Know your weed before you try to kill it. The point of the above statement is, vegetative burn downs will only work for some weeds. I have to agree with Marcos on this one. Sometimes the situation is so dire, the only feasible and economical option is chemical control. Once you get your weeds under control you can then establish your organic program. There are "organic" alternatives that would work, but are many times prohibitive due to cost (top soil replacement), or time (solarization). One could also argue that the two "organic" options will actually be more detrimental in getting an organic program established than the chemical route. Bottom line, assess the site and conditions and find the most effective means of getting your organic program established that will fit the budget of your client.