I live in Rye, N.Y and I also have been having similar problems with many of my 2 dozen potted citrus. I have had most for 2-3 years and they have seemed healthy. Fought off citrus red mites last winter with horticultural oil and pre-treated them in September before bringing them into the house this year. They are in a sun room with ambient temps around 70 degrees. They get east and southeast sun for half of the day (when there has been sun). They are potted in 5 gallon pots with either a mix of citrus/cactus soil or Moisture-control potting soil mixed with orchid bark stuff. The pots have extra drainage holes and the plants are not allowed to sit in dishes with water.
They grew really well this summer with lots of flowers and fruit. The new leaves appeared chlorotic so I fed them as best I could with iron and minor elements as well as a general plant food (Ran out of the citrus foliar and citrus liquid I had brought from Florida several years ago.) They never really looked great, but.... I top-pruned them so they would fit in the house with the 2 dozen Rosa Sinensis I have and brought them in. I've since obtained citrus foliar feed (but can't spray while in the house) and some granular citrus food. I put about a teaspoon of this on them after bringing them in having read that citrus are "heavy feeders" and they looked kind of "pale".
They soon began dropping leaves which I assumed were the previous year's growth but the leaf drop continued and several plants began to get shriveled leaves which turned brown and the trees failed. No bugs, the soil smelled normal. I unpotted one of the "sick" plants (a Buddah's Hand) and found mostly roots, none circling the pot, no rotten roots or bugs in the soil. I cut away some of the larger roots and put the plant in a clean pot with the above mentioned soil mix and I'm waiting for something to happen.
So, basically I have had citrus trees with leaf drop of 1) Apparently healthy leaves; 2) Leaves that shrivel and brown around the edges before falling; 3) Branch tips that wilt and don't respond to watering (if previously dry).
Somehow I thought I had saved my "grove" last year after the mites, but I'm unable to fix their problems now. I'm attaching some pix with the hope that you can somehow advise me since what I read on your blog seemed entirely reasonable for Hibiscus and Citrus.
I would be eternally grateful if you could help me manage my indoor citrus.
Thank you for writing. I can’t even imagine how nerve-racking it must have been to deal with the red mites given the amount of citrus you have potted up. I’m so glad you were able to control the situation. Let me share with you my knowledge of citrus, along with the aid of some literature here in our library, and hopefully I can provide some new information to help you.
Regarding the sunlight it sounds like the room has a strong enough exposure that your many citrus get the hours of direct sun that they need. I assume in summer you place them in an area with an equal amount of direct sun.
A constant temperature of 70 through the winter sounds good. Your citrus could probably even tolerate colder temperatures, into the 60’s or upper 50’s, but I understand that you still want to enjoy your sun room as well. The only other thing about the temperature, as reiterated in my research, is that you want to make the temperature transition as smooth as possible when moving the plants indoors or out. I assume the sun room gets warm during the summer, but in the fall if the room stays around 70 then I would move the plants in while it is still nearest to that temperature outside. Of course we also need to take into consideration the nighttime temps in the fall and spring as they can drop pretty low compared to daytime temps. I have seen people leave their citrus outside until it gets in the 60’s or below and then move them into an enclosed, warmer 70 degree room and that can certainly lead to some shock and leaf drop.
Certainly the citrus and/or cactus soil is great because it has such excellent drainage. I would probably add some quality compost to it as well to increase the nutrient value and water holding capacity. I am curious to know more about how the plants in the moisture control and/or orchid bark mix are holding up. Both of those media can hold moisture for long periods of time without drying out and I was taught that all citrus enjoy being in soil that is allowed to dry out a little between watering. I’m sure the extra drainage holes you added make all the difference.
Citrus are heavy feeders, but you will want to concentrate more on when they are feeding most heavily. During the warmer months when the days are longest and the temperature and humidity is at optimal levels your citrus are going to be most active. In combination with the fact that they are all containerized, this means that fertilizing is most important through the spring and summer. Most citrus fertilizers have a higher first and third number, which means higher doses of nitrogen and potash compared to phosphorous. The nitrogen is important for the green of the plant, the development of foliage for photosynthesis. Potash is important on the cellular level because it aids the overall structural health of the plant. During the summer I would fertilize regularly every two weeks with a citrus fertilizer or another fertilizer as long as it is high in nitrogen and potash. If your local garden center does not carry a product specific enough I was able to find a number of adequate fertilizers for sale online. This time of year, since the plants are inside and still getting ample light, I might still fertilize but definitely less than during the summer. Since chlorotic foliage is an issue I might switch to a foliage fertilizer, with a high nitrogen content and lower phosphorous and potash, and apply lightly and sparingly through the winter. Usually I tell people not to fertilize in the winter at all but in your case it might help. Even though your citrus are evergreen they are still in more of a resting period this time of year. Over-fertilizing in the winter, when the plant is not active enough to take it all up, you can run the risk of adding too many soluble salts to the soil and that can inhibit nutrient uptake even if nutrients are present in the soil. I doubt you have, but if you fear you have over-fertilized, you do have the option of flushing the soil with a heavy watering and leaching out excess salts.
Aside from the fertilizing, the only other issue that seems most pressing that I haven’t mentioned is the humidity. Dry winter air is brutal on our indoor plants so increasing humidity in the room is the first thing that came to my mind when you mentioned shriveling leaves and wilting tips. I have oversized glazed or plastic saucers with gravel below all my indoor plants. Excess water that collects in the gravel “buffer zone” evaporates in the immediate area around the plant and should help those dry and wilting tips. Some times I will even put a little water solely in the saucer if I know the soil is moist enough. This winter I went a step further and finally bought a humidifier and all of my plants look so much better. Increased humidity will not only increase the health of the new growth and the older growth to hold on, but it will also help combat pests like the red mites who thrive in drier conditions. I think even a spray bottle with a fine mist would be a help during these dry months if that isn’t already part of your routine.
I hope this information helps give you some new insight and is not too repetitious to what you have already read or know yourself. If you have more questions send them my way or feel free to call me in the office during the work day to discuss. Good luck.