A reason why it is important to know when the plants will be ready to harvest is because prior to harvest, you will have the opportunity to minimize the amount of nutrient remaining in the plant.
Not everyone flushes (also called leeching) plants prior to harvest, but you may want to. The plants themselves and the hydroponic media or soil the plants were grown in will store some of the nutrients that the grower has supplied them with.
In order to flush nutrient from plants, you feed them water only (pH adjusted) for one or more feedings just before they are harvested.
When timed correctly, you are able to wait until just before you are going to harvest, and give them water with no nutrients. In this way they are allowed to grow for the maximum amount of time (larger harvest) before being flushed.
The plants will use up the nutrients they have stored in them and growth won't slow very much. If nutrients are not flushed from marijuana plants, you will be ingesting some.
This might not be healthy especially if smoked. In addition the resulting marijuana might taste bad, and may also be hard to ignite. Flushing will also reduce the harshness of the smoke, making go down easier and reducing coughing.
If you are growing hydroponic marijuana, start flushing about 4-7 days before harvest. This can be done by changing the nutrient solution and using only distilled water or water filtered via reverse osmosis. Just adjust the pH and don't add nutrients.
Some hydroponic growers will change the water two or more times before harvest because the plant and media may still hold nutrients after the first flush. There are products available to help flush hydroponic and soil plants quicker.
With soil you need to change to distilled or reverse osmosis water about 7-14 days before harvest. With coco coir start flushing about 7 days before harvest. With amended soil where you don't add nutrients, there is no need to flush.
When growing indoors, if you know that the marijuana strain will be ready to harvest 10 weeks after flowering starts, switch to water only feeding about 9 to 9.5 weeks after the plant starts flowering.
Flowering starts when you can first see if a plant is male or female. It is not when the 12/12 light cycle is initiated. It takes 10-14 days after starting the 12/12 light cycle for marijuana plants to start flowering.
When growing outdoors with soil, if you know that the marijuana strain will be ready to harvest in the middle of october, switch to water only feeding 1-2 weeks prior to harvest, about the beginning or first week of october.
Part 01) See the full hydroponic marijuana nutrient article here.
Part 02) Nutrients and hydroponic marijuana introduction here.
Part 03) Water and hydroponic nutrients here.
Part 04) Preparing hydroponic nutrients here.
Part 05) Measuring hydroponic nutrient solution - EC, TDS, PPM here.
Part 06) Hydroponic nutrient strength here.
Part 07) Hydroponic nutrient solution temperature here.
Part 08) Flushing or leeching hydroponic nutrient here.
Part 09) Amount of water for hydroponic marijuana plants here.
Part 10) Hydroponic nutrient list of materials here.