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Background: I have a near-one year old lemon tree sapling (kept indoor), which was very healthy until December. I then left the country for one month, leaving it to my flatmate -- who overwatered it. The sapling got (I assume) root rot, and lost some leaves.

Now, I am back and trying to "heal" it: I water it less, changed part of the soil in the pot, and am using a grow light (this one) turned on and pointed at it during the day. But while it is now making a new (small) branch at the top, some of its previous leaves still are getting brown (see picture below), and one fell yesterday.

Is it because of the light?

enter image description here

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  • That grow light is unlikely to put out either enough light or enough heat to cause the damage I see on the right-front leaf. It's only 7 watts.
    – Wayfaring Stranger
    Jan 24, 2017 at 17:19
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    Where are you and how cold does it get outside? Is your window facing any direct sunlight and if so, for how many hours? How big is the pot, and what is the growing medium used?
    – Graham Chiu
    Jan 24, 2017 at 18:49
  • I am in New York, and the window is facing East (not a lot of light in winter, most of it between 9am and 2pm, and temperatures between -2—5 Celsius outside; my office is heated, so it's warmer inside, maybe 12-15 Celsius near the window). The pot is reasonably big, I changed it in November as the tree had overgrown its previous own. As for the medium, you mean the soil? It's soil for gardening/potted plants I bought, including (IIRC) nutrients.
    – Clement C.
    Jan 24, 2017 at 18:53
  • Your tree needs more light in winter, and misting of the leaves. See this question gardening.stackexchange.com/q/17773/1894 lemon trees don't go dormant in winter as they're native to Asia.
    – Graham Chiu
    Jan 24, 2017 at 20:12
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    They like high humidity, and it gets really dry in winter. Some people suggest water in a plate under the tree but that sounds doubtful to me whether it's going to raise the humidity locally. So regularly mist the leaves with water when inside.
    – Graham Chiu
    Jan 24, 2017 at 23:50

5 Answers 5

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I think it is just over-watering. It takes some time for the plant to find its equilibrium again.

Keep it dry and possibly on a cold and shadow place, to simulate winter, to let it restore the roots and the stem.

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If your tree has been overwatered for a month and has rotting roots, then you need to trim the rotting roots off before it spreads to damage your healthy roots. This means you need to remove it from the pot and inspect the roots. See if there are soft or smelly roots present.

Your citrus should not have developed this problem if it had had a freely draining citrus mix and you just used an ordinary potting mix which is incorrect. These often have water retaining gels which makes the situation worse. Now is the time to change it. This usually means removing as much of the existing mix from around the roots as possible so that you don't create a pot within pot situation, and repot with a citrus/cactus mix.

The light you used is a LED so can't burn leaves as they're not supposed to produce heat. But as citrus plants are of tropical and semi-tropical origin, they want full sun in non-tropical regions all year round. This means providing artificial lighting as discussed in this answer, and regular misting with water as they prefer high humidity. Some people suggest putting a plate of water with pebbles below the tree to increase the humidity but that seems far fetched that that would work. They also like some ventilation so some people will put a fan on their indoor citrus trees for a few hours a day.

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  • Citrus can be put in shadows (and relativelly cold) for 4-5 months without problem. It was so done in my regions, and see also the "fashionable" en.wikipedia.org/wiki/Orangery few centuries ago.
    – Giacomo Catenazzi
    Jan 25, 2017 at 7:56
  • @GiacomoCatenazzi "Orangeries were generally built facing south to take advantage of the maximum possible light, and were constructed using brick or stone bases, brick or stone pillars "
    – Graham Chiu
    Jan 25, 2017 at 8:18
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    but on very north, not so sunny cities, and many many plants put together (in winter). My family citruses, producing lemons, and very old,doesn't get so much sun in winter: packet together, and not so big and sunny place (other plants shadows the windows).
    – Giacomo Catenazzi
    Jan 25, 2017 at 8:30
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    Any tree outside is going to get far more light then inside.
    – Graham Chiu
    Jan 25, 2017 at 9:45
  • My potted lemon certainly drops leaves and shoots upwards if overwintered too dark. In a mild winter (southern UK) I've had more success leaving it in an unheated greenhouse, though I'd still lift it in for a serious cold spell.
    – Chris H
    Jan 27, 2017 at 9:27
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To put it simply:

I doubt its the light. I doubt it's the water (kind of). I bet it's the soil.

