I do a monthly (first of the month) 'maintenance' flush to remove (salt) build-up. Fox Farms (what I use) has a habit of leaving a little bit of salt build-up. Nothing major, but I chose to address it before it becomes a potential problem. Everything in both the growth and flowering growroom(s) get's the flush (1 to 1 1/2 times the pot volume) with plain, properly ph'd water. Sometimes this schedule lines-up with my plant maturity level and harvest dates, sometimes not. I do plan ahead, and adjust my watering schedule so as not to promote root rot at this stage.

Regardless of when I did my last flush, I try and stop all nutrients the week before harvest, and flush with a 1:1 ratio of properly ph'd water. This includes discontinuing the molasses. The last week in flower, molasses won't do you enough good to justify continued use. It's done it's job, and for flavor's sake...time to flush.

When you smoke the plants (in a pipe) when properly flushed and cured, the ashes will be a fine, light grey color and powdery. The more nutrients that remain in the meds, the more the ashes will be darker black, clumpy and almost resinous, and leave a chemical (ammonia) taste. Not sure where the ammonia flavor comes from...but doesn't urea (horse pee commonly used in ferts) smell like ammonia...?

Remember, you are flushing the soil, not the plant. There's no spiggot hook-up sticking out of the stalk to directly flush the plant, so restricting what is available in the soil (or reservoir if hydro) is the only help you can offer. When you don't flush, you'll either have to accept the harshness of the smoke, or cure over a longer period of time.