There are many plants that help to some extent. None can be considered a cure. They are at there most effective in the absence of conventional treatment. I've tried a few, including cinnamon (no noticeable effect) & bitter melon (karela, initial slight improvement, then negative affect.)
The most effective treatment I've tried is reduced carb. I doubt very much if you will find anything natural that offers significant benefit compared with reduced carb. The people tested in that study had high values before the test, so improvement is not difficult in poor control situations.