grow light hours for vegative growth stages?

[40def]

so i have been growing indoors for a few years now and i have always grown my plants on 18 hours of light and 6 hours of darkness from seedlings till im ready to flower them. over time i have heard many people mention growing under 24 hours of light untill ready to flower but could not tell me why they preferred 24 hours of light to 18/6 and couldnt tell me the growth difference between the two. has anybody attempted to use both that can tell me what the benefits of growing 24 hours a day be?

[ian]

Short day plants (plants that flower during short days) require a certain amount of daylight to stay in vegetative state. This is why if it is very bright for a few days in winter, dormant short day plants still do not go into vegetative growth because the number of hours of daylight is too small.

18/6 is not a magic number. This is the typical range at which any short day plant will remain in vegetative state while still growing a robust root system. Most species range anywhere from 12 to 17 hours required for vegetation with variation in individual species especially among crops grown at different latitudes. At near-equatorial latitudes, short day crops typically vegetate in the low range where in higher latitudes, longer day periods are required. Compare for example the growing season in Central America compared to Alaska or Argentina. This is also why crops bred for a particular region don't necessarily do well at other latitudes (flower to early/too late).

24 hour light cycles leave no dark period and therefore no time for plants to commit to root growth. The dark period is also when C3 plants give up excess CO2 and other non-photosynthetic activity. One might use 24 hour light periods for a short time if root growth is desired to be stunted and shoot growth to be accelerated. This would not be good for more than a couple of weeks as the shoot growth would expand more than the roots could provide water for.

Day neutral plants like tomatoes will typically do fine in 24 hour light followed by 16-18 hour light all the way through fruiting. If you use 24 hour light at first with short day plants, it works well to cut this back after they have grown to a satisfactory height by 1/2 hour each day for three weeks or to reach a flowering stage.

Personally with short day plants, I prefer going 16/8 for a week, stepping down to 14/10, and then gradually stepping down to 11/13.  The fertilization scheme is updated weekly during this period from N to P to K heavy fertilizer. Nutrients are another issue as short day plants will use more Nitrogen in veg daytime, Phosphorus at night for root growth, and Potassium for flowering. Most charts for nutrients have a gradual change where half grow/half flower is used and with short day plants, this is where the light would be changed.

It is entirely possible to suddenly shift the light from something like 24/0 to 18/6 and then suddenly again to 12/12. Although this is the easiest to do, plants don't necessarily respond well to this treatment, especially for the first couple of days they may not be at peak photosynthetic activity.