India Flint, guru of Eco-printing suggests this technique. Put your fabric into a jar, clean or not because whatever was in there before may make an interesting mordant. Squash in your leaves etc and cover with water. Add an extra mordant if you want, then seal it up and leave it in the sun for some time.
In the small jar on the left I have cotton mull, screwed up tight and wound with string to hopefully create some variation in pattern. This potion contains NZ flax pods and seeds, crushed, and a tea bag for mordant. In the jar on the right, I have unbleached calico folded with morning glory flowers in between layers, then more morning glory flowers crushed on top. For a mordant I used alum. Both jars are filled with rain water. I took this photo the same day I put the mix down, already you can see a lot of dye extracted. India doesn't say exactly how long, but I'm choosing to leave mine for at least one month (I can only resist the temptation of opening them earlier by leaving the country!).