Flushing isn't always necessary, and there are plenty of people who don't do it.
Some people grow 100% organic. Only natural inputs that are safe to consume in the end product. Others prefer to use synthetic nutrients and supplements for more intense flavor profiles and bigger yields.
There's nothing wrong with either method, but if you're growing with synthetic nutrients and supplements, you'll most likely want to flush your plants before harvest.
What is flushing?
When you flush a toilet, it removes all the contents to make the bowl clean again by rinsing with water. Flushing your plants is similar.
In order for nutrients to work, they need to be absorbed by the plant and spread throughout its system. With continuous use, it is possible that some residue or left over trace nutrients could still inside when you harvest.
Simply put, when you flush your plants, you cease use of nutrients and only give your plants water. As your plants absorb and circulate the water, it cleans out (or flushes out) the excess nutrients inside.
When do I flush?
There are three different opportunities to flush during your plants' growth cycle, but one is a flush you want to avoid and that's nutrient lockout. If you use too much of one nutrient, it can get "locked out" by your plants. The nutrient won't be nearly as effective, and can even harm your plants if you don't flush them out with water to remove the excess nutrient buildup.
The two occasions you normally flush are between the vegetative and flower cycles, and before harvest.
Because your plants require different nutrients when they are flowering, it's common practice to flush out the vegetative nutrients that you used during the early stages of growth. The same goes for before harvest.
You don't want nutrient residuals inside the plant to come out in your final product. It can impact the taste, the smell, and of course can make it unhealthy to consume depending on the nutrient used.
How early you flush before harvest is up to you, but a popular time is two weeks before harvest to ensure the plants are completely flushed. Flushing can also add a unique look to your plants, with changing colors in the last couple weeks from the plant getting flushed out.
Do I have to flush?
No. In fact, if you're growing in amended organic soil you should not flush at all. Flushing dense, nutrient-rich soil can cause flooding, and it can destroy the beneficial ecosystem that the plant thrives in which in turn can damage them.
And like we said earlier, you don't have to flush depending on the nutrients you use and in what concentrations. There are plenty of nutrients that are safe to consume in your end product. But if you use pesticides, fungicides, or synthetic nutrients with non-organic ingredients, it's important to make sure they aren't there when you harvest.