I have a long area of established phlox that hang over a wall. They are getting thin and stringy and I think they need to be trimmed. Wen do I cut back phlox in MD and how short should I cut them?
You can cut your Phlox back any time now through early Winter.
Just give them a hard prune by cutting them back to about 3 to 4 inches in height.
They will grow back in the spring.
https://www.gardeningknowhow.com/ornamental/flowers/phlox/growing-garden-phlox.htm
Should you trim all brown from phlox before spring?
Does deadheading increase blooms?
Deadheading can help the plant look more tidy and encourage more flowering, but does not effect the fragrance of your plant.
https://www.gardeningknowhow.com/ornamental/flowers/night-phlox/growing-night-phlox-plants.htm
Deer have eaten the tops off my Tall Garden Phlox. Will they bloom this Summer?
It's doubtful you will have a 2nd blossom form this season.
https://www.gardeningknowhow.com/ornamental/flowers/phlox/growing-garden-phlox.htm
Can I Plant tall phlox in containers?
Yes, though you will need to repot every few years and likely divide the plants.
https://www.gardeningknowhow.com/ornamental/flowers/phlox/growing-garden-phlox.htm
what can I do to get them to flower
You may have too much nitrogen in your soil, this could be caused by over fertilizing.
Too much nitrogen will cause lots of green leaves and no flowers. If you fertilize you should use a balanced fertilizer, like 10-10-10.
By adding phosphorous you can encourage or boost flowers. Bone Meal is a good way to do this.
You can do a soil test to determine the nutrients in you soil.
Here are some links with more information.
https://www.gardeningknowhow.com/ornamental/flowers/phlox/growing-garden-phlox.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/phosphorus-plant-growth.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/bone-meal-fertilizer.htm
just planted 6 lilac variegated floc and had 3 torrential early summer rains which knocked most of the blooms off. should i dead head and hope for more blooms?
Go ahead and deadhead as with most plants this encourages new flower development.