natural fiber as Shade cloth

Hi, shade cloth can be verry useful but unfortunately it is often (always?) made of some sort of plastic. I know these are cheap and quite durable and do not degrade verry fast, but eventually they will degrade and then you end up with plastic in the garden, and having to buy new.
What natural fiber is the most Uv resistant, lets trough enough sunlight and is not super expensive? I would use it in the summer in central Portugal, if the cloth is stretched out (good air circulation) an does not touch the ground I do not think mold would be a big problem.

Do you have palm trees in Portugal? Palm leaves are very common natural shade cloths in the tropics.
If you don't have palm trees,  you can make a frame using     four light poles, say 2x2 meters. Weave some wire on it. Put the trellis on the vertical and plant fast growing vines, like beans.
When the beans has covered most of the trellis, cut the beans close to the ground. Wait a few days until the leaves has fallen leaving only the stems.  
Move the frame, vine stems and all, to wherever you want the shade.
When the poles got rot, reuse the wires.

Hi, thanks for your interesting answer! It makes more sense using original plants than woven material, but I just didn't think about that option before. We have only one palm and i dont think it grows fast enough in our climate. But we do have a lot of eucalyptus around, their leaves keep hanging on the branches for a long time so i could use those. The vine option is also verry interesting. I guess a living deciduous vine that leaves trough light in winter and shades in summer is also possible

Great question!

Linen and nettle fibres are your most solar resistant natural cloth.  That's why they were so popular for drapes in the past.  But expensive.  Maybe you can get some at a second-hand shop as they sometimes have old table cloths.  

Next up, I would say wool and hemp.  Wool lasts about 300 hours of direct sunlight, before beginning to decay - that's about two summers average - maybe four where I am, less where the sun is stronger.  However, if the wool was processed in a large, industrial scale mill, I would cut that estimate in half due to some of the chemicals used in the processing.  Hemp depends on how it was processed.  Sometimes they use a harsh chemical to soften the hemp which causes it to degrade very quickly - one to two years.  Less common, but sometimes still available, are traditional/mechanical processed hemp which can last four to 6 years in those conditions.  

Cotton is probably the least durable in these conditions, but possibly my first and last choice.  It cheapest and it degrades quickly.  You can often find cotton sheets in secondhand shops.  I would expect this to last one year for something like sheets.  Duck cloth (a kind of canvas good for making tents) lasts me about three years but doesn't allow enough light through for the plants, so I just put this to shade during the heat of the day, not all day.  Something I've been worried about is the high amount of chemicals (both agricultural and manufacturing) used to make cotton cloth.  I'm wondering if they will dissolve into the soil when I compost the cloth.  For this reason, I'm moving away from using cotton in the garden and trying to find more linen.  

Hi, thanks for your clear answer on different natural fibers! I will keep an eye open for natural fibers in the second hand stores