Growing good spinach is a bit like growing lettuce: A nice, fast growth period, when the weather is just right, is what produces the best crops. There’s no hope for plantings that have started to go to seed; this can happen if the weather is too hot, the length of day is too long or if the seedlings are allowed to dry too much or otherwise stunted. The answer seems to be in making several plantings throughout the year, enjoying the times that the crop does come in well, and not getting too upset about the inevitable percentage of crop failures.
Spinach grows best in cool weather, so most people plant spinach in the very early spring and try to get a crop up and harvested before the weather turns hot. Some people like to plant spinach in late summer and let it mature in the cooler, shorter days of fall.
Here in Rapallo, Italy, I can do either spring or fall spinach planting, and I also have the blessing of being able to overwinter plants most years. I find that the older, overwintered plants are by far the heaviest producers, and when things start warming up in spring, overwintered spinach plants will produce much bigger and many more leaves.
Although commercial growers most often grow spinach on deep, alluvial river-bottom soil (one can see that sort of very fine grit on the unwashed leaves of freshly harvested spinach at the market), spinach is fairly tolerant of a wide range of soils. My strange mix of yellow mud and rocks manages to produce fine spinach crops.
Spinach doesn’t like too much acid, which is the case in some of my beds, where I’ve been extremely zealous about adding organic material. Although adding lime is the classsic advice for neutralizing acid soils, I like to use eggshells and wood ashes to get the same effect.
Spinach is a heavy feeder, and as one would expect for a green leaf crop, it uses a lot of nitrogen. I mix compost and manure into the soil, and then use a liquid fertilizer once a week on the plants. One problem with mixing in a lot of organic material is that creates a situation that is ideal for the growth of the diseases such as Pythium, which can be a problem for young seedlings. I’ve found that if I prepare a bed and leave it alone for a month before seeding, then the balance of soil organisms will re-establish and the ever-present Pythium won’t be a serious problem. Some people think pre-covering the planting bed with black plastic has a similar calming effect. When I’m cultivating the bed prior to planting, I try to dig extra deep where the spinach will be planted. While lettuce has a fairly shallow and finely fibrous root system, spinach plants have much thicker and longer tap roots, so a deeper soil is preferred.
Spinach is one of the few crops that I like to direct-seed into the growing bed. I soak the seeds overnight in a nutritious solution such as SuperThrive or compost tea.
It used to be common advice that people should plant a succession of plantings to ensure a long harvest season. That’s a good idea, but way more work. I prefer to just plant it all in one shot, and if there is a large excess, the spinach greens can be lightly steamed and then frozen for later use.