Harvest spinach when the leaves are tender and big enough to eat. Store spinach cold and moist.
Spinach is ready for picking 40 to 65 days after sowing.
How to Harvest Spinach
Cut spinach leaf by leaf—cut the outer leaves first allowing the inner leaves to grow larger–or cut away the whole plant one inch (2.5 cm) above the soil. Either way, the plant will keep producing new leaves as long as temperatures are cool.
Spinach grows best between 60° and 65°F (15°-18°C)—commonly during spring or autumn. Plants commonly flower (bolt) and stop producing when temperatures reach the high 70°sF (21°+C). If temperatures rise into the 80°sF (26°+C), start picking outer leaves immediately; this will briefly delay bolting. Very warm temperatures turn spinach bitter.
Where temperatures are consistently greater than 80°F, grow New Zealand spinach (Tetragonia teragonoides) which looks and tastes like spinach but thrives in warm temperatures.
In mild-winter regions, spinach will often produce through the winter. In cold-winter regions, grow spinach under a plastic tunnel or in a cold frame. In a cold frame, you can keep spinach from freezing by covering plants with straw or hay. Overwintered spinach will give you an early spring harvest.
Harvest spinach as close to meal time as possible for the best flavor.
Cut spinach with a garden scissors or serrated bread knife.
Soil often sticks to the ruffled leaves of spinach. To clean leaves fill a sink with lukewarm water and swish the leaves around then lift the leaves and set them in a colander to drain; repeat this process if soil remains. Air dry or pat leaves dry with a cloth or paper towel before storing.
How to Store Spinach
Store spinach cold and moist, 32°-40°F (0°-5°C) and 95 percent relative humidity. Place spinach in the refrigerator in a perforated plastic bag in the vegetable crisper section. Spinach will keep in the refrigerator for about 10 days.
Spinach that is stored too cold or too long will develop brown spots on the midrib and the leaves will wilt and yellow.
More tips at How to Grow Spinach.