What type of material to tie a knot at our ceremony?

Rope? Appropriate for a farm. Take a trip to the local farm and ranch store. I'm sure you'll find something great.

Like a rope of some kind?

I would say a satin or silk rope, but really, anything is going to degrade in the sun, especially on a door that is either behind a glass storm door, which will amplify the effects of UV on it, or exposed to the elements, which will also accelerate degradation.

Have you considered hanging it on the inside of the door or on your wall?

What about the pretty cords that they use for drapery tiebacks? They come in a variety of colors and are sold at BB&B and probably at Joann's.

For one of my handfasting ceremonies, near a river, they used a white nautical line.

Thanks everyone for all of the ideas! I found red and white nautical rope which should work just fine and look great too.

We are also doing a handfasting ceremony. Make sure to try a run through with your officiant with this material to make sure it will work. Our officiant picked ours out, so we know it will wrap well and not chafe.

I bought nautical rope from Amazon.

http://www.amazon.com/FMS-Triple-Strand-Twisted-Cotton-Natural/dp/B0167TMNKW?ie=UTF8&psc=1&redirect=true&ref_=oh_aui_detailpage_o02_s00