers below 50°F (10°C)
- Monitor with soil thermometers
Nutrition Strategy
- Apply phosphorus-rich bone meal at first bud sight
- Avoid high-nitrogen fertilizers during heading
- Use liquid seaweed biweekly
- Test soil pH monthly
Moisture Control
- Provide 1.5 inches water weekly
- Use drip irrigation at soil level
- Mulch with straw for consistency
- Check soil at 4-inch depth daily
Apply some phosphorus as the buds begin to show. I mix in some bone meal with the topsoil as I head them each week. Phosphorus contributes to the development of cell structure, resulting in firm heads. Avoid nitrogen-rich fertilizers at this stage, as they tend to encourage greenery rather than head growth. Soil testing will help prevent imbalances.
MAINTAIN SOIL MOISTURE. That means a drip system and a layer of mulch. If plants experience a water shortage after heading starts, the stem becomes hollow. I even water at dawn, pouring water onto the vacant ground and measuring the quantity with rain gauges. The soil should have the proper consistency and feel like a wrung-out sponge. If you over-water, the roots are smothered; if you under-water, you get fibrous heads.
Heat Protection As soon as temperatures become high, put on 30% shade cloth at least, before the day gets hot, and I hang mine over portable frames. This reduces the temperature by 5 to 10°, even at 90°, preventing bolting. Take the coverings off on cool mornings. Constant protection of this sort keeps the veggies sweet and juicy.
Provide spacing for air circulation. Crowded plants tend to form small heads as more resources are diverted to outward leaves. I space all my transplants in a square bed at least 24 inches apart in all directions. Prune the lower leaves weekly to improve airflow and reduce humidity around the forming heads. Good spacing naturally mitigates disease pressure.
Choose types suited to your climate. Directly-seeded 'Belstar' forms succulent heads reliably even in fluctuating temperatures; 'Arcadia' is sweeter but likes cold better. I test new kinds each season, and carefully check the results in the coming crops. A knowledge of suitable varieties makes the problem of head formation much easier.
Read the full article: When to Plant Broccoli for Best Results
