this is straight from greg green's cannabis grow bible....
PRE-FLOWERING AND FLOWERING
BY NOW YOU'VE MANAGED TO SET UP THE BASIC growing environment and experimented with modifying and controlling it to promote better, stronger plant growth. You will have observed your plants forming a number of nodes and a small leaf mass at the top, which you know is going to form the next set of leaves and branches. Your leaves should be flat and stretched out to receive as much light as possible across their surface area. If they are, then your plant is enjoying its environment. If not then maybe you should consider turning to the Problem Solver in Chapter 13 to see what has gone wrong.
You should also note that almost everyone makes mistakes the first time growing. Very few first-time growers get to this stage without experiencing at least one problem, so don't feel bad if you didn't get it right the first time.The trick to growing healthier, more potent plants is to keep growing (and reading this book).
During the vegetative growth stage your plants will begin to grow quickly and produce more leaves and new branches. The stem will also grow thicker. This is the point when your plants begin to really look like marijuana.
Then, one day you will notice that your plants appear to be doing more than just growing vertically and producing leaves. You take a closer look and there appears to be new leaf growth at most of the node regions between the stem and the branches. Your plant is now developed enough to receive more light energy and covert this energy into more side branch growth. These new growths produce more leaves, branches, and eventually flowers. This type of new growth at the stem's node regions is called lateral branching or secondary branching. This is really where the extra node regions begin to take shape.
After a few more weeks of this secondary growth your plant is looking more bushy and certainly has more node regions. It is during this time that your plant has reached sexual maturity and is ready to show sex. How long this takes depends on the strain you are growing but after the seedling stage has finished you are looking at a time period of 4 to 8 weeks vegetative growth. With Sativa strains this can take much longer.
At a certain stage towards the end of vegetative growth the plant enters its pre-flowering phase and, as a grower, you need to tailor your grow space and gardening approach to this new stage in your plants' life cycle.The next section explains how to identify the pre-flowering stage.
PRE-FLOWERING AND EARLY SEXING
Recall that during pre-flowering, plants start to exhibit their sex. As a grower, you should be hoping for as many females as possible. Pre-flowering occurs at the node regions. Towards the end of vegetative growth you need to check your plant nodes for what is called calyx development. A clone will carry the exact same genetic makeup as the plant it came from, so if you know your clone's history you will already be able to predict it's sex.
CHECKING FOR CALYX DEVELOPMENT
Choose a plant. First of all examine the node regions of the plant where the branches meet the stem. You are looking for very small pod shaped organs here at these regions.* If you don't find any here then move outwards along each branch checking each node region until you come to the tip. If you do not find calyx development then your plant has not reached its pre-flowering phase yet. You need to wait for it. Calyx development will come in time.
There are three early indicators of plant sex, but they are not 100 percent accurate.So remember, these methods can fail, but are often accurate predictors of your plants' sex.
First Early Sexing Method
If you've been growing the same strain and all the seeds were started at the same time, then you may notice that some plants are taller than others: the smaller plants tend to be female and the taller ones tend to be male. You can separate these plants into two sections in order to see how good your guesswork was when you do definitively identify sex. The other thing to note is that male plants generally start to pre-flower before females. If you have taller plants that are producing new growths before the smaller ones then the taller plants are probably male.
Second Early Sexing Method
A good way to identify plant sex at an early date is to examine the calyx* with the aid of a very fine magnifying glass. If the calyx is raised on a small, short stem then it's probably a male. If the calyx isn't raised on a small short stem then it's probably a female.
Third Early Sexing Method
'Force-flowering' is probably the best early-sexing method. To force-flower a cannabis plant, simply take a cutting and place it in a cup of water or a cloning medium, such as rockwool. Expose the cutting to 12 hours of light followed by 12 hours of total darkness. The cutting should flower and display its sex — however the plant must be mature enough to present its sex. An immature plant will not show sex because initial calyx development is not photoperiod-related. Plants normally mature around the forth week of vegetative growth because sex is not genetically determined until the third week of growth.This also applies to 'feminized seeds,' which can, and often do, turn out to be male.** If your plants are exhibiting calyx development, then this is a suitable method of determining the plant's sex.
These methods are NOT 100 percent accurate. Later in this Chapter we will explain how to definitively identify the sex of your marijuana plant.
WHEN TO FLOWER?
Your plant will remain in the pre-flowering stage between one and two weeks. During this period, the new growth regions begin to change shape depending on the plants' sex. It is during this shape change that you can properly detect your plant's sex.
Pre-flowering is a sign that your plant is mature enough to start flowering. As a grower you have a simple choice to make: Do you want to flower now or do you want to continue vegetative growth? Here are a few issues to consider before you make a decision:
• Most cannabis plants can be kept alive for up to 12 years by simply keeping a light on the plant at all times. Even if the plant only receives light for a few hours a day it can still live for a long time. It all depends on how the plant is treated. These plants will grow to a certain height and then form into a bush. Eventually they will stop producing branches and will spend the rest of their lives growing new leaves to replace the old ones. By keeping the plant in vegetative growth longer, you allow it to reach its optimal size of vegetative growth and the plant will stop growing. Most growers flower before this, when they see calyx development, in order to speed up the growing process. For example: A plant that shows sex at the forth week of vegetative growth can be kept in vegetative growth for a few more weeks to allow the plant to generate more node regions (leading to more branch and leaf growth). When the plant is flowered, this extra stage of growth should help the plant to achieve optimal results however the grow time is extended by a few more weeks to obtain this.
• Bud production does not increase at the same rate as plant growth. Bud production depends on your growing environment, your strain's genetic makeup and the amount of nodes the plant has. All nodes are potential bud areas, but every strain has a genetic threshold for bud production.
• It is possible to get more bud with lots of plants that are flowered as soon as they're mature (which also keeps them shorter and smaller), than extending vegetative growth with less plants until they reach their maximum height and size. The time frame for the shorter option also produces more bud turnover per year.
Keeping these things in mind, you can either choose to flower now or choose to keep your plant growing until it reaches its size threshold before you start flowering. If you take the longer route, prepare to have the space for it, because in the flowering stage, some cannabis strains can more than double in height and width.
If you want your pre-flowers to flower you only have to do one thing: introduce the 12/12 light schedule.