Hi all,

I'm in zone 7a near Wash DC and have my container citrus in full sun outside all year except winter they're inside next to a sunny window.

My question is--is flavor improved greatly if the last say month of ripening the fruit can be outside in full sun? I.e. should the ripening times be kind of synchronized to not have the last key month or two of ripening be during dead of winter when it has to be inside next sunny window?

If that is true, I wonder if fall and summer varieties would be preferable for container citrus that's dragged in and out for winter to let them ripen the last couple months outdoors in full sun???

There was just a post about overwintering citrus in complete darkness, with a comment that fruit ripened normally. Maybe they can comment on flavor. If it tastes good in total darkness you should be okay with more than that.

PersianMD2Orchard, it doesn't matter whether you are growing an early, middle, or late maturing citrus cultivar inside your house during the winter months. The sun light coming through a southern window provides more then enough energy to mature the fruit. The maximum amount of energy that a citrus tree can generate (use) from the photosynthesis manufactured by the sun shining on its leaves is at a level of 650-PAR, which is only 1/3 of full sunlight. In other words, for citrus trees, the amount of energy it can produce, and use, by photosynthesis maximizes at 650 Par (on a completely sunny day PAR is about 2,000). Therefore, the light coming through the southern window is more than enough. Further, the total amount of the energy required to grow and mature a citrus fruit from bloom to a mature ready to eat fruit, is manufactured by only the 3 or 4 closest leaves to the fruit. Therefore, your indoor tree will receive many, many times more energy then it needs to mature the trees fruit. HOWEVER there is a caution: If you set a citrus tree directly near a southern window in order to receive the direct rays of the sun, be SURE, that the container's soil (the tree's root zone) stays at, or above 70F to avoid WLD (Winter Leaf Drop). Welcome to the Citrus Growers Forum, we are glad to have you as a member. Citrus growing is a great hobby, enjoy your tree. - Millet

Thanks very much for the responses and that is good news so I don't have ripening time limit when choosing new cultivars to add. I know with other fruit the last month or two so can be key with developing flavor and sugars etc. It kind of makes sense to me that citrus would need less sun at final ripening stage than other fruit since they are used to ripening in colder weather.

I do have to ask as a follow-up to understand this better--are you saying citrus only need 1/3 full sun year round? Can they really reach peak production and flavor with that low of sun year round? Also does Wash DC 1/3 sun day = 650PAR or southern California 1/3 sun=650 PAR? Just making sure I'm not missing something here. Sounds a little too good to be true if citrus can flourish with 1/3 sun here?

Citrus trees originated in South East Asia area, and grew as under story trees beneath the taller trees. Place your tree in an area where it can get the most light possible. A citrus tree generates its food from the light falling on its leaves up to the light range of 650 PAR. Giving the tree more light will not cause the tree to produce more food (energy). However, longer periods of light, such as lighting your tree at night will keep the tree producing energy (food) for itself. As to your California/Washington DC question, PAR is the same the world over. It is the ranges of light from 400 to 650 nanometers the light spectrum used by plants, which is about 1/3 of the full light spectrum. . As an aside note, to insure your tree does well during the winter, if you not familiar with the indoor winter growing of citrus term WLD you can look it up using the search function on this forum. - Millet.

With regards to your question on PAR varying with location:

PAR - photosynthetically active radiation - is a measure of the amount of light (specifically, that part of the spectrum which is photosynthetically active) at a given moment. PAR varies based on a number of factors, including (in no particular order) time of day (this affects the angle of the sunlight, and thus the length of the light's path of travel through the atmosphere), season/latitude (again, affecting the angle), altitude (affecting the depth of atmosphere through which light must travel), atmospheric conditions (water vapor and particulates block light), shading, reflectivity or absorptivity of surrounding objects, etc...

So, if it is pitch black, PAR is zero. If it is midday on an absolutely clear day, then the PAR would be at its highest - in the range of 2-3,000. When Millet said that citrus can generally only use 1/3 of full sunlight, he was referring, I think, to this latter scenario. Importantly, he wasn't referring to the length of exposure to sunlight, but to the shading at full sunlight - i.e. at midday. That is, he wasn't talking about a citrus plant that is in complete shade for 2/3 of the day and full sun for the other 1/3, but one that is 66% shaded at midday on a clear day. While a citrus tree would suffer no reduction in productivity because it is 66% shaded at midday on a clear day in summer, that doesn't mean that it would be at optimum efficiency if it were in 66% shade early or later in the day, or on a cloudy day, etc..

Ultimately, what is important for a tree's productivity is that it be at or above that 650 PAR threshold for as long a time as possible.

To look at your example of So.Cal. and D.C.:
On midday on a clear day, a tree at either would have more than enough sunlight.
There are, however, differences:
1) D.C. is a slightly higher latitude. This means that on the summer solstice, its days are actually longer than So. Cal., but in the winter its days are somewhat shorter. Overall the total hours of daylight are (roughly) the same. However, the angle of sunlight varies, and overall the lower the latitude, the more solar radiation over the course of the year.
2) Weather is different; D.C. is generally more humid, and has more cloudy days, thus reducing the sunlight available to plants.
(To get a relative sense of the effects of 1 and 2 on the amount of solar radiation an area gets, do a search for an insolation map.)
3) Temperature differences: Cool temperatures (as well as other environmental factors, like availability of water and nutrients, or even excessive heat) can effective the photosynthetic efficiency of the plant, independently of how much light it is receiving.

As to the confluence of all these factors in your particular case, short of using a light meter, I would defer to Millet in his suggestion that a southern exposure generally provides plenty of light during a northern hemisphere winter.

Wow ! That is a keeper. I took a screen shot of that last post to save in my picture album for a long time. Great stuff. After reading it three times I think I understand. Thanks. Tom