As your sunflowers and flowering sweet peas (for example) grow you'll need to support them to prevent wind damage.
Use natural string (such as jute twine from Nutscene, which is made in Scotland) which is plastic-free and home-compostable.
Fix the supporting string in place with a bow (like a shoelace), so you can easily untie and re-use / remove and store for next year.
After a good few seasons the natural string will weaken and snap - it can then be composted.
This is another step towards zero-waste gardening...yay!