When to Plant Spinach
Spinach requires 6 weeks of cool weather from seeding to harvest; this generally spring or fall, though many gardeners have better luck in the fall.
Although seeds can be started indoors, it is not recommended, as seedlings are difficult to transplant.
In the fall, sow seeds when the soil is 70°F or cooler. See our fall planting calendar.
Many gardeners can grow spinach throughout the winter if they protect the young plants with a cold frame or thick mulch, then remove the protection when soil temperature in your area reaches 40ºF in spring. Remove the mulch to harvest some spinach, then replace the mulch.
If planting in the early spring, sow seeds as soon as the ground warms to 40°F. (Cover the soil with black plastic to speed its warming.)
To distract leaf miners, sow spinach seeds and radish seeds in alternate rows. Leaf miner damage to radish tops does not affect their root growth.
Common spinach cannot grow in midsummer as it’s not cool enough. (For a summer harvest, try New Zealand Spinach or Malabar Spinach, two similar leafy greens that are more heat tolerant.)
How to Plant Spinach
Sow seeds 1/2 of an inch deep every 2 inches and cover with 1/2 inch of soil.
Plant in rows 12 to 18 inches apart or sprinkle over a wide row or bed.
Sow every couple of weeks during early spring for a continuous harvest.
GROWING
Water spinach to keep soil constantly moist.
Use row covers to maintain cool soil and deter pests.
When seedlings sprout to about 2 inches, thin them to 3-4 inches apart. You can eat the thinnings.
Beyond thinning, no cultivation is necessary. Roots are shallow and easily damaged.
Water regularly and mulch to retain moisture.
When plants reach one-third of their growth, side-dress with a high-nitrogen fertilizer, as needed. Nutrient deficiencies may appear as yellow or pale leaves, stunted or distorted growth, a purpling or bronzing of leaves, leaves dropping early, or other symptoms.
In early spring and late fall: Spinach can tolerate the cold; it can survive a frost and temps down to 15ºF (-9°C). (See local frost dates) Young spinach is more tender; cover if cold temps are in the forecast.
RECOMMENDED VARIETIES
There are four main types of spinach suited for spring and fall plantings.

Baby-leaf style spinach is tender, with small-size leaves. The variety ‘Baby’s Leaf’ is good for containers; ‘Catalina’ is heat-tolerant and resistant to downy mildew.
Savoy spinach has curly, crinkled, dark-green leaves, e.g. ‘Bloomsdale.’ The ‘Winter Bloomsdale’ variety is a crinkled-leaf, fall variety, tolerant to mosaic viruses.
Semi-Savoy has slightly crinkled leaves and can be difficult to seed. ‘Melody’ is resistant to cucumber mosaic virus and downy mildew; mildew-resistant ‘Remington’ will grow in spring, summer, or fall; ‘Tyee’ can be planted in spring or fall, and is resistant to downy mildew.
Smooth- or flat-leaf (also called plain leaf) varieties have spade-shape leaves. ‘Giant Nobel’ is a plain leaf variety and an heirloom that is slow to bolt; ‘Nordic IV’ is bolt-resistant.
Malabar Spinach (Basella alba), a vine, and New Zealand Spinach (Tetragonia tetragonioides), a perennial, are two heat-tolerant leafy greens that resemble common spinach; both are heat-tolerant. Grow them in the summer, when common spinach can’t take the heat.
HARVESTING
Harvest a few outer leaves from each plant (so that inner leaves can develop) when leaves reach the desired size, or harvest the entire plant, cutting the stem at the base.
Don’t wait too long to harvest or wait for larger leaves. Bitterness will set in quickly after maturity. Be aware of day length and heat: Increasing daylight (about 14 hours or longer) and warmer seasonal temperatures can cause spinach to bolt (develop a large stalk with narrower leaves and buds/flowers/seeds), which makes the leaf taste bitter. 
If spinach starts to bolt, pull the plant and use the leaves. Or try to slow the bolting: Pinch off the flower/seed heads, keep the soil moist, and provide shade.