Last Updated on October 9, 2026
How to Start Broccoli Seeds Indoors (Spring and Fall)
Start broccoli seeds indoors 8 to 6 weeks before your last spring frost and plant them out 4 to 2 weeks before it. Sow 1/4 to 1/2 inch deep at room temperature, grow the seedlings cool under bright lights, and get them in the ground after about 4 weeks, before they get big and rootbound.
Those are the windows the Plan My Garden planner uses for broccoli. For a fall crop, the planner counts back from your first fall frost instead: start seeds about 13 weeks before it and set plants out about 8 weeks before it. To turn either window into dates, look up your frost dates by zip code or enter your last frost on my seed starting calendar.
The spring broccoli timeline
| Step | When | Basis |
|---|---|---|
| Sow indoors | 8 to 6 weeks before your last frost | Plan My Garden planner |
| Seedlings come up | About 2 weeks after sowing | University of Minnesota Extension |
| Harden off | 10 to 14 days before you plant out | My transplanting guide |
| Plant out | 4 to 2 weeks before your last frost | Plan My Garden planner |
| Ready to plant | 4 to 6 mature leaves | Utah State University Extension |
Pair the sow week with the plant-out week
Broccoli spends less time indoors than most transplants, and that matters more than the exact sow date. Sow 8 weeks before the frost and plant out 4 weeks before, and the plants get 4 weeks indoors. Sow 6 weeks before and plant out 2 weeks before, and it is the same 4 weeks. University of Maryland Extension gives cabbage, broccoli and cauliflower 4 to 6 weeks indoors.
What to avoid is the long combination: sowing at 8 weeks and holding the plants until 2 weeks before the frost. Older plants are more likely to button; Iowa State University Extension says large, older, rootbound transplants are more likely to button than young plants, and that plants that button do not form usable heads.
Broccoli germination temperature
Broccoli seed is not fussy about warmth. These are the ranges from my seed germination temperature chart, which uses the UC ANR California Master Gardener Handbook table.
| Soil temperature | Broccoli |
|---|---|
| Below 40°F | Below the minimum |
| 40°F | Minimum |
| 60-85°F | Optimum range |
| 95°F | Maximum |
A normal room sits inside that optimum range, so you can skip the heat mat; University of Minnesota Extension says not to use bottom heat for broccoli and that the seeds should germinate in about two weeks.
Step by step
- Moisten the mix. Wet the seed-starting mix in a bucket before you fill anything. A mix with compost in it, like Coast of Maine, works well here.
- Pick cells or blocks. If you are just starting out, I recommend cell packs. Maryland Extension suggests cells of 4 to 9 square inches for broccoli and cabbage, and a 2-inch soil block (2 by 2 inches, 4 square inches) is at the small end of that.
- Sow 1/4 to 1/2 inch deep. That is Minnesota Extension’s depth. I put two seeds in each cell or block.
- Cover, but skip the heat. A clear dome or plastic wrap keeps the mix moist. Set the tray somewhere at normal room temperature.
- Uncover and light up. Take the cover off as soon as the seeds sprout, which helps prevent damping off. Hang the lights 2 to 4 inches above the seedlings and run them 12 to 16 hours a day on a timer. Maryland Extension says tomato and cabbage family plants are prone to stretching toward the light, so keep the lights close.
- Keep them cool. After germination, Maryland Extension says seedlings can be grown somewhat cooler: 65-70°F days and 55-60°F nights. Iowa State says cole crops like broccoli grow best at 60-70°F.
- Thin to one. Snip the weaker seedling at the soil line with scissors. Minnesota Extension warns that pulling it can damage the roots of the one you keep.
- Water from below. Pour water into the tray and let the cells drink it up through the holes. For soil blocks I add about an inch of water to the tray daily or every other day.
Feeding is usually not needed; Maryland Extension says most potting mixes contain enough fertilizer to grow a seedling for at least 6 weeks, longer than broccoli spends indoors. The full method for every crop is in How to Start Seeds Indoors, and the supply list is in my step-by-step seed starting guide.
Hardening off and planting out
Broccoli goes out before your last frost, so it needs to be tough when it leaves the house. Start hardening off 10 to 14 days before planting. Give the seedlings a few hours outside out of direct sun at first, then a little more time and sun each day. Minnesota Extension adds two broccoli details: reduce watering once the plants have four or five true leaves, and give them wind protection while they adjust. A cold frame makes the job easier.
Plant out 4 to 2 weeks before your last frost. Utah State says transplants should have 4 to 6 mature leaves and a well-developed root system. Set each one at the same level it grew in its pot, as Minnesota Extension does. Broccoli does not need its stem buried the way tomatoes do. My transplanting guide covers watering in and mulching.
