Spraying on its own probably encourages the weed, as it creates a new seed bed with little competition. The best method of control is hand pulling and mulching, with repeated follow up to remove new seedlings while allowing native species to grow and spread. Slashing, just before the flowers mature (February/March), will work to a degree, but must also be followed up, as a lot of the plants will grow again. The good news is that where the native bush is healthy and there is reasonable natural cover, the ragweed remains much sparser. However, if these are left to seed, following severe drought the ragweed will seal the deal before the native cover can re-establish.
The answer is to get rid of every single plant, wherever it is. Go home yankee. If you would like to help clear this weed from Logan, please contact Barry Fitzpatrick on 3287 9268. Logan City Council is keen to support all bushcare groups.