Spinach likes to grow when it's cool. You would usually plant it about 6 weeks before the last frost of the season. It also likes nitrogen-rich soil, and needs room for a very long taproot. The seeds don't store well, so always start with seeds that are packaged for this growing season. Also, remember the size of the seed dictates how deep you should plant them, so spinach seeds should be planted no deeper than 1/2 inch into the soil.