There is more to harvesting marijuana plants than just cutting them down. Before harvesting, the plants can be given plain pH adjusted water only, with no nutrients.
This is to remove any nutrient that has built up in the plants themselves, and the soil or hydroponic media they were grown in. After the plants are harvested, large leaves have to be cut off and the plants have to be manicured and dried.
A good way to determine when to harvest a plant from a particular marijuana strain, is to find out from the person or company you got the seeds from, what they recommend.
With outdoor marijuana strains the recommended harvest time will often be stated as what month to harvest the plant, for example late october.
With indoor marijuana strains the recommended harvest time (sometimes called the flowering period) will be stated as the number of days or weeks after the plant has started to flower until the time it should be harvested, for example 56 days or 8 weeks.
When grown indoors, the most commonly cultivated strains take 7-10 weeks from when they start flowering until they are ready to harvest. Some less common strains can take 12 weeks or longer.
Even if the person or vendor you got the seeds from tells you when they recommend harvesting the plants, you might want a better idea so you can judge for yourself what is best.
Female marijuana plant flowers look like this. As the plant gets closer to maturing, the pistils, they look like hairs, will turn from white to a darker color, usually brown or red, sometimes yellow or maybe another color.
When 50%-80% of the pistils have darkened, most strains are ready for harvest. Other clues include: a stronger aroma than usual from the plant, a frosted appearence around the buds, calyxes (seed pods) swelling.
In addition to these clues, you can better determine when to harvest by using tools to enlarge specific parts of the plant that are too minute to be seen by the unaided human eye.
The items necessary for observation include inexpensive microscopes to monitor certain female marijuana plant trichomes and the round head that develops on them.
Trichomes are small appendages that look like hairs, they are produced by marijuana and other plants. Capitate trichomes are resin glands. They are the part of the cannabis plant that contains the most THC.
Monitoring capitate trichomes and the round heads that develop on top of them will help you to best judge when the plants are at their maximum potency and ready for harvesting, by letting you know when THC production has reached its peak.
All marijuana strains produce trichomes that start out clear, some have a slight yellow tinge. As they age, most strains will then turn a milky translucent color. Towards the end of flowering they will turn light brown or in rare cases a reddish color.
See this and this for images of what trichomes look like and how monitoring them can help you determine harvest time. The trichomes can be found in their largest concentration on the buds of flowering female marijuana plants.
To help determine when the best time to harvest is, keep an eye on the round head on top of the trichome on the buds on each plant.
For maximum THC content and the strongest high possible, harvest your plants when at least 50% to 75% of the round heads on the top of the trichome have turned from clear to a cloudy milky translucent color, and less than 20%-30% have turned light brown.
Waiting until more than 20%-30% of the round trichome heads have turned light brown will result in marijuana that produces a less cerebral high with a bit more of a sleepy body stone feeling.
If you harvest when over 50% of the round heads are light brown colored, the THC content of the marijuana will suffer and the high will be diminished.
You can harvest the plants by cutting them down at the base of the stem, with pruning shears or some other type of cutting tool, just above where the plant meets the hydroponic media or soil. Or you can cut off each branch or bud separately.
All buds on the plant will probably not mature at the same time. It is best to monitor the largest buds at the top of the plants and harvest when a majority of the premium buds are ready.
This will allow you to harvest the entire plant at the same time. Alternatively, since different branches or buds on the same plant may mature at different times, you can monitor and harvest each branch or bud separately.
After the first crop you can alter the harvest time and see what difference it makes if you harvest sooner or later than you did before, if you grow that marijuana strain again.
There is another type of trichome that can be found on marijuana plants, they are called cystolith trichomes, or cystolith hairs.
This type of trichome is mainly found on leaves. They look like small hairs with no round head at the top. Cystolith trichomes are not a source of THC.
One thing to keep in mind, if you are a first time grower or relatively new to growing, is do not be over-anxious and harvest too early. With every strain you have a period of several weeks where you can harvest and still produce good marijuana.
After the marijuana plants have been harvested, large leaves should be cut off and they should be manicured and dried. Cutting off large leaves, manicuring, and drying the marijuana harvest is a very important part in the cultivation process, instructions are located here.
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 by flushing them.
Flushing, also called leeching, can improve the taste. It will also reduce the harshness of the smoke, making go down easier and reducing coughing. Also, it will eliminate health risks associated with smoking or ingesting nutrients.
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.
Marijuana plants need nutrients to help them grow, mature, and produce THC. In order to flush nutrient from plants, you feed them water only, pH adjusted, for one or more feedings just before they are harvested.
City and well water will most likely have some elements or compounds present in them, so it is best to used distilled water or reverse osmosis water for this purpose. This holds true for hydroponic and soil grown marijuana.
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 will not slow down. 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 pH, no 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.
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 in soil, you can stop watering the plants 2 days prior to harvest. This is not essential but it can speed the drying process. Do not stop the flow of water to hydroponic plants until you are ready to harvest them.
You probably won't be able to judge the exact optimal time to switch to water only feeding to flush the plant and then harvest it, the first time you grow a strain.
Once you have grown a strain, you will be able to better estimate the best time to start giving it distilled or reverse osmosis water to flush nutrients, and when to harvest future crops.
As an example, let's say you want to harvest your crop when THC production has reached a peak, that's when about 50%-75% of the trichome heads have turned from clear to a milky translucent color.
So you wait until you see about 40% to 50% have turned to a milky translucent color, then stop nutrients and supply the plant with water only, for 7-14 days with regular soil or 4-7 days with hydroponics, and then harvest.
At harvest time, take a look at the trichomes and make a note of what proportion of the trichome heads had turned to a milky translucent color, and what the resulting marijuana was like.
If you are satisfied and happy with the marijuana you harvested, repeat the process in the same way next time you grow that strain.
If you are not satisfied with the results you get, try altering the time when you harvest on future attempts at growing the same strain.
Be aware, there are products to help flush hydroponic, soil, soilless marijuana plants quicker. They can help speed the flushing process and allow the plants a few more days to grow normally.
In addition to flushing just before harvesting, most plant flushing products can be employed earlier in the life of the plant if you think too much nutrient has built up in the hydroponic media, soil, or the plant itself.
Marijuana for medical purposes should always be flushed to remove nutrients. Some people who grow for other purposes don't flush the plant at all, or have some kind of reduced nutrient regime leading up to harvest time.
An option to cutting off all nutrients prior to harvest, is to cut the amount of nutrients the plants get in half. Do this for the last week with hydroponics, or the last two weeks with regular soil.
If you are going to experiment with different nutrient flushing methods, keep some bud from each method so you can compare the finished product to determine what you think is best, or if you think it makes any difference at all.