Spinach

Quick facts

Day length, heat and bolting

Bolting is the plant sending up a seed stalk. The leaves change from oval to arrowhead-shaped, small flowers appear and the leaves turn bitter (Cornell's e-GRO alert). Spinach is an obligate long-day plant: it needs long days to start flowering. The critical day length is 13–14 hours, depending on cultivar (e-GRO). Heat then decides how fast it happens. e-GRO calls high temperature the most common cause of premature bolting and says it can come quickly above 80°F; plants kept cooler and on days shorter than 13 hours as seedlings bolted later.

Extension pages for home gardeners describe the same pair of triggers: spinach bolts as day length and temperature rise in spring (UMD), quickly during the long days of late spring or summer, with warm weather speeding it up (Illinois), and with long days and warm summer temperatures (USU). UMD adds stressed or overly mature transplants and close spacing as bolting risks.

How long is "long"? The figures disagree because cultivars do. The Organic Seed Alliance's seed-production manual says bolting is initiated primarily by day length, that ancestral forms bolt under less than 14 hours of light, and that most modern European varieties wait until close to 16 hours, roughly the day length near the summer solstice in northwest Washington. In that account heat speeds flowering after it has started rather than starting it.

In a small garden that means growing spinach while days are short and cool: sow early, use slow-bolting ("long-standing") varieties for spring (Illinois, USU), and make fall the main season. For summer greens, Montana State suggests New Zealand spinach; UMass points to beet greens or Swiss chard.

Spring timing

Illinois adds that soil prepared in fall can be seeded over frozen ground or snow in late winter, with seed germinating as the soil thaws. Find your dates on frost dates explained; the planting calendar applies UNH's transplant offsets and Montana State's direct-sowing window.

Succession, fall crops and overwintering

• Succession. Every 2–3 weeks (Michigan State) or about every 2 weeks (UW) while it stays cool; UMD suggests several small plantings several days apart.
• Fall timing. 6–8 weeks before the first fall frost (Montana State); about 6 weeks (UMass); July to early August in Michigan (Michigan State); mid-August for a late-September harvest in Wisconsin (UW). USU aims for plants to mature 1–2 weeks after the expected first frost.
• Count back. Michigan State's fall bulletin starts from the first frost date and subtracts days to harvest (60 for spinach), which it counts from germination, not sowing.
• Summer sowing. Soil above 70°F germinates poorly (UMass), and USU says above 80°F reduces germination. Illinois suggests refrigerating seed 1–2 weeks before summer or fall sowings.
• Overwintering. If the ground freezes before plants mature, mulch with hay and remove it as temperatures warm for an earlier harvest (Colorado State). USU wants 4–5 true leaves before winter, heavy mulch, and mulch removed after snowmelt. UMD says late-fall sowings can overwinter in mild areas and plants can be harvested into December with protection.

Sowing and spacing

For grid layouts, compare square foot gardening spacing. Michigan State notes that spinach's shallow roots compete poorly with weeds.

Containers and small beds

• Pot size: 1–3 gallons, 4–6 in deep (UMD), or at least 2 gallons, 4–6 in deep (UW–Madison). See container sizes for vegetables.
• Mix: commercial potting mix, filled to the top, with no rocks in the bottom (UW–Madison).
• Light: at least six hours of sun for containers (UW–Madison); UMD says spinach tolerates partial shade of 4–6 hours and grows well in full sun in spring and fall.

Water, mulch and feeding

• Keep moisture uniform (UMD); 1–2 in a week, as uneven moisture makes leaves tough (USU).
• Light-colored, soil-cooling mulch (Michigan State).
• Spinach's nutrient need is "high" (UMD) or "medium" (Michigan State). UW side-dresses about 1/4 lb of 5-10-5 per 10 ft of row once plants are 2 in tall; USU, 1/4 cup of 21-0-0 per 10 ft at thinning.

Types, harvest and storage

Savoy types have crinkled leaves, flat or smooth types are easier to clean, and semi-savoy falls between (Colorado State). Michigan State says most gardeners prefer smooth-leaf because grit collects in savoy crinkles.

• Leaves: pinch outer leaves when large enough, taking no more than 50% of a plant's leaves (Michigan State); from about 3 in long (Illinois).
• Whole plants: cut at the base (Michigan State); cut to 2 in above the soil and plants with an intact growing point regrow (USU).
• Bolting plants: make a final harvest when flower stalks appear (UMD), as leaf quality drops quickly (Illinois).
• Storage: near 32°F and high humidity: 2–3 weeks (UMD), 1–2 weeks (Michigan State, USU).

Common problems

Michigan State also lists leafhoppers, loopers, damping-off, anthracnose and Cercospora leaf spot.

This page gives no pesticide rates. If you use any product, follow the label exactly and ask your county extension office.

Where sources disagree

Small-space checklist

• Sow about 4 weeks before the last frost, 1/2 in deep, in soil of at least 40°F.
• Choose slow-bolting varieties for spring and pick before days run long and warm.
• Re-sow a short row every 2–3 weeks while it stays cool.
• Make fall the main season: sow 6–8 weeks before the first frost.
• Get 4–5 true leaves before days drop below 10 hours; mulch to overwinter.
• Pots of 2 gallons or more, 4–6 in deep; even moisture.

More crop pages: lettuce, tomatoes and peppers.