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When should I sow my broccoli seeds indoors and when should I transplant outside?

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Look up the last frost date for your zip code here. Duluth, for example, has a last frost date of May 21.

Or, as Ed Staub points out in a comment below, for a better picture of the situation go to the original source of the data at the National Climatic Data Center (1971-2000):

  • View the data sheet for your state.
  • In the "State and Station Name" column, find the observation station that is nearest and/or most like your location weather-wise.
  • In the row for 32°F, read across to the 50% column. (This means that there is a 50% chance of a temperature below 32°F after that date.)
  • This date is a reasonable time to transplant broccoli.
  • If you are going to cover the transplants -- which is an effective cabbage looper protection -- then you might be able to transplant a week or two early depending on how heavy your row cover fabric is.

You can transplant broccoli outside a week or two before the last frost date, and start the seed 4 to 6 weeks prior to the transplant date.

About a week before you transplant, start hardening off the plants: first set them outside on a moderate day for a couple of hours, avoiding winds or too much intense sunlight. Then set them outside for longer periods each day. This lets them acclimate to outside conditions so they don't get a sudden shock.

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  • I'm in zone 4b so I'll probably put them in Feb. 12th along with Cabbage and Parsley. Thanks :)
    – Logicalunatic
    Commented Jan 29, 2012 at 23:34
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    @LogicaLunatic: See this answer for info about what the USDA Zones don't tell you -- specifically your last frost date, which is the thing you need to know. Your zone is not relevant in this case. FWIW, I'd start the parsley 2-3 weeks earlier than the broccoli, and the cabbage seeds perhaps a week after the broccoli, though I'd transplant them all at about the same time. (Parsley seeds take a long time to germinate.)
    – bstpierre
    Commented Jan 30, 2012 at 2:13
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    A better source for frost dates is to go to the source - the National Climatic Data Center (1971-2000). With global warming, you have to worry about stale data - I found one site listing dates from 1988, which are likely to be a week or two off. I don't see where the Old Farmer's Almanac gives a date, and the NCDC tables are a lot richer, too.
    – Ed Staub
    Commented Jan 30, 2012 at 3:48
  • @EdStaub: Thanks, +1 to your comment. I've edited the answer to include that link.
    – bstpierre
    Commented Jan 31, 2012 at 12:45

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