Spinach is a crop similar to lettuce but it is higher in iron, calcium, and vitamins than most cultivated greens, and one of the best sources of vitamins A, B, and C. Growing and harvesting spinach is easy. Keep an eye on your plants. Harvest when leaves reach the desired size. Don’t wait too long to harvest or wait for larger leaves; bitterness will set in quickly after maturity. The whole plant can be harvested at once, and cut at the base, or leaves may be picked off plants one layer at a time, giving inner layers more time to develop.