Lemons require 8-12 hours of sunlight daily to prosper, 5-6 hours to just survive . Grown indoors, a South or west facing window is best . Not that I'm comparing apples to oranges, but citrus plants unlike other fruits do not normally go through a period of dormancy or hibernation in the winter, but will tolerate slightly lower light conditions during this phase of slower growth.
Supplemental light , such as grow-lights or fluorescent plant lights will help them produce better, especially if your location is not optimally sunned.
Night time temperatures no lower than 55°F and daytime temperatures around 70 - 80°F are ideal. They will usually tolerate temperatures hovering above 32 degrees for a few hours or heat over 100 degrees so long as they are well watered. Temperatures should however , not be a major problem in an indoor environment. Temperatures below 55 will will invoke a dormancy , extended periods below 55 could result in their premature demise.
You should be using a light [low clay], well draining soil mixture with an abundance of peat, and perlite or vermiculite. Adding additional perlite or vermiculite to any soil you purchase is advisable.
You can also add wood chips, redwood shavings or even hamster bedding and semi-sterile compost [Not from your back yard compost heap] in moderation . Using dirt from your yard is a bad idea.
A common problem with indoor citrus trees is chlorosis. Symptoms of chlorosis are yellowing leaves , or yellow spots on the leaves. The leaves frequently turn yellow while the vein remains green.
Using a balanced fertilizer such as 18-18-18 or a fertilizer specifically designed for Citrus is advisable.
If plants get too dry, salts accumulate in the soil. Under natural conditions these salts would be moist and soluble and harmless to the plants.
Water regularly to keep the soil moist, not saturated, just moist. A layer of decorative sterile mulch such as bark or any other organic mulch is advisable to retain soil moisture. Allow the potting soil to slightly dry between watering, not "DRY-OUT" just slightly dry. Allow the surface of the soil to get dry, but water the plant so there is ample moisture around the roots.
I prefer to water modestly 2 - 3 times weekly , test the soil by hand - stick your finger in about a half inch to be certain it is semi dry before adding more water. Over watering is just as deadly as under watering
Most indoor environments have relatively very little humidity as opposed to their natural environment. Any building that has heat or air conditioning will usually have no more than 15% humidity at most. Citrus plants need much higher levels just to survive and 50% or better to thrive and produce.
Symptoms of Humidity deprivation are
1. Plants begin to wilt.
2. Leaves develop brown edges.
3. Flower buds do not develop .
4. Flower buds drop from the plant before opening.
5. Flowers shrivel soon after opening.Significantly increasing humidity indoors is good for the plants , but it's not good for you or your home unless of course you like mold, mildew, bacteria, mites and so forth.
There are however a number of ways you can increase humidity for the plants without seriously damaging your personal environment.
1. You can add moisture with a humidifier.
2. Cluster your plants in tight groups. All Plants naturally exhale moisture through their leaves in a process known as transpiration, Citrus trees are no exception. By grouping citrus as well as other indoor plants together, you add to a more suitable environment in the immediate growing area.
3. Misting the trees foliage with a simple spray bottle is good way to help citrus cope with the indoor environment.
4. You can add moisture with a humidity tray, a pebble filled tray with water added to the top of the pebbles. As the water gradually evaporates, it raises the humidity in its immediate vicinity enough to improve conditions for the tree.
Lemon trees are susceptible to spider mites, mealy bugs and scale. Check the trunk for mealy bugs and scales. To treat spider mites, spray your plant with insecticidal soap or horticulturist oil, which will smother the insects. clean the foliage, top and bottom sides of the leaves as well. Neem Oil is effective against Scales and Mealy bugs. Rubbing alcohol applied with a Q-tip will also work in mild infestations.