I regularly see photos of teasels (fullonum) identified as fuller's teasels, but in spite of the name, it is the teasel called 'sativa' that was traditionally used to raise a nap.
Yes, they are both teasels, but the teasel heads are quite different and while you *can* raise a nap - sort of - with fullonum, the head really doesn't do that great of a job. On the other hand, you can find fullonum teasels quite commonly in many places, growing in ditches, so you can collect a lot of them and if they break down quickly, it's not that big a deal.
Fullonum heads have soft, straight, pliable bracts, the head is frequently shaped more like a clove of garlic (conical) while the sativa have very firm, stiff bracts with a bit of a hook at the end. The shape is more sausage like and they can be quite large. The bigger teasel head in the photo above is nearly 6 inches in length.
At some point I learned that the sativa heads must overwinter on the plant to 'harden' and are harvested in the spring. (I no longer remember where or from whom I learned this.)
Sativa were imported to North America for the woolen industry and are 'invasive' so it is NOT recommended that someone get seeds (any teasel seeds) because they will take root and over run native vegetation. If you want to try growing some, I'd recommend growing them indoors in order to control the spread of seeds.
Sativa heads are no longer commonly found growing 'wild', although there are some, I'm told, still growing in areas in New England. I managed to source some sativa heads in the 1980s and used to bring them with me when I taught, until it became difficult to transport vegetation across the border. But I recently unearthed my stash of teasel heads during a really deep dive cleaning up my storage area. I gave most of the teasels to my new student, who is interested in creating cloth that will be fulled, and the two in the photo above went to another guild member who is involved in SCA.
I cannot find the little book by Beverly Gordon called The Final Step which has the most information I've found on teasels and their use in brushing a nap on wool, although the website I linked to above has about the 'best' information I've found on line. Most just show the fullonum heads, not sativa, and talk about brushing a nap, implying that the fullonum heads are the ones historically used.
It isn't difficult to find information online - if you know what you are looking for and don't stop at the first website that appears on the search results. I spent a good 20 minutes yesterday digging and found primarily fullonum information. This morning I changed my search terms and the website linked at the beginning of this post popped up.
It was only *because* I knew what I was looking for that I was able to determine that this website had the information I was looking for.
We may have a world of information at our fingertips, but it is still up to us to look beyond the very first site provided and make sure that we are getting 'good' information.
Since sativa heads are extremely difficult to source now, a dog brush will work just fine.