My lemon trees seem to love a lot of light and a lot of water. But they live outside, in the harsh Australian sun; and very dry, very sandy, arid (practically hydrophobic) soil, where nothing else will grow.

I don't know what soil it's in at the moment, but your description is consistent with dieback from root-related trauma. My bet is that there's now something else (fungus, nematode, arthropod, pathogen, bacterium, whatever) living and thriving (or dying and rotting) in that waterlogged substrate; and it's causing collateral damage to (if not targeting directly) the roots.

If it were mine: I would literally uproot the entire thing, wash the roots off (maybe with distilled water if possible?), try to let them dry out a little bit and plant it (the same day) in some drier and / or very well drained soil. It will likely be a significant shock. But you've soiled your soil. You need to remove it from its current environment and put it somewhere inhabitable to whatever (probably micro) organism it is, that's doing the damage.

Hope that helps.

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  • I feel quite anxious about uprooting it. I don't have a lot of experience, and care deeply about the plant... I would rather avoid doing something so drastic if I can avoid it
    – Clement C.
    Jan 26, 2017 at 3:43
  • Well, it's your plant, so that's your prerogative. But you asked for advice to save your plant and now you have it. I'm sure it'll be fine. Surgery is drastic, but if you have rotting, parasite ridden, gangrenous flesh, you need to operate; clean it all out before it claims a limb. If you need to amputate something, I suggest you do it before it claims a life. But you have the subject in front of you, so you be the judge. The choice is yours. If it makes you feel any better; it probably grows best suspended in mid-air with an aerosolised water/nutrient solution routinely sprayed at the roots..
    – voices
    Jan 26, 2017 at 5:28
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Plants breath through openings on the underside of the leaves called stomata. With grow lights and with the sun if heat becomes excessive the plants can't keep up with transpiration and the plant will close the stomata to slow down the water loss. When the stomata close to slow down evaporation it also restricts the cooling mechanism at the same time. This causes heat to build up and the plant leaves will become cooked like tacos. When you put citrus in the window and add a grow light it can cook the leaf faster if you don't provide the correct RH and the correct air movement.

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Citrus prefer tropical, subtropical, or Mediterranean climates. Indoors in NYC in the winter the climate is more like a desert, arid and dry. Google vapour pressure deficit calculator to understand the relationship between air and relative humidity

Indoors there is not much breeze and citrus leaves will increase 20-30F in the window. You can check this with an infrared thermometer. You need to provide a fan to move air around.

The leaves are curling down to reduce photosynthesis and reduce transpiration. The curled leaf traps moisture that is being exhausted from the stomata.

The wrinkled leaves are heat and stress related from too much moisture being lost.

Watering it more or less will not compensate for the arid growing conditions. Changing the potting soil can't fix the low humidity problem.

LED lights certainly do produce heat. They also produce UV light. The UV light can burn the leaves.

Between 54 and 72F there is not much root activity. There is a linear relationship between 72 and 85F. Heating the roots can increase the transpiration rate.

The tip of the leaf is probably sunburn but could also be fertilizer burn.

Putting a pan will raise the RH a few degrees. Citrus really wants 60 RH or higher. Indoors you can raise the humidity with a humidifier or you can lower the temperature.

Misting helps. Water with 90F water. If you water with room temperature water it will shock the plant.

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  • I removed your link as it did not add to the question.
    – kevinskio ♦
    Jan 25, 2019 at 11:24

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