How to Grow Broccoli from Seed: Fall Planting Guide

By Paul Smith, pemdasgarden.com

Fall broccoli is the vegetable garden's best-kept secret. Heads that mature in cool weather are sweeter, tighter, and far less bothered by the cabbage worms that plague spring plantings. Growing broccoli seeds for a fall harvest just means counting backward from your first frost — this guide does the math with you.

Broccoli (Brassica oleracea var. italica) is a cool-season crop that thrives when heads form in 60 to 70°F weather. Heat above 80°F during head formation causes loose, bitter, "ricey" heads. Fall's naturally cooling temperatures deliver the ideal finish — you just have to start early enough in summer.

Count Backward from First Frost

Fall broccoli transplants go out 6 to 8 weeks before your first expected fall frost, which means starting seeds indoors 10 to 12 weeks before that frost date (4 to 6 weeks of indoor growth plus transplant time). Worked examples:

• First frost ~Oct 15: start seeds indoors around late July, transplant late August.
• First frost ~Nov 15: start seeds indoors late August, transplant late September.
• Zones 9-10 (frost ~Dec/Jan): you can direct-sow in September and grow straight through winter.

Check your county extension's frost dates rather than guessing — a two-week error in summer sowing is the difference between great heads and none.

Starting Seeds in Summer Heat

Starting a cool-season crop during the hottest weeks of the year is the crux skill. Sow 1/4 inch deep in seed-starting mix and germinate at 75 to 80°F — broccoli sprouts fast, in 5 to 10 days. Then move seedlings somewhere cooler immediately: a shaded porch, a basement under lights, or indoors. Leggy, heat-stressed seedlings never recover into vigorous plants.

Harden transplants off for a week before planting out, and set them slightly deeper than they grew — broccoli roots along the buried stem. Space 18 inches apart in rows 24 to 30 inches apart.

Soil, Feeding, and Water

Broccoli is a heavy feeder. Work 2 to 3 inches of compost into the bed and add a balanced organic fertilizer at transplanting, then side-dress with nitrogen when plants are half-grown and again when small heads appear. Soil pH of 6.0 to 7.0 is ideal; clubroot disease is worse in acidic soil, so lime if your pH runs low.

Keep water steady — about 1 to 1.5 inches per week. Drought stress at head formation causes premature flowering (bolting). Mulch 3 inches deep once transplants establish to hold moisture and cool the roots.

Pest Defense: Row Cover From Day One

Imported cabbageworms and cabbage loopers — the green caterpillars — are the main enemy, and fall pressure is lighter but not zero. Drape lightweight floating row cover over transplants on day one and leave it until harvest; it excludes the white butterflies that lay the eggs. Check weekly for harlequin cabbage bugs in warm regions, and use cutworm collars (cardboard rings) around each stem at transplanting.

Harvest the Head, Then the Side Shoots

Cut the central head when it is tight and firm, before individual flower buds open — typically 4 to 8 inches across depending on variety. Cut at an angle about 5 inches below the head, and the plant responds by producing side shoots for weeks afterward. In mild climates these secondary harvests can out-yield the main head. Stop harvesting when hard freezes arrive, though mature plants tolerate frosts down to about 28°F and light frost actually sweetens them.

Common Mistakes

• Starting seeds too late in summer. Small transplants cannot size up before day length collapses — count backward carefully.
• Letting seedlings bake. Germinate warm, then grow cool; heat-stressed starts stay stunted.
• Skimping on nitrogen. Pale, slow plants at head formation mean underfeeding.
• No row cover. One unprotected week can mean caterpillar-riddled heads.
• Waiting for a giant head. Overmature heads flower and turn bitter — cut when tight.

Garden tools that help: build your fall schedule with the Planting Calendar, and check whether your crop still has time with the Last Chance to Plant tool on our gardening tools hub.

Frequently Asked Questions

Can I direct-sow broccoli for fall?

In zones 8 and warmer with long autumns, yes — sow 10 to 12 weeks before first frost and thin carefully. Elsewhere, indoor-started transplants are far more reliable.

Why did my broccoli bolt instead of heading?

Heat or drought stress during head formation, or transplants that were root-bound or chilled below 50°F for extended periods as seedlings. Steady water and timely transplanting prevent most bolting.

Will broccoli survive frost?

Established plants handle light frosts (down to ~28°F) and taste sweeter for it. Row cover extends the harvest into harder freezes.

How long do side shoots keep producing?

Often 4 to 6 weeks after the main head in cool weather. Keep picking — every shoot harvested signals the plant to make more.

Keep growing: Browse all vegetable seeds, plan the rest of the cool season with our kale and spinach guides, or return to the Growing Guides hub.