Watch for the spinach leaves to reach 4 to 7 inches long with about four to seven leaves per plant. This typically occurs around 50 days after sowing. When temperatures reach 75 degrees Fahrenheit, expect faster growth. Do not wait until the seed stalk develops, and don't wait until the leaves get too big -- the bigger the leaves, the more bitter the taste.
Cut the entire spinach plant off with a sharp, clean knife, cutting at the base. In California coastal areas where temperatures are mild, especially up north, you might reap a second harvest. Therefore, cut the plant off about 3 inches above the soil line, just beyond the growing point.
Twist and snap the outer spinach leaves off with your hands as they reach mature size as an alternative to harvesting the entire plant at once. Allow the center leaves to develop further for a continual harvest until the seed stalk develops.