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This weed is very common in western Idaho, but I have yet to find out what it is. I haven't seen it online in any weed database I've searched. Yes, as large as it is, it's growing in a crack in the cement patio. It's been in the area for as long as I can remember (probably since at least the 80's). It can get a lot taller than this (maybe 7 feet tall or so). If you break any part of it, white latex leaks out. It's an annual. I think the seeds are cottony. It's one of the first weeds to grow in the spring, and it's around up until at least the first frost. The stems are spiny. The edges and veins of the leaves are spiny.

Picture of a spiny unidentified, but common, weed.

Brōtsyorfuzthrāx
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  • Think it might be some sort of thistle. – OrganicLawnDIY Jun 27 '15 at 00:09
  • After reviewing the answer I accepted, I have this to say: It is sometimes called milk thistle (not to be confused with the edible milk thistle that is good for the liver). This is good to know, because I'm growing the edible milk thistle, and I think local residents in my area perhaps think that I've been talking about this weed the whole time (or else milk weed, which is different, of course). – Brōtsyorfuzthrāx Jun 27 '15 at 05:27
  • Well, I guess both milk thistles are supposed to be edible. – Brōtsyorfuzthrāx Jun 27 '15 at 05:38
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    Too many plants are called milk thistle: https://en.wikipedia.org/wiki/List_of_plants_known_as_milk_thistle – GardenerJ Jun 27 '15 at 10:37

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Lactuca serriola, Prickly lettuce. Native to Europe and Asia, but it's found almost everywhere now because, as you have found, it's terribly capable of growing wherever it lands. It generally is an annual although there is some potential for them to overwinter if conditions are right. All of your description matches well with the plant including the fluffy seeds.

GardenerJ
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    Great compost heap additive, let it grow until just before it flowers. Also, the young, tender leaves make good salad ingredients. – Fiasco Labs Jun 27 '15 at 05:53