Winter is the most difficult season for acid-loving houseplants such as the azalea in the banner photo. They generally have to "drink” tap water during that time period, and tap water tends to be hard (alkaline).
Also, the combination of chilly windowsills and too much water can deplete oxygen from their soil. Both that and the already mentioned alkalinity limit the plants' intake of iron. Without enough iron, they can’t make chlorophyll. When that happens, chlorosis appears on the youngest leaves, often turning them yellow while their veins remain green.
Plants which might suffer from this problem include any which prefer acidic soil, such as azalea, camellia, crossandra, gardenia, ixora, and citrus trees. The symptoms are likely to be worse if, like me, you don’t get around to repotting often—which can cause a build-up of calcium in the soil.
For a quick fix, apply liquid iron chelate. The type I purchased includes small amounts of sulfur, copper, and zinc as well. The instructions recommend measuring 1 teaspoon of the minerals into 1 quart of water before pouring the resulting mixture into your houseplants' soil.
In my experience, the treatment—which is supposed to be repeated every couple weeks or so--doesn't make the leaves quite as green as they should be, but it does help. To completely solve the problem, you still will need to address the issues which cause it.
If possible, water your indoor plants with rainwater instead of tap water, preferably rainwater which has warmed to room temperature. I attempted this one year, by filling a large number of gallon jugs in late autumn, lugging them inside, and shoving them under a table in the basement. My supply was exhausted quickly, though, and I didn’t have the stamina to keep dipping more out of the icy rain barrel.
However, those of you who live where freezing temperatures are uncommon should have access to rainwater year round. Of course, you're probably able to leave your potted plants outdoors year-round too!
The rest of us can allow our hard tap water to sit in its can for about a day before we apply it to our plants. Most of the calcium should sink to the bottom and we can throw out the lower inch or so of water after using the rest.
We also should make it a point to cut back on the amount of water we give our plants during winter, so their soil will retain more oxygen. Those of us with a tendency to over-water can keep susceptible species in an acidic fast-draining mix, such as that intended for cacti and citrus trees.
I generally don't feed plants over the winter because they aren't growing much then. If you do, be sure to select a plant food intended for acid-lovers which is low in phosphorus and contains micronutrients such as iron, magnesium, and sulfur.
Keep in mind also that manganese deficiency can mimic iron deficiency. So, if treatment with iron doesn’t help—or even seems to make the chlorosis worse—it may be necessary to apply manganese chelate instead.
Plants will bloom despite yellowing foliage, as you can see from the photos in this article. But those flowers would look much better against a background of evenly green leaves rather than splotchy ones!
Photos: The azalea banner photo and the ixora photo are my own. The iron deficiency photo is by Scot Nelson and the gardenia photo by Anna, both courtesy of Flickr Creative Commons and this license.