Flushing should begin about 14 days before harvest by stopping all nutrients and using only pure water to feed the plants. By providing no nutrients, you force the plant to rely only on what is left in the growing medium to feed on. The actual act of flushing is achieved by over-irrigating the medium until the nutrients inside are dissolved and washed out the bottom of the container. The best way to do this is with a two-step flush technique. (The process is an easy one.)
First, flood the grow medium with a heavy dose of water and wait a few minutes to allow all of the salts (nutrient buildup) to break down. Then add more water to chase out the first dose. By waiting a few minutes after adding the first dose of water, you're allowing enough time for the water to dissolve the salts. As salts decompose, they can be effectively flushed out by the second dose. Traditional leaching usually employs only the first flush of water, which isn’t always adequate for complete dissolution.
A few days after flushing, you should notice signs of nitrogen deficiency. The leaves will go from dark to light green, eventually turning completely yellow. Another sign is a reddening of the leaf stems, starting at the center of the leaf where the blades come together.
Test your flush by snapping off a leaf and tasting the juice that flows from the stem. If the taste is bitter, there's still plenty of food in the plant's system. When the juices are clean and taste like pure water, the plant is clean enough for harvest. The bitterness is from nutrients and other chemicals that you definitely do not want in your smoke.