Harvest broccoli when the heads are green and firm and before the florets have begun to open.
Broccoli heads are clusters of flowers. The florets are ready for harvest 10 to 13 weeks after sowing and plants produce buds for 6 to 8 weeks.
Once flower heads open to yellow flowers the buds will be past their prime and can become bitter flavored and mealy—though edible.
To know when broccoli is ready to pick, run your finger over the green central flower head, if it feels tight you can delay picking, but once the buds start to loosen start the harvest. Broccoli florets can be cut 1 to 2 weeks before they mature.
Broccoli heads can be as large as 4 or more inches (10 cm) across, but smaller heads are very flavorful. Cut heads with an inch or more of stem attached; both the head and stem is edible.
A few days after the main stem is cut, the plant will begin to form smaller heads on side sprouts. These will be your second harvest. The secondary sprouts will be smaller than the central bud, but very edible and tasty.
Whenever you harvest, it is important to leave 2 or 3 inches (5-7 cm) of stem behind. New buds will form on these stems.
Check the plant every 2 or 3 days for harvestable buds. Keep cutting buds and the plant will keep producing new side shoots with bite-sized buds until a frost kills the plant. Break or cut these new, smaller buds off near the main stem. If buds open to yellow flowers, cut them away immediately otherwise the plant will soon stop producing
Harvest shoots with a sharp knife and cut stems at an angle so that moisture does not collect on stems allowing fungal diseases to grow.
Time broccoli planting so harvest happens in a cool time of the year. Temperatures in the 80sF (26°+C) will cause buds to open. If you live where there is a short spring and long summer, it is better to plant broccoli in late summer for a fall harvest.
Store broccoli in a cold and moist place, 32°-40°F/ 0°-4°C and 95 percent relative humidity. Cold and moist storage is a challenge. Refrigerators provide the cold, but they also dry the air. Place broccoli in a perforated plastic bag in the crisper section of the refrigerator.
Unwashed broccoli will keep in the refrigerator for about one week. Washing before storage encourages bacterial head rot.
Growing tips at How to Grow Broccoli.