As I see the nighttime temperatures dropping to the low 50s in southeast PA I started thinking... would it be more cost efficient to heat a small outdoor greenhouse or run grow lights indoors? I have to put my plants somewhere out of the way because the damned cats will destroy them if they get the opportunity. Basement, attic, or similar.

I would imagine that the cost of running a heater outdoors would far exceed the cost of running some flourescent lights, but I really don't know anything at all about greenhouse energy efficiency.

Zone 6-7.

I had the same question run through my mind this year as I dived head first into citrus and other edible tropicals.
I suppose much of the time the answer is related to how much indoor space you would have available and how many plants you have/how much square footage you would have to light.
Another factor would be the height of the plants.
I have just finished my "indoor greenhouse" in my basement and have squeezed as many plants as I can into a 25 sf space. I found the biggest limitation was the height of the plant- I can't fit anything more than about 5 1/2 foot tall including the pot.
I am using ceramic metal halide lamp on a light mover. Much less expensive than greenhouse space. My house is already heated-no extra cost there. Only time will tell how successful growth will be.

Indoor space is not a problem, I could house hundreds of plants between the basement and attics. There is little to no natural light available in these locations, though. I believe flourescent lights are much cheaper to run than metal halide, but it might be worthwhile to run more expensive lights that run host just so I don't have to otherwise heat the rooms they are in. I think at $4/gal, oil heat likely costs more than electric. Good 'ol nuclear power around here.

"that run *hot*"

If you have a basement, you may want to know that you can store your citrus trees there without any light for the winter ( if it is cool--like 50-55).
The trees will not grow of course, but will survive with little attention.

flourescent (CFL) lights may seem cheaper to run, but you need more of them they dont make much light ,the spread of the light accross and through the plants is not good

I have some cfl lamps (flourescent) and the light meter measures 200 at about a meter away from each one, after that its not measuring anything, they need to be very close to the plants, my flourescent lights are listed as 250w lamps, about 5 inch in diamiter and a foot long

The sodium or metal halide on the other hand, i have 600 and 400 watt lamps, infront of one , 2 meters away from the 600w i get 2000 on the light meter

The sodum or mh lamps produce heat as well reducing the need for heating, i use themostatic heater mats under he plants to warm the pots as well so the roots are warmer

Dispite having a south exposed large window in my house, in the months of Dec and Feb I experience a lot of leaf drop. I think this is due to house temps in the high 60s and low humidity. This year I am going to take Skeeters advice and move some of them to my basemement, one less thing to worry about.

Frank

Yeah but I thought that the fruit only developed if it was warm and well-lit?

Citrus trees will go into almost complete dormancy with low light and temps in the 50s. Storage in basements over winter is one way to get the trees through the winter--the down side is you will get no growth, but most people do not get growth during the winter--yet risk exposure to unbalanced conditions of light and temperature that cause loss of leaves.

Starloc-- the benefit of CFL is that they produce more light for the same wattage (power). Citrus maximise their photosynthetic potential at light levels of about 650 micro moles/meter square. The problem with metal halide lamps is their heat--so you cannot put them close to the trees. Light intensity decreases exponentially with distance (1/4 the light at twice the distance). The low wattage of CFL means that you can put the light closer to the leaves without damage and get the same amount of light to the tree--with less power.

For those that are trying to maintain growth during winter, it is better to extend time of light exposure than intensity of light.

Two other points--In order to get your tree to bloom in the spring, it must be exposed to cold and allowed to rest-- about 800 hrs below mid 60s (50 even better). If you are going to expose the tree to additional light, you must heat the roots to at least 65--otherwise the roots are not functional and you risk loosing leaves.

If I keep it well lit and warm can I expect existing fruit to develop, or will it fall off or never mature?

Also, it looks like the efficiency of fluorescent lights isn't that much better than halide, at least at a glance. Maybe my source is bad that doesn't make sense to me.

I would not use compact fluorescent, but the 4ft tubes as I already have a half dozen fixtures to hold them.

I don't have any experience with indoor fruit production---all of my fruit producing trees are inground, but they generally produce fruit in winter and the cold exposure increases sweetness in the satsumas. Lemons don't get sweet, but do color while exposed to cold.

From what I have read here from those that do have indoor producing fruit like Meyer Lemons, they should maintain the fruit to maturity. What kind of trees do you have?

Brian asked...."If I keep it well lit and warm can I expect existing fruit to develop, or will it fall off or never mature? .....

brian, small fruit falling from the tree of failing to mature, is not caused by keeping the tree too warm through out the winter. To answer the first part of the question, a citrus tree requires between 750 to 850 accumulated hours of temperatures below 68F in order to set a good bloom in the spring.

As far as halide lights, they have never given any of my trees a problem because of their generated heat. I have counted the heat that they produce an extra benefit. Every winter I place the light 12 inches above the tree, and then move it down 1 or two inches each night until it is 2 or 3 inches above the tree. I like halide lights, they work great for citrus. Halide is the only type of light I use. My Marisol Clementine tree has produced 5 growth flushes each year for the last three years, due to winter lighting. - Millet

I'm in the cold storage camp. I would love to have year round flushes of growth, but I can't swing the extra dough for the power bill. I have a screened in porch that I put plastic sheeting over. Last winter my place got down to 17.6 F, porch temperature was 33.2 F. I provided no additional lighting, I watered about every month, and I had very little leaf drop. It worked great, but now I have more trees, and no more porch.
Well, I've got about two months to figure out which ones are going to come inside. I'd rather not have them inside because in dormancy they are almost on auto pilot. In SC, in terracotta pots, in summer, I water almost EVERY day!

Cheers to you other citrus freaks..................
Scott

I asked the wrong question... I intended to ask if existing fruit will ripen in the hibernation scenario. As in, spring bloom, green fruit in the fall, and indoors all winter but with no light as was suggested in this thread. I can't imagine how the fruit would develop at all without new sugar, so I would expect that one of the two winter growing conditions in the "sticky" post would be required to ever get ripe fruit from an indoor/outdoor citrus.

Existing, yet still young immature fruit, require the energy that is produced by a citrus tree from sunlight, water, and continued nutrition to continue their growth and become large mature fruit. A tree setting in cold temperatures or cold/dark regions cannot provide this, and all growth stops. - Millet