Fiona and I were working a series of events for Glasgow Landservices in eight different parks throughout the city that year.
We only used native plants (and native animal hair).
Somewhere I have a list but, from the top of my head it included ……
Nettle,
Honeysuckle,
Ivy,
Fireweed,
Bedstraw (that came from my garden before anyone shimpfs)
Soft rush
Field rush (60 fathoms of this Fiona made for a Kishie)
Lake rush leaves (very beautiful twine)
Cattails leaves, as above
Chestnut bark,
Lime bark
Elm
Oak (with trees, it's three fold; either whippy roots, peelable (or rettable) inner bark or fibrous timber that can be 'stripped' )
Ash
Walnut (sent to us from a friend in England)
Sycamore
Clematis
Hop vines
Elder
Oat grass leaves (again a very beautiful twine)
Wild rose
Willow bast (this and lime are honestly two of the best)
Brambles
Spruce roots (not native is spruce, but pine works too, we just had heard of spruce)
Flag iris (makes a beautiful flexible rope)
Small veriscolour iris (again a beautiful rope)
Flax (beautiful, absolutely superb twine. Didn't grow enough for rope)
Hemp (we cheated on this one, and bought plumbers hemp, well oakum that was to be used in the boatyard, native though)
Daffodil leaves (beautiful colours as they dried out fully)
Grass ( several varieties with this. Leaves make string, but the whole makes a rope, and it's a good rope)
Heather
Broom
Mallow
Dockens
Sticky willie/cleavers (just because it was there. Dried out well though)
Bracken
Crocus leaves (another beautiful twine)
Bog cotton
Lobellia (it's a flax family plant, and it's remarkably good)
Privet (nice, but better for basketry we reckoned)
Potentilla
Clover
We had a go with seaweeds and with the long cress from a burn too. Both work, but they need a power of drying.
Not really worth the bother tbh, but we did make cordage.
I played around with scented plants to create twines for tying up seasonal wreaths as well. Everything from meadowsweet to sweet gale, from thyme to rosemary. They all work, they all hold together as twine, and while I wouldn't put any real weight on them, they did tie properly and take knots. My hands smelled wonderfully for a couple of weeks

Strawberry runners (the little wild ones run riot over some burnsides) didn't really twist well, they kind of rolled rather than twisted, but they plaited together into a braid, same with the buttercups when we could pull up an entire line of them. Really needed four to each strand, iirc.
and not really plants

but it was to hand, so we used it…..
Dog hair,
Highland cattle hair,
Horse hair.
I'll add in the others if I can find the list
It's a fun way to look at the world for a bit, "Can I make cord from that ?". There have been more than a few threads on making the stuff too.
http://www.bushcraftuk.com/forum/showthread.php?t=120400 for instance.
atb,
Mary