Blooming:
To a certain degree, the flowering of all (perhaps I should hedge and say nearly all) plants is dependant on tissues reaching a certain ontogenetic age before tissue differentiation into flower structures is possible. Ontogenetic aging is roughly a measure of the number of cell divisions that must occur before flowering. It varies from plant to plant. An illustration of its influence can be seen in the fact that plants propagated from cuttings invariably flower/fruit sooner than plants of the same species from seed. The tissues in the cuttings have seen many cell divisions and are ontogenetically older than tissues from seed, thus they are capable of flowering sooner. It is interesting to note that cuttings taken from basal portions of a plant are more vigorous because they are ontogenetically younger than cuttings from top branch tips. However, the cuttings from the ontogenetically older wood at the upper parts of the plant will flower sooner.

Photo-period:
Plants have more than a single internal clock to fool when forcing them to flower. There are Circadian and endogenous rhythmics that effect the ability of a plant to keep track of photo-period. Even when you think you have succeeded in tricking the endogenous rhythm of a plant into thinking day length is what you've programmed it to be, there is another rhythm keeping time in the background, ready to reset the clock at a few moments notice. I'm not sure how much you know of photo-periodism & the auto-rhythms of plants, but even short light exposure while the plant is in a receptive phase (photophilic) of the endogenous rhythm can undo your programming work.

Illustration: I grow about 75 tropical trees and 25 houseplants under fluorescent lights 18/6 from late Sep until mid-May. The only other source of light in my basement grow area is a single small window at the far end of the room that is mostly obscured by a planting outdoors, yet all the plants invariably know when nights begin to shorten, and they all begin to grow in earnest in late March, after the vernal equinox. These changes are controlled both by the plants timing mechanisms and the plants ability to measure the increasing photo-period - even though the window allows almost no light to enter the room.

Good luck, Eric.

Al

You got some great info from Tapla, I will try and fill in some blanks for you as I can.

Right now my plants are under 24-hr flourescent lights while they are in their vegetative state.

If your plants are annuals or simply plants you only want for one year then practically speaking this is fine and usually will result in faster growth. If you are growing a plant you will want to keep for more than 2 years I do not recommend the 24 hour light schedule as this screws up the cycle plants are 'wired' for. In other words if you want fast growth followed by bloom/fruit as quickly as possible and after that the plants can be tossed then no worries.

What I was hoping to do is force my plants to flower early if possible. It's my understanding that this happens when the plant's photoperiod has been reached (the amount of darkness hits a certain length). This isn't true with all plants though, is it?

No, it isn't.

Aren't some plants' flowering schedule based on the age of the plant?

There are numerous things believed to trigger various plants to enter into the next stage of their life cycle. Light period is only one and it is the primary influence for only some plants.

Does anyone know a site that lists the needed photoperiod for different types of plants?

Unfortunately I know of no such site. However, google is your friend and searching on the plant name followed by 'photo period' should turn up info if the plant is at all photo period sensitive.

As Al indicated though, if you wish to manipulate photo period sensitive plants you need to keep them in darkness. If they 'see' the outside even through a shaded window they don't get fooled. A windowless room with the door always closed when you want the plant to think it is night works well if you have that available. Alternately you can cover your plants with breathable, dark material when you want them to think it is nighttime, but that can get to be a pain and it doesn't take forgetting to do it more than once or twice and the plant reverts back to it's vegetative cycle.

I hesitate to say this, but if you really want to understand what it takes to use darkness to force plants into flowering visit overgrow.com. Be forewarned the site is about growing marijuana which is why I hesitiate to recommend it, but marijuana is a plant with strong photo period sensitivities and therefore there is a ton of discussion on that topic. If I knew of any other site where people discuss the ins and outs of manipulating light duration for plants I would mention it instead, but I don't.

Thanks Al and username_5!

Yes I've visited overgrown back when I was interested in hydroponic topics and read over a lot of other topics including photo periods, which is why I was curious about force flowering in other plants. Apparently with marijuana you can force flower the plant as soon as you like, but they stress that ANY kind of extra light will disrupt the photo period -- this could be the reason (among others) your tropical trees and houseplants aren't being fooled, Al.

I'm going to set my small plants on timer in a contained room and see what happens :)

but they stress that ANY kind of extra light will disrupt the photo period

This hasn't shown to be true in my experience. Consistent, low intensity lights do not appear to take photo-period sensitive plants back to a vegetative state. By consistent lights I mean a distant street light that is always on after dark as well as moon light. Sunlight, even well shaded and irregular indoor lighting do though.

I don't claim to understand why, it has simply been my observation.

I'm going to set my small plants on timer in a contained room and see what happens

Good for you, experience is the best teacher. If you can track down where your plants grow natively (or where they originated from) this may help you in experimenting with photo periods. If you can find out what time of year the plants bloom there you can also find out how long day/night length is at that time and then mimic it. Alternately if you have grown these plants outside in your yard and know when they bloom you can use day/night length durations based upon that as well.

If you want to experiment with even more factors than light/darkness duration you can try other things like damaging plant roots or putting them in the fridge for a day or so. Some plants react to damage by stopping whatever they were doing and putting all their energy into reproducing. This means flowering or ripening fruit. Other plants react to temperature changes. These are 2 events that are fairly straight forward in manipulating.

anyway, whatever you do, have fun.

anyway, whatever you do, have fun.

Of course! :)
I'll give those stress effects a try depending on the plant. Guess there will be some research involved so maybe I'll setup an excel sheet :)

Don't forget that light quality and quantity (wave length and foot candles) is just as important as light duration (duration). For some plants, these two are the critical factors in plant production.

You can find specific information regarding artificial light requirements if you research each plant that you are attempting to force, on an individual basis.