I prefer to fertilize my Mandarin oranges and Tangerines by a slightly differing schedule from other citrus in my collection. Fertilize annually starting in the early spring when new growth has begun to appear and repeat this process and in Mid spring and again in late summer .
One-year-old Mandarin orange trees should be fertilized with an additional cup of either ammonium sulfate or citrus tree fertilizer divided equally over the three feeding sessions. Increase the application to 2 cups for 2-year-old trees and continue this regimen for the third year. By the 4th year cut back to 1 cup twice annually - starting with the early spring and ending in late summer. It's okay to add some other fertilizer from time to time but don't get carried away. Excessive fertilizer can do more harm than good. Fertilizer developed specifically for citrus trees is naturally a good choice.
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.
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 a 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.
If you are delving into indoor gardening ~ indoor plants you might want to consider a hygrometer, an indoor temperature and humidity monitor.
Mandarin orange trees are susceptible to spider mites, mealy bugs and scale. Check the trunk for mealy bugs and scales. Wipe the foliage periodically with a damp paper towel or sponge. Dust will attracts pests, they hide themselves and their egg clusters in the dust while parasitizing the plant . 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.