Spinach is a cool weather crop that grows best when daytime temperature remain consistently below 75°F–commonly in spring or fall.
Young plants will bolt when exposed to temperatures below 40°F, but mature plants can withstand temperatures as low as 20°F.
Spinach is ready for picking in about 40 days; for a continuous harvest, sow spinach every 14 to 21 days
• Planting Spinach. Sow spinach seed ¼ inch deep and 1 inch apart; thin plants to 6 inches apart when seedlings are 1 to 3 inches tall. Spinach can be grown in intensive beds (to help retain moisture), or space rows 1 to 3 feet apart. Plant about 30 plants for each person in the household. Thinnings can be added to salads.
• Temperature. Spinach grows best when temperatures range between 60°F and 65°F. Spinach is an excellent choice for fall gardens since mature plants can withstand frost; sow spinach about 8 weeks before the first expected frost. Mature spinach leaves will not freeze until temperatures hit 20°F. Overwinter spinach by covering plants with 8 to 12 inches of straw; or grow spinach through the winter in a cold frame. In spring, plant spinach early to harvest before temperatures climb; increasing day length can also cause spring spinach to bolt. In warm climates, plant spinach in the shade of tall or trellis-grown crops such as corn, squash, beans or peas to help prevent bolting. Protect plants with shade cloth if the temperature goes above 80°F; mulch in the fall to lower the soil temperature.
• Sowing tips. Soak spinach seeds in compost tea for 20 minutes before sowing to speed germination. In warm weather, place spinach seeds in the freezer for two days then moisten and refrigerate them for two more days before sowing. The optimum temperature for germination is 70°F; spinach seed will germinate more slowly to temperatures as low as 35°F. Spinach will not germinate in soil temperatures greater than 85°F.
• Site. Spinach grows best in sun to partial shade. Sow spinach in light soil that is well drained. Prepare planting beds by working aged compost to a depth of 4 inches; remove clumps of soil or organic matter before sowing. Add blood meal–rich in nitrogen–to the planting bed to promote rapid growth. Spinach grows more quickly in sandy soil and more slowly in clay soil. A soil pH range of 6.0-7.0 is optimal. Mulch plants to cool the soil in warm regions.
• Winter crop. Grow spinach through the winter under a cold frame, row covers over hoops, or straw mulch. Sow seed 6 weeks before the first expected frost so that plants get up to size before frost or freezing temperatures come. Sow winter crops in raised beds to ensure quick drainage. Choose hardy varieties for winter growing. In warm winter regions, grow spinach through the winter without cover.
• Feeding. Spinach is a heavy feeder. Feed spinach with compost tea, manure tea, or fish emulsion when plants have four true leaves. Mix 1 tablespoon of fish emulsion and 2 tablespoons of kelp extract per gallon of water; use about one cup per one-foot of row on a weekly basis until plants are about 4 inches tall; then feed two more times before harvest.
• Water. Spinach grows best in evenly moist but not wet soil. Water spinach every other day in light sandy soil; water once a week in heavy soil. Consistent moisture will ensure rapid growth and prevent bolting. In warm regions, mulch around plants to slow soil moisture evaporation. Where soil is slow draining, plant spinach in raised beds.
• Bolting. Spinach will bolt (form flowering stalks and go to seed) in warm temperatures and increasing day length. If a central stalk starts to form but temperatures have not started to warm, cut the whole plant back to just above the soil line and let the leaves regrow.
• Leaf miners and other pests. Leaf miner larvae tunnel through spinach leaves and leave a trail of light-colored squiggles or blotches. Pick and destroy this foliage; keep the garden clean of leaf debris. Protect young spinach plants from leaf miners, flea beetles, and aphids with row covers as long as temperatures are moderate. Row covers can be removed in chilly weather. After harvest, turn the planting bed to destroy larvae in the soil.
• Succession cropping. Sow spinach every 10 to 14 days to ensure a steady harvest of tender young leaves; harvest of one plant extends over two to six weeks. Spinach is ready for harvest in about 40 days.
• Spinach Harvest. Start harvesting spinach when plants have at least six leaves that are 3 inches long. Pick the outside leaves first and the inner leaves will continue to grow or cut the entire plant just below the soil level with a garden knife. Cutting outside leaves will extend harvests, particularly with fall and winter crops. Baby spinach leaves are tender and tasty.
• Spinach Varieties. Winter spinach varieties include ‘Bloomsdale Longstanding,' and ‘Cold Resistant Savoy.' A good fall variety is ‘Tyee' which is mildew resistant. ‘Melody' has smooth, dark green leaves and does not turn bitter tasting. ‘Avon' is semi-puckered leaves and has good heat tolerance; puckered leaves are nutritious. Smooth-leaved varieties are more tender and easier to clean of dirt and grit for eating raw in salads. Varieties with savoyed leaves are generally more heat tolerant and disease resistant.
More tips: Spinach Growing Problems Troubleshooting.
More growing tips at How to Grow Spinach.