Good morning,

I want to start running my lights (200 watt compact fluorescents) at night, and was wondering if citrus, like my african violets & streptocarpuses, do best with a number of dark hours or if it doesn't matter.

Plants are directly under the fixtures are located in the family room that has windows/skylights, but not much direct sun. Suprisingly, (to me anyway) I'm finding that the lights are a bit too bright/annoying to me to have them on during the day which is why I'd like to switch to running them at night. Thanks

Steph, "Does citrus require a dark period; optimal hours of lighting" -- what a great question! Some plants do require dark periods in order to flower. However, citrus are day neutral and flowering is not controlled by the amount of darkness. However MANY plants respire during the night hours. Respire, or cellular respiration is the process in which the chemical bonds of energy-rich molecules such as glucose (sugars) are converted into energy usable for the life of the tree. Citrus trees do respire, but I am not sure if they require dark to do so. However, you certainly do not need to have your light on during the day. The PAR value (Photosynthetically Active Radiation), for citrus is maxed out at only about 1/3 of full sunlight. In the winter I turn my light on at dark until 10:00 PM. Maybe Dr. Manners can add some light (no pun intended) on whether citrus require darkness. - Millet

It means that beyond the 1/3 full sunlight any additional light will not increase photosynthetic activity. The photsynthetic apparatus is at max capacity, or is at saturation unless other variables are modified.

It is not related at all to 1/3 of daylight hours, rather to intensity of light.

Ok, this maybe a stupid question, but is "full sunlight" the same in the summer as in the winter...? Or does it max out at 2/3 of winter "full sunlight" for example.

Just seems strange to me....

sunlight intensity varies by day time, season, lattitude, elevation and average cloud cover.

http://www.jgsee.kmutt.ac.th/exell/Solar/Intensity.html
http://www.thermotechs.com/usdata.htm

I have measured net solar radiation that peaks at about 1,200 Watts per square meter in the Philippine rice fields. So I guess, about 350 W/m2 would be the saturation point for citruses. In literature, for healthy Citrus sinensis plants and adequate CO2, and optimal water supply and optimal air temperature, the saturation would be at Photosynthetic Photon Flux Density of about 600 µmol.m-2.s-1

plants like cacti, perhaps pineapples, would require dark nights as they only open their stomates during the night and closes during the day. These are plants that uses the CAM pathway of photosynthesis.

Grasses like corns, bamboos, sorghum are C4 type of plants, they have a lot higher PAR saturation point compared to C3 plants like citruses. The C4 plants have also higher photosynthetic rate at higher temperature while the C3 plants have photosynthetic decline above certain temperature.

As Millet have mentioned, cultivars that are day-neutral and are not affected by daylight length, you could almost set them to what ever light exposure you want. In UC Davis phytotrons, I have observed the lights on citruses kept for more than 18 hours during the winter. But if you have pineapples and other succulent (most probably CAM plants), it is not a good idea to put them together with citruses when using abnormally prolonged light exposure.

Hi Millet,

Great info as always. A little clarification if you would..............If you run your lights from dark till 10 (about 5 hours?) does that depend on your plants getting any other natural light? Hypothetically, if they only got the 5 hours of supplemental lighting, would that be enough for them to flower & fruit or just maintain foliage? Also, please share your lighting setup.

I was also wondering about the effect of the tops of the plants getting (what seems to me) the bulk of the light, but not so much the bottom of the plant. I have a meyer lemon that is more vertical than bushy. It has five nice-sized lemons on it, but they are on the bottom branches and that area doesn't seem to have much of the light shining on it at all. How is the photosynthesis actually made/used? Does the top take in "enough" to keep the whole plant going?

I do apologize for the both the volume and the "newbie level" of questions that I post. This is my first winter bringing in the plants and I am trying to do the best for them............and, honestly? I just find the whole thing facinating! Thank you so much, Stephanie

Steph, all of my trees are in a large greenhouse. So at present they are in full sunshine from 7:00AM until 6:30PM when the sun sets. I then light only two of my trees, the Marisol Clementine, and the Cara Cara Navel. Both of these trees were planted in the ground December 18, 2005. They are now about 3 feet tall and 2-feet wide, with perhaps 20 to 30 side branches. Both are very full healthy trees. I use 1000 watt HID lights from sun set until 10:00 PM, which gives the trees 14 hours of light a day. Both trees have already flushed new growth 4 times so far this year, and I'm hoping for a 5th flush before years end. I have a picture of the Marisol Clementine on the In Ground section of this forum, dated somewhere around December 2005. I intend to put another picture of the same tree on December 18, 2006 when the tree has been planted for one year. The growth is stunning. Concerning your question about your extra lighting hitting mainly only the top portion of your tree. The energy produced from the leaves on the upper part of a citrus tree goes to support new growth and provides energy (food) for the upper part of the tree. The leaves on the lower (bottom) portion of a citrus tree, supports roots and lower trunk growth. When people quickly prune the lower branches off from the bottom of the tree, in order to make the tree look like a standard instead of a busy type, they are greatly sacrificing root and trunk growth. Leave the lower limbs on as long as possible before pruning them off. About your tree's healthy survival over the winter. Don't worry so much about the foliage portion of the tree. It is the root system that does all the dirty work, and the leaves and branches get all the credit. The best advice I can give you for winter care of your indoor citrus tree, is keep the container's soil temperature at least at 64F (18C), and if you see any trouble raise the container's soil temperature up to 70F (21C). When the roots are not happy no part of a citrus tree is happy. Take care, I always enjoy you postings. - Good luck. - Millet

I am learning a lot in this series. With excellent questions by Steph and a wise answer from Millet and Joe.
Thank you guys.
Benny

I second Ben's applause.
I must have spent an hour thinking about the questions and
studying the answers.
Thanks to all the participants.