Our plan for growing spinach is two fold. First, we'll grow spinach inside in flats for transplanting in early spring and again in early fall. Second, we'll direct seed spinach early in the spring and again in the early fall, probably late September, much of which will be put up - canned or frozen.
Obviously, the most important consideration when growing spinach is the temperature. Spinach does not germinate or grow well when the summer heat settles in. It has to be cool - 50F-70F in order for the seeds to germinate well. And it is very cold hardy! Clemson University says spinach can survive 20F without injury, which makes it perfect for extended season plantings under plastic.
Many years, a covered crop of spinach would survive here in the midwest until around Christmas. This doesn't mean it will continue to grow in the cold temps, but it will stay fresh and ready to be harvested well into the winter. And like a lot of the cold hardy vegetables, it's crisp and sweet when harvested in the cold weather.
We have some fall planted spinach that was planted in single rows. When (or if) the plants start growing in the spring we'll thin the plants gradually, harvesting every other one as we go to give those left more room to grow. Eventually we'll harvest the remaining mature plants to process for storage.
We'll try planting in a wide row with "broadcasted" seed this year too. Basically sprinkling the seed as evenly as possible over a 2 foot wide row with a drip irrigation tape running down the center. The question is how bad the weeds will be and if the cost of weeding a thick planting of spinach is made up for by the heavier harvest per row foot. If I were making my own compost I would try putting down a couple inches of good compost on the bed first (which should keep many of the weed seeds from germinating), then seeding the spinach, and finally covering the seeds with a 1/2 inch layer of weed-free compost. (Properly made, compost will be weed free due the high temps the decaying compost achieves). Maybe I'll buy enough good compost to try this method of production this year.
Spinach, like most vegetables, does best on well drained soil with a lot of organic matter. According to Territorial Seed, spinach does best on neutral to slightly alkaline soils, optimally pH 6.5-7.5. It requires even moisture, so irrigation will be helpful.
An important consideration that I hadn't thought of is that nitrogen is more slowly released when it is cold, which is when the spinach is most likely to be growing and therefore in need of the nitrogen. Territorial suggests, "Because nitrogen cycling is slower during cooler temperatures, use fertilizers that release nutrients quickly such as blood meal, fishmeal, bone meal, composted chicken manure, or feather meal."
As mentioned previously, extending the season for spinach here in the midwest is not difficult.