Many gardeners launch the gardening year by planting spinach as soon as the soil can be worked, usually four to six weeks before the average last spring frost. Thin these early plantings carefully and supply steady moisture; crowded spring crops are more likely to bolt, as are those that are stressed for water. Karen Sisky, commercial director at Johnny's Selected Seeds in Albion, Maine, explains, "Dry soil stresses the plant, which tells it that it's time to stop growing leaves and start making seed."
Extend the spring harvest by planting again every two weeks until about mid-spring. For these later plantings, be sure to choose varieties described as bolt resistant or long standing -- they'll hold at least a week longer in the garden without going to seed than regular varieties.
Planting spinach in mid- to late summer for a fall crop is a good way to sidestep the problem of bolting altogether. Karen Sisky recommends sowing seed six to eight weeks before your first expected fall frost. This gives the plants time to reach a harvestable size before cold weather sets in, which slows growth significantly. You'll be able to pick from those plants until a hard frost in the North (even longer if they are protected by a cold frame) and all winter in climates where the temperature lows average in the mid-20° Fs.
Mid fall sowings may not germinate well, as soil temperatures inch up above 80° F. When the weather is hot, sow seed more thickly to ensure a good stand. I try to plant my fall crop when a cool spell is predicted. I've also found that it helps to dampen the soil well a day or so before planting. Shade cloth, supported above the seedbed on stakes, helps keep the soil cool, too. Be sure to water once or twice a day until the seeds germinate if there is no rain. If all else fails, you can start spinach seedlings indoors and transplant.
Jeff McCormack of Southern Exposure Seed Exchange in Earlysville, Virginia, leaves some of his fall-planted spinach crop in the garden through the winter, by far the easiest way to harvest an early spring crop. "'Bloomsdale Long Standing' easily survives our zone 7 winters unmulched. Over-wintered plants are ready to grow right away, just as soon as the weather warms a bit in spring, so they're more productive, with bigger leaves than spring-sown spinach," he explains. He recommends side-dressing with a nitrogen fertilizer as soon as the plants begin to grow in spring.
In Maine, Karen Sisky has also successfully overwintered fall spinach crops. "In areas with very cold winters, it's especially important to go into winter with plants that are already well established, 6 inches tall or so," she advises. "If you don't have a dependable snow cover to protect the plants, mulch them after the soil freezes," she adds. Shredded leaves, straw or spunbonded row covers such as Reemay work well to protect overwintered crops. Uncover the plants about the same time you would sow your first spring crop of spinach.
In my northern California garden, I sow a small bed of spinach especially for over-wintering at the beginning of October. (A September 1st sowing provides my fall and winter harvests.) Though the plants for overwintering will be only a few inches tall when cold weather arrives, they survive our mild winters with no mulch; in fact a mulch would cause the plants to rot.
Stretching the Spinach Season Spring Crops
Soil pH of 6.0 to 7.0
Soil temperature of 35? to 75? F for germination; 70° F is optimum.
Air temperature of 60? to 65? F daytime and 40? to 45? F nighttime for optimum growth.
Provide rich soil and steady water for quick, succulent growth.
Plant seed 1/2 inch deep, spacing plants 1 inch apart in rows 1 foot apart. Or, broadcast seed over a wide bed. Thin to 3- to 4-inches apart.
Harvest by picking individual plants, or cut the whole plant an inch above the crown and allow it to regrow for a second crop.
The most important spinach-growing lesson I've learned is that it's crucial to plant at the proper time. Spinach requires six or seven weeks of shorter, cooler days to produce a harvest of succulent leaves. Spring crops can be particularly tricky: 14- to 16-hour days cause even very small spinach plants to bolt to seed, and this process is accelerated as temperatures climb into the 70? and 80? Fs.
Actually, I've learned to think of these requirements as advantages. Spinach seed will germinate in soil temperatures as low as 35? F and some varieties are hardy unmulched to the low 20? Fs, which means you can have tasty greens in both late fall and in early spring, when garden-fresh vegetables are too scarce. In fact, there are several windows of opportunity for planting spinach, each yielding a harvest at different times in the cool months of the year.