When to Plant Broccoli Seeds: Spring and Fall Dates

Step 1: Fix the last spring frost for your garden

Every broccoli date starts from one number, the average last spring frost for your area. Broccoli is a cool-season crop, so it goes out before that date, not after it.

Find the date for the weather station nearest you, then use it as week 0. The rest of this article uses a made-up last frost of April 20 and a made-up first frost of October 15, so you can follow the sums.

Step 2: Work out when to plant broccoli seeds indoors

Extension offices give different weeks, and the gap is wide. The table shows what Utah State, Iowa State, New Hampshire and Illinois say.

The New Hampshire numbers give a clear sum. Broccoli is safe to set out 2 to 4 weeks before the last frost, and seedlings need 4 to 6 weeks before that. With a last frost of April 20, the set-out window runs from March 23 to April 6. The sowing window runs from 6 weeks before March 23 (February 9) to 4 weeks before April 6 (March 9).

Convert the three sources that count from the frost date. Illinois gives 4 to 6 weeks before it, New Hampshire gives 6 to 10, and Utah State gives 8 to 10 (6 to 7 weeks before a planting date that is 2 to 3 weeks before the frost). The sowing date lands anywhere from 4 to 10 weeks before the last frost, a six-week spread.

Choose the middle of the range your own state publishes, and note in a notebook how the seedlings looked on set-out day.

A seed packet tells you how long broccoli takes. Your frost date tells you when to start counting.

You can also sow broccoli straight into the garden. Seeded broccoli can go in at the same time as transplants, 2 to 3 weeks before the last frost, according to Utah State. Sow seeds a quarter to a half inch deep, and thin when the plants have three or four true leaves.

Soil warmth for sprouting

Broccoli seed sprouts in cold soil, slowly. A University of California extension garden note lists 40 F as the minimum soil temperature for germination, 60 to 85 F as the best range and 95 F as the maximum. The note says that outside the best range seeds take more days to come up, and in extreme cases may not come up at all. A probe thermometer in the bed settles the question before you sow.

Seedlings are easy to move. A Clemson factsheet lists broccoli among the vegetables that transplant easily, unlike beans, corn and carrots.

Step 3: Set out sturdy seedlings before the last frost

A Utah State fact sheet says a transplant should have 4 to 6 mature leaves and a well-developed root system before it goes outdoors. Bigger seedlings are not better. The same sheet warns that larger, older transplants tend to form small, early heads, which it calls buttoning.

Harden the seedlings first. Acclimate transplants outdoors for several days before planting, says Iowa State. Utah State adds that broccoli is not seriously damaged by temperatures down to 28 degrees F.

Plant 12 to 18 inches apart in rows 2 to 3 feet apart, which is the Utah State spacing. Give the bed fertile, well-drained soil. Iowa State puts the ideal pH between 6 and 7.

Step 4: Count back from the first fall frost

Fall broccoli runs on a different count. Transplants for a fall crop go in anywhere from late June to early August, says Iowa State, and harvest lands between mid-September and late October. Start the transplants indoors 4 to 5 weeks before planting outdoors.

Iowa State also gives the growing time. Plants are ready to harvest 60 to 75 days after transplanting. Put the two together and you can work out the whole stretch.

• Short end: 4 weeks indoors is 28 days, plus 60 days in the garden, makes 88 days from seed.
• Long end: 5 weeks indoors is 35 days, plus 75 days in the garden, makes 110 days from seed.

With a first frost of October 15, count 75 days back and you get a transplant date of August 1. Count 60 days back and you get August 16. Seeds start 4 to 5 weeks before each, so the sowing window runs from June 27 to July 19.

How hot is too hot for broccoli

Heat shapes both seasons. Cole crops grow best between 60 and 70 degrees F, says Iowa State. Days of 86 degrees F or more with nights of 77 degrees F or more can prevent broccoli from forming heads.

Its advice for spring is to avoid planting too late, so crowns form before the hot weather arrives. The same sheet goes further, saying yields are highest when temperatures stay at or under 75 degrees F, and that high summer heat gives loose, bitter heads.

A Kansas State guide lists broccoli as a hardy crop with 60 to 80 days to first harvest. For a fall crop, Utah State says to choose early varieties and plant 50 to 75 days before the maturity date you want, and notes that harvest can come several weeks after the first fall frost.

When these dates are the wrong tool

If you buy nursery transplants, skip the sowing math and use only the set-out window. If your state extension office publishes broccoli dates for your county, follow those, because they come from local weather. Free extension charts and printables are enough for a single broccoli bed.

The counts above assume a real freeze season. Where freezes are rare or summers are very hot, ask your county extension office for sowing weeks and do not count from a frost date.

The Winter Plan helps when you want the weeks counted for you across many crops and a page to write them on. It uses whichever last frost and first frost you enter, with a table of last and first freeze dates (10, 50 and 90 %) for 108 NOAA stations, so the dates above are only examples. In late winter the spring week list holds the sowing week, the season calendar has day boxes for set-out day, and in midsummer the fall countdown holds the sow date for the fall crop. The notes for next winter page is where you write how the seedlings looked on set-out day.