Broccoli is semi-hardy on my vegetable frost tolerance chart, which has you cover it on nights below 28°F. Minnesota Extension suggests a row cover over spring plantings that gives protection against cold. Spacing is one plant per square, as on How Many Broccoli Plants per Square Foot?
Why broccoli buttons, and how to prevent it
A button is a tiny head that forms soon after planting, on a plant that never grows a real one. It is a stress problem, and most of the causes trace back to the seedling stage.
| Cause | Prevention |
|---|---|
| Transplants too old or rootbound | Plant out after about 4 weeks indoors. Utah State says larger, more mature transplants are prone to button and to pick plants with 4 to 6 leaves. |
| Long cold spells after planting | Harden off well and cover during cold snaps. Iowa State lists prolonged temperatures below 50°F as a buttoning stress. |
| Dry soil | Water every week that rain does not give an inch, which is Iowa State’s guideline for cole crops. |
| Poor soil | Iowa State names infertile soil as a cause. Add compost every year, as in my transplanting guide. |
| Held too long in the tray | Illinois Extension says plants that stay too long in seed flats may produce button heads soon after planting. |
Heat is a different problem. Iowa State says hot weather, 86°F or more in the day and 77°F or more at night, can stop broccoli from forming a head at all, and warns against planting too late in spring. Getting spring broccoli out on schedule, weeks before the last frost, is the fix.
Starting broccoli seeds for a fall crop
Fall broccoli is timed from your first fall frost, not your last spring one. The planner’s fall schedule sets broccoli out about 8 weeks (57 days) before your first fall frost and starts the seeds 5 weeks before that, about 13 weeks before the frost. That 5 weeks indoors is in line with University of Maryland Extension’s broccoli guide, which says fall transplants take about 4 to 6 weeks from seeding to setting into the garden.
| Fall step | When |
|---|---|
| Start seeds indoors | About 13 weeks before your first fall frost |
| Set plants out | About 8 weeks before your first fall frost |
To check the window against a specific variety, use Illinois Extension’s method: count back from your first fall frost by the variety’s days to harvest from transplanting, plus about 10 days. That gives the set-out date, and the seed-starting time comes before it.
Minnesota Extension explains why fall works so well: summer heat hurts a developing head, but heat while the leafy plant is growing is fine, and by the time heads form the weather has cooled. It suggests varieties of 60 to 85 days for fall, so the heads form in cool weather, and says broccoli keeps growing after the first light frosts. Harden fall seedlings off the same way, starting in the shade. If you cover them after planting, Minnesota Extension says to use a lightweight row cover in summer so it does not get too hot underneath. My fall crop planting guide covers what to plant where the summer crops finish.
Where to get broccoli seed
If you need seed, Wood Prairie’s organic broccoli collection carries De Cicco, Waltham 29 and Purple Peacock. Use code PMG5 for 5 percent off.
Quick questions
When should I start broccoli seeds indoors?
8 to 6 weeks before your last spring frost, then plant out 4 to 2 weeks before it. For a fall crop, the Plan My Garden planner starts seeds about 13 weeks before your first fall frost and sets plants out about 8 weeks before it.
How long do broccoli seeds take to germinate?
About two weeks, according to University of Minnesota Extension. My seed germination temperature chart puts the broccoli optimum at 60-85°F, with 40°F as the minimum and 95°F as the maximum.
Do broccoli seeds need a heat mat?
No. University of Minnesota Extension says not to use bottom heat for broccoli, and normal room temperature sits inside the 60-85°F optimum on my germination chart.
How deep do you plant broccoli seeds?
1/4 to 1/2 inch deep, which is what University of Minnesota Extension recommends.
Why did my broccoli make tiny heads right after planting?
That is buttoning, a stress response. Iowa State University Extension lists prolonged temperatures below 50°F, dry conditions, infertile soil and large, older, rootbound transplants as causes, and says plants that button do not form usable heads.
Get your spring and fall broccoli start weeks
The planner turns your zip code’s frost dates into broccoli start-indoors and transplant dates, spring and fall. Plan My Garden is free to start: enter your zip code and it finds your frost dates, draw your beds, pick broccoli and everything else you want to grow, and it places each plant at square-foot spacing and builds a planting schedule (start indoors, transplant and direct sow dates) from your frost dates.
Related: How Many Cabbage Plants per Square Foot? · How Many Cauliflower Plants per Square Foot? · How Many Kale Plants per Square Foot? · Seed Starting Calendar
