Bolting does not occur just due to the plants being exposed to high temperatures.
There are other reasons for bolting to occur as well as the formation of stunted flower heads referred to as “buttons”
Bolting is the broccoli plant's response to stress – it is attempting to quickly produce seeds before dying.
When your broccoli starts to bolt, the actual cell structure of the plant changes. Unfortunately, once the process has started, you cannot stop your broccoli from bolting. Attempting to remove the bolting stems or flowers will simply cause more to take its place.
Broccoli grows best in soil with temperatures between 65- and 75-degrees F. (18-24 C.). Warmer soil temperatures than that, and the broccoli will bolt, or go to flower. Hot weather (86°F in the day and over 77°F at night) can also cause broccoli to grow no crown of flower heads.
Mulching the soil around the plants as they are growing can help keep the soil temperatures from rising too quickly. Straw is a good to help insulate the soil and prevent it from getting too warm while the plants are developing.
Root stress is another cause for broccoli to bolt and not form large heads.
Root stress such as becoming rootbound or being damaged, can also cause broccoli to bolt, or go to flower.
When starting broccoli indoors make sure that you do not start the plants too early or make sure that the pot that they are grown in provides enough room for the roots to develop without completely filling the pot.
When planting out door especial in containers provide a depth of 12 to 15 inches and a width of 18, to accommodate feeding roots and mature plant dimensions
To properly mature broccoli needs full sun, or a minimum of 6 hours of sunlight per day. Too little light and the flower head will be loose and not tight.
Broccoli prefers a soil pH between 6.0 and 7.0, and if the pH is outside of these limits’ broccoli may not set heads.
For the best growth and largest heads, be vigilant and constant with regards to fertilization. You should apply a nitrogen-based fertilizer (21-0-0) in the amount of ½ cup (118 ml.) per every 10-foot (3 m.) row, four weeks after transplanting or thinning.
This will stimulate more vigorous plant growth. Apply an additional ¼ cup (59 ml.) when the heads reach the size of a quarter. Then, when the main head has been picked, apply additional fertilizer 6 inches (15 cm.) to the side of the plant and irrigate into the soil. This will encourage side shoot development.
Do not use fresh manure as it may contain harmful bacteria, and may increase weed problems.
Temps below 30° and above 75°F may spell disaster in the form of frost and heatwaves that cause stress on developing plants, and may result in the failure of the broccoli plants to form heads.
Buttoning of broccoli is when the plant develops small heads or “buttons” when the plant is immature.
Buttoning of broccoli occurs in young plants when they are exposed to several days of cold temperatures of around 35 to 50 degrees F. (1-10 C.).
Cold temperatures are not the only reason for poor broccoli heads.
When using transplants, they should have four to six mature leaves and a healthy, well developed root system prior to setting out. Larger, more mature transplants tend to form tiny, early heads (buttoning) that flower too soon.
The use of Hot caps and floating row covers fabric help protect tender seedlings and transplants from frost and may combat broccoli buttoning problems.