How to Start Broccoli Seeds Indoors: Step-by-Step for Success

Broccoli is a cool-season crop that thrives when given a head start indoors. By starting seeds indoors, you can get an earlier harvest, avoid temperature extremes, and ensure your plants are healthy and well-established before they face the outdoor elements.

I’ve been growing broccoli from seed for more than 30 years in raised beds and containers, and I’ve learned that indoor seed starting is the single best way to guarantee strong, productive plants. Here’s a clear, step-by-step guide based on both horticultural best practices and personal experience.

Table of Contents

Step 1: Know Your Timing

Start indoors: 6–8 weeks before your region’s last expected frost date.
Transplant timing: Broccoli seedlings grow best when transplanted outdoors 2–3 weeks before the last frost, while daytime temperatures are cool.

I’ve found that in mild climates like my Sonoma Valley garden, I can even start a second round in midsummer for a fall harvest.

Step 2: Choose the Right Seeds

Select a variety suited to your climate and taste:

For spring harvest: ‘Packman’ or ‘Belstar’—fast-maturing and dependable.
For heat tolerance: ‘Green Magic.’
For extended harvest: ‘Calabrese’—produces side shoots after the main head.

From trialing dozens of varieties, I can tell you that matching the variety to your growing season is crucial for avoiding bolting.

Step 3: Prepare Containers and Soil

Use clean seed trays or small pots with drainage holes.
Fill with a fine-textured seed-starting mix (not garden soil) for better aeration and root growth.

Over the years, I’ve noticed that quality seed-starting mix dramatically reduces damping-off disease and boosts germination rates.

Step 4: Plant the Seeds

Sow seeds ¼ inch deep.
Space seeds about 2 inches apart in trays or plant 2–3 seeds per cell.
Lightly cover and mist to keep soil evenly moist.

Step 5: Provide Light and Warmth

Keep soil temperature between 65–75°F for germination.
Place seedlings under grow lights or in a sunny south-facing window for 14–16 hours of light per day.

In my setup, I use adjustable LED grow lights to keep the bulbs just 2–3 inches above the seedlings. This prevents leggy, weak stems.

Step 6: Thin and Care for Seedlings

Once the first true leaves appear, thin to one seedling per cell or pot. Water when the top of the soil begins to dry—never let them dry out completely, but avoid soggy conditions.

I feed my seedlings a diluted organic liquid fertilizer every 10–14 days to keep them growing steadily.

Step 7: Harden Off Before Transplanting

About a week before planting outdoors, gradually introduce seedlings to outdoor conditions—starting with a few hours in shade and working up to full sun. This prevents transplant shock and helps the plants adjust to wind, temperature swings, and direct sunlight.

My Experience in Broccoli Seed Starting

As a year-round vegetable gardener in Sonoma Valley (Zone 9) with decades of hands-on experience, I’ve trialed many broccoli varieties, experimented with indoor lighting setups, and fine-tuned my timing to match seasonal changes. My NEW (Narrow bed, Equidistant planting in Wide rows) method ensures my transplants thrive in raised beds, producing dense, flavorful heads. This guide reflects not only research-backed horticultural practices but also the lessons learned from countless successful broccoli harvests in real gardens.