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Plants thrive in moist, well-drained soil high in organic content. Gardeners should irrigate soil when the top layer feels dry to the touch to prevent both dryness and waterlogged conditions.
Garden phlox is susceptible to a disease called black root rot. Caused by the fungal pathogen Thielaviopsis basicola, this disease often lives in soil for years. It is most partial to a soil pH between 5 and 8.5 and thrives in soil temperatures between 55 and 70 degrees Fahrenheit. Infected phlox plants display yellowing, wilting, leaf drop and slowed growth. As the disease progresses, roots begin to decay and display black areas of dying plant tissue. To control black root rot, maintain sanitary conditions in the garden, particularly by sterilizing tools before moving to a new plant. Applications of a fungicide with an active ingredient such as thiophanate-methyl offer effective control when applied at the first sign of a problem. However, gardeners may need to contact a licensed professional as this fungicide may be unavailable for home use.

Yes, deadhead the plants often.

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Seeds can be sown in fall.

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The proper method will be removing the flower, and then the stem as it dies, but cutting all the way down all at once will not hurt anything. In fact, it will save the plant some energy in the long run.