Interesting to see a Eureka dropping leaves. My largest Eureka is not next to a window at all, but the room is bright and warm on sunny days and not dropping any leaves. Have you checked for mites or the condition of the roots? What type of soil are you using? How are you checking to see when to water? Are you fertilizing at all? I stopped organic fertilizing in September, but think I need to use a liquid fertilizer now that my trees are indoors.

There are some small mites which appear to be red mites. Doesn’t appear to be a huge infestation but there definitely are some.

For soil, I am using a mix of mostly bark with a little perlite, sand, and sphagnum moss. Watering when it feels dry an inch or so below the surface. I did notice the root ball itself sometimes appearing to get drier before the soil along the sides of the rootball so I’m trying to monitor now based more on the root ball. As for fertilizer, I did a small application of granular fertilizer over a month ago.

Could be winter leaf drop. http://tropicalfruitforum.com/index.php?topic=13332.0

Just watered it the other day and number of leaves falling off have increased since watering it. Does that suggest the root ball got too dry?

Winter leaf drop is due to too much light on the leaves and not enough warmth on the roots. You may decrease light on the leaves or increase heat on the roots.

I will try to heat the roots but wouldnt you still need the light? I would think that decreasing the light would be harmful. Would it help to both decrease light and warm roots?

This is what Millet says in the link I posted above:

Keeping the trees in a room with high humidity seems also to slow down
WLD but cannot prevent it. Also a place more in the shade, to minimize
leaf activity, slows down WLD. Keeping the root temperature below 64 F
(18 C) but at or above 59 F (15 C) seems to work best for stopping WLD.
The plant functions are minimized, but water and nutrient uptake for
leaf activity is high enough to support the breathing process and leaf
surface cooling by water evaporation. If WLD persist, force the root
temperature higher, around 70 F (21 C) this should stop leaf drop.

So, he suggests a root temp of 59 - 64 F and keeping the tree "more in the shade". (I would give it bright indirect light.) If that does not work, then increase the root temp to 70 F.

Be sure to keep those blinds up so the plant is getting all the sun possible. I think the leaf drop is a result of low light levels.

Millet has grown citrus in containers for many years and is the moderator of the Citrus Forum on the Tropical Fruit Forum. I would trust what he recommends.

But has he ever grown citrus in a northern state?

Winter sun in FL, and in OR are not the same.

FWIW, I have been growing citrus inside for 55 years. They need sun in December.

Does Colorado count?

don't forget Ohio. For me

Depending on the temperature of the tree's root zone, iochroma's advice of ""keep those blinds up so the plant is getting all the sun possible"" is a recipe for a complete utter disaster.

Thanks for everyone’s input and advice. I have tried getting more sunlight and supplementing with a grow light over the last several weeks but it didn’t seem to really help. Continued to lose leaves. Trying to warm the soil now so we will see what happens. What is the best means to warm the soil? I have a heating pad but it doesn’t seem terribly warm.

All of my other citrus are still fine without significant leaf drop at all. That’s what’s so weird. Watering schedule and soil medium is same for all of them.

Silica, good to see you again.

I have used a seed starter mat to warm containers.

Maybe the one I have is defective. I know they aren’t supposed to heat much but the one I have you can barely feel any heat even directly touching the mat. Any recommended brand?

I have used this one: https://www.amazon.com/gp/product/B013HFKRKW/ref=oh_aui_search_detailpage?ie=UTF8&psc=1 but any one will do. I also use this controller with it: https://www.amazon.com/Hydrofarm-Germination-MTPRTC-Controller-Thermostat/dp/B000NZZG3S/ref=sr_1_6?ie=UTF8&qid=1546231686&sr=8-6&keywords=jump+start+seed+starter+mat

In my understanding, WLD happens when the roots are cold (usually below 12c) and the leaves are receiving too much/sufficient amount of light that would heat the surface up. As a response to this warming, the stomata will have to release water to cool the leaves down. Since the roots are too cold to absorb and provide water to the leaves, the tree will drop the leaves instead. You can either lessen the light or warm up the roots to help with leaf drop. Seedling heat mats work just right for this purpose.

Millet's article has some great points. However I cringed when I read the line, "The temperature tables from the book Biology of Citrus show that citrus stops root growth and root function if the soil temperature drops below 54.5 F (12.5 C)." That statement is misleading in the context of the article and gives people the false impression that their is 'substantial' root activity down to 54.5 F.

From, Kenneth B. Bevington and William S. Castle, University of Florida, Institute of Food and Agricultural Sciences, Citrus Research and Education Center, "The most intense root growth occurred when soil temperatures were above 27°C, and was limited at soil temperatures below 22°C" (see their tables in the link below).

You have to decide whether your winter goals are overwintering and keeping the plant near dormancy or continuing the growth cycle through the winter. Indoors no matter what the season it is very hard to encourage root activity as most homes are heated/cooled to 70F. When I have dug up a citrus that is 1 or 2 years old I have been shocked at how little root activity has occurred. This is why I am transplanting this weekend into hot galvanized pails so I can keep the roots in the growing zone.

Annual Root Growth Pattern of Young Citrus Trees in Relation to Shoot Growth, Soil Temperature, and Soil Water Content

Millets' comment "Keeping the trees in a room with high humidity seems also to slow down WLD but cannot prevent it." What he could have added is reducing humidity below a threshold will trigger WLD. Why? because in the winter a heated home will have a large enough VPD caused by dry heated air and low RH which citrus can't tolerate for extended periods of time. In this condition the leaves will evaporate moisture faster than transpiration can replace and will trigger winter leaf drop.

Interestingly I have had much less leaf drop since the original post. Though no new leaves have emerged just yet, there has been a burst of new bud activity which makes me think it’s heading in the right direction.

If you are growing indoors a digital temperature and humidity meter and an off contact infrared thermometer are absolutely indispensable. You can buy them on Amazon cheap. Here is an example of a critical situation I was able to address having these tools.

Outdoor temperature -11 C
Indoor temperature 22.5C
Heated roots 29C (germination heating pad)
Container hot galvanized pail

RH near leaves 50%
South facing window 12:00 PM
Hardiness Zone 6a
Date Jan 13th
Leaf temperature 35C (critical) (95F)

At 35C and 50% RH the plant is going to exhaust roughly 3x the moisture it would at 20C. By adding a fan the temperature dropped immediately to 24C. (75F). That is an immediate 20F drop in leaf temperature.

I have one of those infared thermometers too. Love it!