These are gig 5-6 foot thistles. If I don't kill the seeds, they will multiply like crazy. I assume drying them out with the sacks will seem to kill the seeds, but I wonder if they'll just just dry up and regrow once the conditions are right.

Why do you want the seeds? Are they still on the plants?

Drying probably won't work. Heat will do it.

If it is a non-native plant, then I would cut the heads off before they flower, and work to replace the plants with something with native and similar wildlife value.

kill it with fire

If you're going to the trouble of bagging the heads, just cut 'em off and throw them in the trash...

Then round-up the plant, and destroy the remains with fire...
Only way to be sure short of nukes.

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Paul F.
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A sword never kills anybody; it is a tool in the killer's hand. - Lucius Annaeus Seneca c. 5 BC - 65 AD
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Good is the enemy of Excellent. Talent is not necessary for Excellence.
Persistence is necessary for Excellence. And Persistence is a Decision.
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Eureka, CA

Most seeds need to be heated to at least 140°F. Seeds from desert plants can survive 170°F. Compost piles can kill seeds at temps down to 120°F because of high moisture.

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In tha 360. MRF User Map

Yeah, likely won’t kill the seeds. You can spray them up to the time that they flower. You can clip the seed heads but the plant will try to grow more. I pull them, trying to get all of the root, and burn them. They grow on a two year cycle and flowers come year two. Get them year one if possible. Easier to pull in the spring when the roots are smaller and the ground moist. If fully grown, the spines aren’t as bad right near the ground.

There are some biological controls:


[extension.missouri.edu]


[ag.colorado.gov]

Round-Up.

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Saint Cloud, Minnesota, where the weather is wonderful even when it isn't.

Quote
mikebw
kill it with fire