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This may be totally off the wall, but the sunflowers growing around my garden are bigger the nearer they are to the street light. I've seen documentaries with marijuana labs in California where they grow their plants all day and all night under the lights. But do street lights have any similar effect?

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Could be drainage? Here in Texas as the lawns turn yellow, there are some pretty obvious green stripes where the water would normally seep out - eg. against curbs. –  winwaed Jul 27 '11 at 1:08
    
@winwaed it really could be drainage, there is a strange half circle in my lawn where the grass grows extra thick. I think I'm just extra surprised by the height of some of my sunflowers. –  Peter Turner Jul 28 '11 at 13:44

2 Answers 2

As far as I'm aware, most plants need a "dark" period for their overall health & well being.

Examples:

Why tomatoes don't set fruit:

It has also been found that tomatoes under continuous light do not set fruit well. The key here is to avoid garden sites that are under lights that remain on all night, such as security lights or street lights.

Re-blooming Holiday Cacti

Interior lights in the home, street lights or even car lights can disrupt the required dark period and cause disappointing flowering.

‘MUMS’ the Word

If you plant mums underneath street lights you may find that they do not bloom as expected as the light will interfere with the required ‘dark’ period.

Holiday Plants

New buds will set as the nights lengthen during fall. This is due to the photoperiodic nature of poinsettia and allows them to bloom naturally around November or December. However, any stray light, such as outside street lights or household lamps, will delay or halt the re-flowering process. The plants must be kept in complete darkness for 14 continuous hours each night starting October 1 or 2 months before you would like the plant to flower.

Autumn Changes in Deciduous Trees

The effect of day length can be observed in tree leaves that grow close to street lights. These leaves stay green longer and are shed later than other leaves on the same tree, providing the tree species is sensitive to day length

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So exactly the opposite effect is true, that's strange. I thought my garden was being helped by the streetlight! –  Peter Turner Jul 28 '11 at 13:45
    
@Peter Turner, I really think it depends on the specific plant, but like I said above, "As far as I'm aware, most plants need a "dark" period for their overall health & well being." –  Mike Perry Jul 28 '11 at 15:44
    
It does depend on the plant, most plants can adapt without a "dark period" (will undergo that part of photosynthesis anyways). Here is a similar question: http://gardening.stackexchange.com/questions/3402 –  WienerDog Feb 15 '12 at 21:53

Regarding your mention of marijuana growth in California: yes, they do grow their plants under 24 hour light. However this is only in the "vegetative" or growth stage of the plant. Once the plant reaches ideal size, the light cycle is switched from 24 hrs to 12 hrs on/12 hrs off to begin the blooming stage of the plant's life (replicating the natural shortening of the days) . So yes, you are correct in that you can grow a plant as big as it will want to get under 24 hrs light, but it will never start to bloom.

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