It can be difficult to know when to harvest broccoli. Heads form slowly and remain on the plant a long time. Broccoli in particular also forms smaller heads as side shoots. Here are some clues for when your broccoli is ready for your dinner table.
For the largest head of broccoli, wait until the central head ceases to increase in diameter. The heads (actually a set of flower buds) should be tight, with a dense set of small, green buds. Flowers do not open all at once, so the first sign of yellow means that the head should be harvested immediately to avoid bitterness.
In most broccoli varieties, numerous side shoots form as well. These smaller heads continue to mature after the central head is harvested, and are the best way to have a second, and sometimes a third crop of broccoli from a single plant. Follow the same guidelines for harvest: Cut the head when it's still deep green and there's no sign of yellow. Side shoots are more open or spreading than a central head.
Once broccoli heads are harvested, and any side shoots you want are done, the plant is finished for the season. Pull it and put it in the compost. Hot spells (highs in the 80s) for more than one week will cause a quick flowering, so watch plants carefully and be prepared to harvest immediately.