Hi, Kyla!
I would avoid plastic, I think the seed heads will be much happier if they can breath and the humidity they exhales can escape.
I think "organza" is the best material. It lets air, sun, rain and gases flow through freely. They have built-in pull-ties. You can get any color or size you want online, or small white bags locally from the "wedding supplies" isle of craft stores or Wal-Mart.
For big seed stalks like those, how about an organza bag big enough to hold a bottle of wine?
Best Organza Bag Source: found by Patti1957
order sizes around 10 to 30 bags
http://www.yourorganzabag.com/...
wine bottle size, available in 25 colors:
http://www.yourorganzabag.com/...
Ten for $3.30.
>> 1. Do I need to let them dry out completely before I bag the seedheads on the plants? (It's pretty wet right there due to recent showers.
Well, make sure the flowers had time open to the air, to be pollinated, but you've already done that if those are seed pods and not buds waiting to open.
If plastic can be used at all, starting out dry would be necessary. With breathable organza (or a foot cut from panty hose) , water can still evaporate, so it would not matter. You should expect them to be rained on more after bagging, it really can;'t matter much.
They DO have to be 100% dry or even more than air-dry before sealing them away or even piling them into a heap. I dry them in paper envelopes once I bring them indoors. (I need newer photos on this article. I found a way to cut "trays" from cereal boxes that hold envelopes upright tidily.)
http://garden.org/ideas/view/R...
I'm also fascinated by desiccants, and I imagine that seeds will store viably for more years if stored drier than 30% RH.
http://garden.org/ideas/view/R...
I've been working on another "desiccant" article, but need better photos. I have it as a blog entry, but the HTML codes seem to have exposed themselves:
http://garden.org/blogs/view/R...
Oh, look! I seem to have written a tip for your very question! I had forgotten.
http://garden.org/ideas/view/R...
"P.S.
http://www.giftsintl-us.com/pr...
Here is another source of organza bags. A few years ago, I thought that the first site had better prices, but I haven't compared shipping cost and minimum orders recently."
Personally, I would not have bothered putting the bags on until some seed looked nearly ripe and was getting ready to fall out. But thinking about it, why wait?