Though there is always a risk that you may overstress a plant by encouraging it to grow too fast, there are also methods you can use to encourage growth that are generally quite safe. There is no guarantee that any single method will cause the plant to respond the way you want it to and it may take a combination of several methods to promote plant growth. In order to understand why different methods work, it is necessary to understand the processes that take place in the plant when the method is employed.
Promoting Plant Growth
Ensure the plant is receiving an adequate water supply. This means not giving it too much or too little. Some plants require almost a constantly moist soil. Others, such as some tropicals and desert species, may prefer the soil to almost go completely dry before watering again.
Add additional light to the plant so that total light exposure is in excess of 12 hours each day. Light causes the plant to produce more sugars through photosynthesis. This sugar is used as energy by the plant, which converts some of that into energy used for growth. Further, plants that get less than 12 hours of light each day are more likely to enter a period of dormancy, where no growth will take place (see reference 1).
Use appropriate fertilizers at appropriate levels. Fertilizers should have nitrogren, phosphorous and potassium, known collectively today NPK. Follow all label directions, but generally fertilizer for house plants needs to be added every two weeks (see reference 2).
Remove competing species if the plant is outside. Soil can only provide so many nutrients and if there are many different species competing for these limited resources, your desired plant growth may be stunted.
Use mulch around plants. Mulch helps prevent invasive species from taking root around a desired species. A study by Washington State University in 2000 (see Resources below) found that in restoration efforts, the locations that had mulch had more vigorous and health plant growth than those that did not. Mulch should be at least 2 inches deep but not directly touch the trunk of a plant in order to help prevent fungal infections.