Citron tree losing leaves
I have two citron trees that I've been growing for about 2 years. Because it gets cold here in the winter (citrons are native to the middle east and north Africa), I have them in large pots and take them inside in the winter and use a grow light. Recently one of them has started losing a lot of leaves. This happened to another tree last year and it ended up dying so I'd really like to try to save this one. In addition to losing a lot of leaves, there is also some discoloration (see the attached images). I use Miracle-Gro Cactus Palm and Citrus Potting Soil Mix and Miracle-Gro Fruit and Citrus Plant Food Spikes as fertilizer. When this first started happening I made sure to water the tree a lot, and the moisture meter says the soil is still moist. Am I under watering? Over watering? Is there some nutrient I'm missing? Also, what should I do about branches that have lost all their leaves? Should I trim them off or should I leave them hoping that the leaves will grow back? Thank you Ari
Using a grow light indoors on high-light lovers such as citrus-family plants is good, as natural light is often insufficient unless the plants are in a greenhouse. Make sure your grow light is powerful enough to make an impact and a light intended for plant growth so the spectrum is the most useful for the plant. Light intensity is difficult to judge by sight since human eyes adjust so easily to varying light levels; reading the specs on the light's packaging can help, though still isn't always solidly useful. Lumens are measured in a way that skews levels towards parts of the spectrum that humans see the most easily, which isn't the same part of the spectrum that plants utilize most. Lights labelled "full-spectrum" are best, especially if you prefer a white light versus the more plant-specific spectral array of lights that are that jarring red-blue combo. Wattage also isn't the best measure of brightness as it matters how energy-efficient the lights are, but in general, fluorescent (especially the high-output types) and LED plant lights are best with regards to the most bang for your buck with regards to brightness resulting from higher wattages.
Citrus-family plants can be more demanding of nutrients than other houseplants, but the use of fertilizer spikes may be complicating matters since some roots could be getting over-fertilized and others under-fertilized given the uneven distribution of fertilizer in the pot. Time-release fertilizers in and of themselves aren't a bad idea, but a more even distribution on and in the soil would work more effectively. Similarly, liquid-applied fertilizers (whether already in liquid form or those that get dissolved in water) also better distribute the nutrients in the root zone. If the plants haven't been repotted in the 2+ years they have been in these pots (were they in this soil when purchased?), it's also possible excessive fertilizer salts have built up in the soil. In that case, repotting them in fresh potting soil in spring will help minimize the root damage that comes with excessive nutrients. While the foliage is showing signs of a nutrient deficiency, there are other possible causes, including excesses of some nutrients that can interfere with the absorption of others.
Nutrient deficiencies can also be caused by improper soil pH (at some point the nutrients become chemically bound to the soil and hard for the plant to absorb) and a compromised root system. Roots that have suffered dieback from either being too wet or too dry for a long enough period will simply not have enough access to the nutrients even if they are present in sufficient quantity. Repotting may also be the answer in this case, but in the meantime you can carefully tap the plant upside-down out of its container and check on the appearance of the roots. Mushy, brown, or malodorous roots often indicate they succumbed to rot from overly-wet conditions; firm, white or creamy roots with nothing but an earthy smell are typically healthy. Roots that are tugged on and easily broken are dead. If you think the plants are not rooted-in enough to this pot size to do this without the rootball falling apart, you could wail until spring.
Citrus grow best when allowed to dry a bit between waterings, so make sure the pot has unobstructed drain holes and that the saucer underneath the pot does not hold water. To keep the plant minimally stressed, keep it in cool conditions (in the low 60s), bright light, and with as much humidity as you can manage. (Many homes in winter have poor humidity, though raising it some can benefit people as well as houseplants.) Typically indoor plants need little or no fertilization in winter, though this plant in particular may have some catching-up to do. This pattern of yellowing could point to several possible nutrient deficiencies; determining which it is can be challenging. That said, any well-balanced fertilizer that contains micronutrients (beyond the N-P-K on the main label; they will be listed if present) such as Iron and Manganese should correct the problem if this is the only cause. New growth should start to look more evenly green; older leaves may or may not green-up, as different nutrients relocate differently in plant tissues and some are only routed to new growth. While the leaves with yellow in them are missing some chlorophyll, don't remove them or their bare branches as they are still partially functional and we don't want to take away too many of the plant's resources. Shorter winter days and dimmer light than summertime levels will result in slow growth indoors even among healthy plants, so do not be alarmed if progress seems slow. The important observation will be that the decline isn't progressing, although keep in mind that there may be a lag time between the cause and effect in the expression of symptoms. For example, if poor roots are to blame, some further leaf loss may happen before the root system has recovered enough to begin reversing that and allowing for replacement leaves to grow.